High Boron-induced Ubiquitination Regulates VacuolarSorting of the BOR1 Borate Transporter in Arabidopsisthaliana*□S

Received for publication, September 14, 2010, and in revised form, December 4, 2010 Published, JBC Papers in Press, December 9, 2010, DOI 10.1074/jbc.M110.184929

Koji Kasai‡, Junpei Takano§, Kyoko Miwa‡1, Atsushi Toyoda‡2, and Toru Fujiwara‡¶�3

From the ‡Biotechnology Research Center, University of Tokyo, Tokyo 113-8657, Japan, the §Research Faculty of Agriculture,Hokkaido University, Sapporo 060-8589, Japan, the ¶Graduate School of Agricultural and Life Sciences, University of Tokyo,Tokyo 113-8657, Japan, and �Core Research for Evolutional Science and Technology (CREST), Japan Science and TechnologyAgency, Saitama 332-0012, Japan

Boron homeostasis is important for plants, as boron is es-sential but is toxic in excess. Under high boron conditions, theArabidopsis thaliana borate transporter BOR1 is traffickedfrom the plasma membrane (PM) to the vacuole via the endo-cytic pathway for degradation to avoid excess boron transport.Here, we show that boron-induced ubiquitination is requiredfor vacuolar sorting of BOR1. We found that a substitution oflysine 590 with alanine (K590A) in BOR1 blocked degradation.BOR1 was mono- or diubiquitinated within several minutesafter applying a high concentration of boron, whereas theK590A mutant was not. The K590A mutation abolished vacuo-lar transport of BOR1 but did not apparently affect polar local-ization to the inner PM domains. Furthermore, brefeldin Aand wortmannin treatment suggested that Lys-590 is requiredfor BOR1 translocation from an early endosomal compart-ment to multivesicular bodies. Our results show that boron-induced ubiquitination of BOR1 is not required for endocyto-sis from the PM but is crucial for the sorting of internalizedBOR1 to multivesicular bodies for subsequent degradation invacuoles.

Although boron is an essential micronutrient for plants,excess boron is toxic. The homeostasis of boron is importantfor plants. In fact, both boron deficiency and toxicity havenegative impacts on agricultural production (1, 2). The onlyestablished function of boron in vascular plants is cross-link-ing of the pectic polysaccharide rhamnogalacturonan II. Thiscross-linking mediated by borate is important to maintain cellwall architecture and is necessary for normal growth and de-

velopment (3, 4). In contrast, the molecular mechanism ofboron toxicity is still unclear, despite several efforts at apply-ing biochemical, genetic, and molecular approaches (5–9).Boron is taken up by plant roots, mainly in the form of bo-

ric acid. Arabidopsis thaliana possesses two distinct types ofboron-transporting proteins, BOR1 (10) and NIP5;1 (11), foreffective boron uptake into roots and transport to shoots un-der boron-limiting conditions (12). BOR1 is an efflux-typeborate transporter with similarities to animal bicarbonatetransporters (13). In roots, BOR1 is polar localized to the in-ner (stele-facing) plasma membrane (PM)4 domain of variouscells (14) and functions in inward transport of boron towardthe stele under low boron conditions (10). NIP5;1 is a mem-ber of the MIP (major intrinsic protein) family and functionsas a boric acid channel for efficient boron uptake into the rootunder boron-limiting conditions (11). NIP5;1 is polar local-ized to the outer (soil-facing) PM domain of root epidermaland root cap cells (14) and increases the permeability of theoutermost PM domain to boric acid for efficient boronuptake.These transporters have been used successfully to generate

boron stress-tolerant plants. BOR1 overexpression in A. thali-ana results in enhanced boron translocation from the root toshoot and improved shoot growth and seed yields under bo-ron-limiting conditions (15). Enhancing NIP5;1 expression byactivation tag improves root growth under boron-limitingconditions (16). Overexpression of BOR4, a BOR1 paralog,reduces boron concentration in both roots and shoots andconfers tolerance to toxic levels of boron (17).Accumulations of NIP5;1 and BOR1 are regulated by boron

availability via different regulatory mechanisms. NIP5;1 isregulated at the level of mRNA accumulation in response toboron concentration in the media (11), whereas BOR1 accu-mulation is regulated through protein degradation. In thepresence of high boron concentrations, BOR1 is rapidly re-moved from the PM and transferred to the lytic vacuolethrough the endocytic pathway for prompt degradation (18).The endocytic degradation system is assumed to be beneficialfor plants to quickly shut down boron transport and avoidexcess accumulation of boron.

* This work was supported in part by Grant-in-aid 19-7094 for JSPS Fellowsfrom the Japan Society for the Promotion of Science (to K. K.), a grant-in-aid for scientific research (to T. F.), and a grant-in-aid for scientific re-search priority areas (to T. F.) from the Ministry of Education, Culture,Sports, Science, and Technology, Japan.

□S The on-line version of this article (available at http://www.jbc.org) con-tains supplemental Figs. S1–S6 and Table S1.

1 Present address: Creative Research Inst., Hokkaido University Kita-ku, Sap-poro 001-0021, Japan.

2 Present address: Food Technology Dept., Nagano Prefecture General In-dustrial Technology Center, 205-1 Aza-Nishibanba, Oaza-Kurita, Nagano-shi, Nagano 380-0921, Japan.

3 To whom correspondence should be addressed: Dept. of Applied Biologi-cal Chemistry, Graduate School of Agricultural and Life Sciences, Univer-sity of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan. Tel.: 81-3-5841-5104; Fax: 81-3-5841-8032; E-mail: atorufu@mail.ecc.u-tokyo.ac.jp.

4 The abbreviations used are: PM, plasma membrane; �B, low boron; �B,high boron; BFA, brefeldin A; EE, early endosome; LE, late endosome;MVB, multivesicular body; Ub, ubiquitin; WM, wortmannin.

THE JOURNAL OF BIOLOGICAL CHEMISTRY VOL. 286, NO. 8, pp. 6175–6183, February 25, 2011© 2011 by The American Society for Biochemistry and Molecular Biology, Inc. Printed in the U.S.A.

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Endocytic membrane trafficking is a key mechanism forcontrolling the amount of protein in the PM. During endo-cytic degradation, the target protein is internalized from thePM to the early endosomal compartments by endocytosis,sorted into late endosomes (LEs)/multivesicular bodies(MVBs), and transferred into vacuoles/lysosomes for degrada-tion (19). Although the details of this mechanism have beenstudied intensively in yeast and mammals (19–22), much lessknowledge has been accumulated in plants. A recent reportdemonstrated that the A. thaliana brassinosteroid insensi-tive-1 (BRI1) receptor and BOR1 are sorted into LEs/MVBsfor vacuolar degradation through the trans-Golgi/early endo-some (EE) network (23).Ubiquitination is a key signal for the homeostatic regula-

tion of proteins. Two distinct forms of ubiquitination areknown: polyubiquitination is used in the proteosomal proteindegradation system, and mono- and multimonoubiquitinationare required for endocytic internalization of transmembraneproteins for lysosomal/vacuolar turnover of membrane pro-teins in yeast and mammals (19). In addition, monoubiquiti-nation has also been reported as a sorting signal of the mem-brane proteins to MVBs in mammals (24). This ubiquitinationis required for binding to the endosomal protein sorting com-plex (ESCRT). In A. thaliana, ESCRT-related proteinsCHMP1A and CHMP1B are required for MVB sorting of thethree auxin carriers: PIN1, PIN2, and AUX1 (25). PIN2 turn-over, which is related to root gravitropism, is regulated byendocytic membrane trafficking and proteasome activity (26).This study showed that PIN2 is modified by ubiquitin andthat ubiquitination depends on proteasome activity; however,whether polyubiquitination or multimonoubiquitination isrequired remains unclear. In addition, no direct evidence ex-ists that ubiquitination is involved in PIN2 degradation (26).Here, we identified a ubiquitination site in BOR1, which is

required for boron-induced vacuolar sorting of BOR1. Wereported previously that BOR4, unlike BOR1, is not degradedby high boron (�B) supplementation (17). Thus, in this study,we searched out amino acid residues involved in boron-in-duced degradation of BOR1 by differences between the BOR1and BOR4 sequences and found that lysine 590 (Lys-590) wascritical for the degradation. Supplementation with high boroninduced mono- or diubiquitination of GFP-tagged BOR1(BOR1-GFP), and a Lys-590 mutation completely inhibitedubiquitination. Furthermore, the Lys-590 mutation did notinhibit the endocytosis step from the PM but prevented MVBsorting of BOR1 for degradation in the vacuole.

MATERIALS AND METHODS

Plasmid Construction and Plant Transformation—Binaryplasmids for plant transformation to express GFP-fused BORmutants under control of the BOR1 promoter were generatedusing Gateway technology (Invitrogen). Construction ofBOR1-BOR4 chimeras and site-directed mutagenesis wereconducted using the in vitro overlap extension PCR method(27). The BOR1-GFP (10) or BOR4-GFP (17) construct wasused as a template, and the primers used are listed in supple-mental Table S1. The PCR fragments of GFP-tagged geneswere subcloned into pENTR/D-TOPO (Invitrogen) according

to the manufacturer’s instructions and sequenced to confirmthat that no PCR errors occurred. The subcloned GFP-taggedgenes were mobilized into pAT100 (14), a destination binaryvector carrying the BOR1 promoter, with LR Clonase (In-vitrogen) according to the manufacturer’s instructions.The resulting binary plasmids were introduced into

Agrobacterium tumefaciens strain GV3101:pMP90 (28).A. thaliana bor1-3/2-1 plants, a BOR1/BOR2 double null mu-tant, were transformed using the floral dip method (29).To generate a T-DNA insertion mutant for both BOR1 and

BOR2, T2 seeds of bor1-3 (SALK_37312) and bor2-1(SALK_56473) were obtained from the Salk Institute (30).bor1-3 and bor2-1 are in the Col-0 background and carry T-DNAs in their exons. The plants were crossed, and thosehomozygous for both T-DNAs were selected by PCR. Theprimers used for PCR for bor1-3were: BOR1 forward, 5�-GCAT-ACCAAGAGCTTAGCAACT-3�; BOR1 reverse, 5�-TGGAGT-CGAACTTGAACTTGTC-3�; and the SALK left border(LBb1), 5�-GATGGCCCACTACGTGAACCAT-3�. Primersfor bor2-1 were: BOR2 forward, 5�-CATGCAACAAGCCAT-CAAAG-3�; BOR2 reverse, 5�-AAATCCAAGAAGAAGCA-GAT-3�; and the SALK left border (LBb1).Plant Growth Conditions—MGRL medium (31) was pre-

pared with slight modifications according to Takano et al.(18) and used with various concentration of boric acid as theboron source. MGRL solid medium was made with 1.5% gel-lan gum (Wako Pure Chemical, Inc., Osaka, Japan) and 2%sucrose in a sterile square Petri dish (140 � 100 mm; Eiken,Tokyo).A. thaliana seeds were surface-sterilized by soaking in

99.5% ethanol for 2 min and allowed to air-dry for a fewhours; then they were sown onto MGRL solid medium. Aftera 2-day cold acclimation at 4 °C, the plates were placed verti-cally, and the plants were grown at 22 °C under a 16-h light/8-h dark cycle.For microsome isolation, plants were grown on vertically

placed MGRL solid medium containing 30 �M boron for 16days and grown hydroponically with MGRL liquid mediumcontaining 30 �M boron for 4 days and then with the mediumcontaining 3 �M boron (�B) for 4 days under a 10-h light/14-h dark cycle followed by treatment with the medium con-taining 100 �M boron (�B, an optimal condition). Themethod for hydroponic culture was as follows. Twenty-fiveplants were placed between two formed polyethylene sheets(15 � 100 � 4 mm), and four sets of the sheets were floatedon 400 ml of MGRL liquid medium in a light-shielded plasticcontainer. The liquid medium was aerated continuously by anair pump and was changed every 4 days.Confocal Fluorescent Imaging—Confocal imaging was per-

formed as described previously with slight modifications (18).The transgenic plants were grown on MGRL solid mediumsupplemented with 3 �M boron (�B) for 4 days. The rootswere cut and incubated with MGRL liquid medium contain-ing boric acid, inhibitors, or fluorescent dye at room tempera-ture, as described in the figure legends (Figs. 2, 3, and 5; sup-plemental Figs. S2, S3, and S4). A stock solution ofcycloheximide or FM4-64 (Molecular Probes, Carlsbad, CA)was prepared with water at 25 or 10 mM respectively, and

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brefeldin A (BFA) (Sigma) was prepared with dimethyl sulfox-ide at 50 mM.

Confocal imaging was performed using an LSM510 (CarlZeiss, Oberkochen, Germany) or FV1000 laser scanning con-focal microscope (Olympus, Tokyo). The excitation wave-length was 488 nm, and emitted fluorescence was detectedwith a 505–530-nm band-pass filter for GFP or a 650-nmlong-pass filter for FM4-64. Signal intensity was monitored toavoid pixel saturation. At least two independent transgeniclines (T2 or T3 plants) from each construct were used to con-firm the reproducibility of the results.Preparation of Microsomal Fractions from Arabidopsis

Tissues—Microsomal proteins were prepared as describedpreviously with some modifications (18). bor1-3/2-1 andtransgenic plants (T3 homozygous-lines) were grown hydro-ponically, as described above. The roots and shoots were har-vested after treatment with 100 �M boron (�B) MGRL liquidmedium. Approximately 1 g of tissues was homogenized in 5ml of ice-cold homogenization buffer (250 mM Tris-Cl (pH7.8), 25 mM EDTA, 290 mM sucrose, 75 mM 2-mercaptoetha-nol, 10 mM N-ethylmaleimide, 1 mM PMSF, and one tablet/50ml protease inhibitor mixture (Complete, EDTA-free; RocheApplied Science)) using a Polytron PT-2100 homogenizer(Kinematica Inc., Bohemia, NY) at 26,000 rpm for 5 s. Thishomogenization was repeated four times. The homogenateswere centrifuged at 8,000 � g for 5 min at 4 °C, and the super-natant was centrifuged at 100,000 � g for 10 min at 4 °C. Themicrosome pellets were resuspended in 200 �l of ice-coldstorage buffer (50 mM Tris-Cl (pH 8.0), 150 mM NaCl, 10 mM

N-ethylmaleimide, 1 mM PMSF, and one tablet/50 ml prote-ase inhibitor mixture (Complete, EDTA-free; Roche AppliedScience)) using a pestle. The protein concentration was deter-mined using the Bradford method (Quick Start Protein AssayKit I; Bio-Rad). The samples were frozen in liquid nitrogenand stored at �80 °C.Immunoblot Analysis—Microsomes containing 20 �g of

protein were dissolved in 2� solubilization buffer (50 mM

Tris-Cl (pH 7.5), 50 mM DTT, 5% glycerol, 5% SDS, 5 mM

EDTA, 0.03% bromphenol blue, and one tablet/10 ml proteaseinhibitor mixture (Complete mini, EDTA-free; Roche AppliedScience)), incubated at 65 °C for 5 min, and then separated bySDS-PAGE using a modified Laemmli system (32) with 8%acrylamide and 0.064% N,N�-methylene-bis-acrylamide. Gelswere blotted onto PVDF membranes (BioCraft, Tokyo) bysemidry electroblotting. The anti-GFP mouse monoclonalantibody GF200 (Nakalai Tesque, Kyoto, Japan) was used at2.2 �g/ml in 2% ECL advance blocking reagent (GE Health-care). Immune complexes were detected with horseradishperoxidase-conjugated goat antibody to mouse IgG and ECLadvance reagents (GE Healthcare), and chemiluminescencewas detected with an LAS-3000 imaging system (Fujifilm,Tokyo).Immunoprecipitation and Detection of Ubiquitination—Mi-

crosomal protein fractions (50 �g) were lysed in 300 �l oflysis buffer (20 mM HEPES-KOH (pH 7.5), 150 mM NaCl, 1mM EDTA, 1% Tween 20, 0.5% deoxicholate, 0.1% SDS, 10mM N-ethylmaleimide, 1 mM PMSF, and one tablet/10 mlprotease inhibitor mixture (Complete mini, EDTA-free;

Roche Applied Science)) and incubated with 20 �l of agarose-conjugated anti-GFP monoclonal antibody (50% gel slurry;Medical & Biological Laboratories Co., Nagoya, Japan) for1.5 h at 4 °C with gentle shaking. After incubation, the beadswere washed three times with lysis buffer, and the bound pro-teins were analyzed by immunoblotting using anti-GFP anti-body or anti-ubiquitin (Ub) antibody. Anti-Ub monoclonalantibody (P4D1; Santa Cruz Biotechnology, Santa Cruz, CA)was used at 0.5 �g/ml in 2% ECL advance blocking reagent.

RESULTS

The Amino Acid Sequence Involved in Boron-induced Deg-radation of BOR1—Our previous studies demonstrated thatBOR1 and BOR4 are distinct in terms of boron-induced endo-cytic degradation and polar localization (14, 17). BOR1 andBOR4 localize to the inner and outer PM domains of variousroot cells, respectively, and BOR1 is degraded by �B supple-mentation (Fig. 2), but BOR4 is not. Therefore, we searchedamino acid residues involved in boron-induced degradationby means of differences between BOR1 and BOR4 sequences.To identify the BOR1 region responsible for boron-induced

degradation, we constructed two BOR1 and BOR4 chimeras.The C-terminal region of BOR1 was replaced with the corre-sponding region of BOR4 from Leu-468 or Glu-641 and calledchimera 1 and chimera 2, respectively (Fig. 1). BOR1, BOR4,and chimeras, C-terminally fused to GFP and placed underthe control of the BOR1 promoter, were generated. To ex-clude the possibility that an endogenous BOR1 or BOR2 (theclosest homolog of BOR1) might disturb the mutational ef-fects of introduced BOR mutants through protein-proteininteractions, the constructs were introduced into bor1-3/2-1,a double BOR1 and BOR2 mutant without BOR1 and BOR2accumulation. The growth defect in the bor1-3/2-1mutantunder boron-limiting conditions was almost fully restored byexpressing BOR1-GFP and was partially restored by BOR4-GFP, chimera 1-GFP, and chimera 2-GFP (supplemental Fig.S1), indicating that these expressed proteins possessed boron-transporting activity. As shown in Fig. 2, BOR1-GFP was lo-calized to the PM of the root cap and epidermal cells in theroot tip with inward polarity and degraded upon treatmentwith �B for 2 h. In contrast, chimera 1-GFP and BOR4-GFPdid not show inward polarity and did not disappear with �B.Although a part of the 1-GFP chimera was observed in thecytosol or endomembrane, most likely endoplasmic reticu-lum, PM localization was confirmed by co-localization with aspecific membrane dye, FM4-64 (33) (supplemental Fig. S4).Chimera 2-GFP showed no or weaker polarity but was de-graded by �B supplementation. These results suggest that theBOR1 sequence from Leu-469 to Glu-641 and Phe-642 to thelast amino acid residue (Asn-704) contains the motif(s) forboron-induced degradation and polar localization,respectively.Lysine 590 Is Crucial for Boron-induced Degradation of

BOR1—We focused on potential ubiquitination sites to iden-tify an amino acid residue required for boron-induced endo-cytic degradation of BOR1. Ubiquitin conjugation to thelysine residue in yeast is involved in endocytosis and degrada-tion of PM proteins such as receptors, channels, and trans-

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porters (22, 34). Therefore, we searched for a lysine residue inthe region from Leu-469 to Glu-641 of BOR1 and found onlyone lysine residue, Lys-590 that is not conserved in BOR4(Fig. 1B). We introduced a mutation to change Lys-590 to A(K590A). The GFP-tagged K590A mutant (BOR1K590A-GFP)was expressed under the control of BOR1 promoter. Severalindependent transgenic plants were generated, and the trans-genic plant lines displayed almost full restoration of the bor1-3/2-1 growth defect under a boron-limiting condition (sup-plemental Fig. S1). The BOR1K590A-GFP showed inwardpolarity similar to the wild-type BOR1-GFP; however, boron-induced degradation was not observed (Fig. 3A). GFP fluores-cence was seen in the PM 4 days after treatment with �B (100�M). This result indicates that Lys-590 is involved in BOR1degradation. For further confirmation, microsomes isolatedfrom shoots and roots of these transgenic plants after treat-ment with �B were subjected to immunoblot analysis usingan anti-GFP antibody (Fig. 3C). A single band correspondingto the expected size of full-length BOR1-GFP or BOR1K590A-GFP (�106 kDa) was detected in all root samples, indicatingthat the GFP fluorescence seen in Fig. 3A was derived from

the full-length BOR1-GFP protein. Boron-induced degrada-tion of BOR1-GFP was observed in the root samples, but deg-radation was not detected for BOR1K590A-GFP, consistentwith the results of GFP imaging. Degradation was not ob-served in either BOR1-GFP or BOR1K590A-GFP in the shootduring this short period of time (30–120 min). In the shoot,another band was observed at a higher weight than 250 kDa,which was assumed to be a trimer or tetramer.We also tested the effects of other lysine mutations in the

BOR1 C-terminal region (see Fig. 1B), including K649A,K689A, and a K649A/K689A double mutant. All three mu-tant proteins, BOR1K649A-GFP, BOR1K689A-GFP, andBOR1K649A,K689A-GFP, fully complemented the hypersensitiv-ity to boron deficiency of the bor1-3/2-1mutant (supplemen-tal Fig. S1) and exhibited normal polarity and boron-induceddegradation (supplemental Fig. S2). These results further con-firmed the importance of Lys-590 but not of other nearby ly-sine residues.We then tested whether Lys-590 is sufficient for boron-

induced degradation of BOR proteins by introducing anN590K or N590K/P591G double mutation into chimera

FIGURE 1. Construction of a BOR1 and BOR4 chimera. A, schematic representation of constructs. The BOR1 topology model predicted by ConPred II (47) isshown at the top. Ten deduced transmembrane (TM) domains are indicated by black boxes, and the letters I and o represent the inside and outside of thecell, respectively. BOR1, BOR4, and two BOR1-BOR4 chimeras, chimera 1 and chimera 2, were tagged with GFP at the C-terminal and inserted under the con-trol of the BOR1 promoter (PBOR1). The white, gray, and black segments represent the BOR1, BOR4, and GFP sequences, respectively. The amino acids posi-tions of BOR1 and BOR4 at the chimera junctions are shown. B, comparison of the C-terminal amino acid sequences of BOR1 (Glu-443 to Asn-704) and BOR4(Glu-449 to Glu-683). The positions of the two chimera junctions are indicated. The BOR1 unique lysine residues are denoted by white triangles.

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1-GFP and N596K or N596K/P597G into BOR4-GFP. Thesechanges represent the introduction of BOR1-type amino acidresidues at these specific positions in the BOR4-type se-quences. These constructs partially rescued the hypersensitiv-ity to boron deficiency of the bor1-3/2-1mutant, confirmingboron transport activity of the BOR-GFP derivatives (supple-mental Fig. S1). These amino acid substitutions did not in-duce boron-dependent degradation in either the 1-GFP chi-mera or BOR4-GFP (supplemental Fig. S3). The chimera 1derivatives, chimera 1N590K-GFP and chimera 1N590K, P591G-GFP, as well as chimera 1-GFP, were localized in the PM andpartly in the endomembrane, most likely the endoplasmicreticulum. The PM localization of these proteins was furtherconfirmed by co-localization with FM4-64 (supplemental Fig.S4). These results indicate that Lys-590 is necessary but notsufficient for boron-induced degradation of BOR proteins.BOR1 Is Ubiquitinated Under �B Conditions, and Lys-590

Is the Principal Ubiquitination Site—We assumed that BOR1ubiquitination at Lys-590 was involved in boron-induced deg-radation. Therefore, we attempted to detect ubiquitin conju-gated to BOR1-GFP and compared that with BOR1K590A-

GFP. BOR1-GFP, BOR1K590A-GFP, and BOR1K649A,K689A-GFP were immunoprecipitated using the anti-GFP antibodyfrom root extracts of transgenic plants after a time coursetreatment with �B, and ubiquitination was detected by im-munoblotting using an anti-Ub antibody (Fig. 4). Two specificubiquitinated protein bands were observed from BOR1-GFPand BOR1K649A,K689A-GFP within 30 min after treatment with�B. The lower and upper bands corresponded to the size ofBOR1-GFP conjugates with one and two ubiquitin molecules,respectively. No apparently ubiquitinated protein was ob-served for BOR1K590A-GFP. In addition, immunoblotting us-ing anti-GFP detected a relatively small amount of diubiquiti-

FIGURE 2. Effects of high boron treatment on the subcellular localiza-tion of BOR1-GFP, BOR4-GFP, chimera 1-GFP, and chimera 2-GFP inroot cells. A, GFP images from the roots of transgenic A. thaliana express-ing BOR1-GFP, BOR4-GFP, chimera 1-GFP, and chimera 2-GFP. Plants weregrown on solid medium with a �B concentration and then incubated with�B or �B liquid medium for 2 h. Scale bar, 50 �m. B, enlarged views of theepidermal cells of A under the �B condition showing distinct polar localiza-tions. The white arrows indicate the direction of the polarity. In, inward; Ou,outward. Bar, 5 �m.

FIGURE 3. Interference with boron-dependent degradation of BOR1 bya K590A point mutation. A, GFP images from the roots of transgenicA. thaliana expressing BOR1K590A-GFP. Plants were grown on solid mediumwith low boron concentration (�B) and then incubated with a high boronconcentration (�B) for 2 h (�B2h) or 5 h (�B5h). Plants were also grown onsolid medium with �B for 4 days (�B4d). Scale bar, 50 �m. B, enlargedviews of the epidermal cells in A under a �B condition. The white arrowsindicate the direction of the polarity. In, inward; Ou, outward. Bar, 5 �m.C, immunoblot analysis of microsomal fractions from roots and shoots oftransgenic plants expressing BOR1-GFP and BOR1K590A-GFP. The plantswere grown on �B medium and then treated with �B for the indicatedtime periods. Microsomal proteins were separated by SDS-PAGE and blot-ted onto PVDF membranes. The blots were reacted with anti-GFP antibody.Control, microsomes from bor1-3/2-1. The letters T and M are the deducedtrimer and monomer, respectively. The sizes of the molecular markers areshown in kDa. Coomassie Brilliant Blue (CBB)-stained membranes are shownas loading controls.

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nated BOR1-GFP compared with non-ubiquitinated proteinin a long exposure image, indicating that the ubiquitinatedBOR1 protein is rapidly degraded and does not accumulate inlarge amounts within plant cells. These results suggest thatBOR1 is mono- or diubiquitinated and that Lys-590 is essen-tial for ubiquitination under a �B condition. Thus, Lys-590 islikely to be the ubiquitination site.Lysine 590 Is Essential for Vacuolar Sorting of BOR1 but Not

for Endocytosis—Boron-induced turnover of BOR1 is medi-ated by endocytosis and subsequent degradation in the vacu-ole (18). To determine which step of the endocytic degrada-tion of BOR1 requires Lys-590 ubiquitination, we comparedthe endocytic trafficking of BOR1-GFP and BORK590A-GFPusing a general endocytosis marker, FM4-64 (33), and themembrane traffic inhibitor BFA. BFA inhibits the transitionof membrane proteins from EEs to MVBs and to the PM butallows endocytosis (35–37).BFA treatment was performed in the presence of cyclohexi-

mide to prevent de novo protein synthesis. After 30 min ofBFA treatment, both BOR1-GFP and BOR1K590A-GFP accu-mulated with endocytosed FM4-64 in the BFA compartmentof root epidermal cells (Fig. 5A). The same volume of di-methyl sulfoxide, the BFA stock solution solvent, was used asthe control, and no effect on traffic was observed. This resultindicated that Lys-590 is not involved in endocytosis of BOR1from the PM; however, it may be involved in subsequent vac-uolar sorting steps (supplemental Fig. S6).To exclude the possibility that K590A simply enhanced

protein synthesis and overaccumulated BOR1 at the PM, wemonitored GFP accumulation in the root vacuoles of trans-genic plants expressing BOR1K590A-GFP and compared thatwith BOR1-GFP to confirm that the K590A mutation inhib-

ited BOR1 transport to the lytic vacuole. In the absence oflight, GFP is resistant to vacuolar proteolysis; thus, dark treat-ment is an effective method of visualizing vacuolar targetingof GFP-tagged proteins (14, 36, 38). The roots of transgenicplants expressing BOR1-GFP and BOR1K590A-GFP were incu-bated in �B and �B medium in the dark for 2 h, and GFPfluorescence in the epidermal cells of the root tip was moni-tored (Fig. 5B). After �B treatment, fluorescence from bothBOR1-GFP and BOR1K590A-GFP was observed in the PM.After �B treatment, GFP fluorescence in the BOR1-GFP-expressing plant was observed in the vacuole as reported pre-viously (14), whereas fluorescence in the BOR1K590A-GFP-expressing plant was not detected in the vacuole but wasretained in the PM. This observation confirmed that theK590A mutation prevents sorting of BOR1 into the vacuoleunder a �B condition.We also investigated the effect of wortmannin (WM) on

trafficking of BOR1-GFP and BOR1K590A-GFP. WM is a spe-cific inhibitor of phosphatidylinositol 3-kinase and induceshomotypic fusions and enlargement of the prevacuolar com-partment/LE/MVB in plants (39–41). In agreement with aprevious study (40), BOR1-GFP was observed in several roundstructures after 90 min of treatment with 16.5 �M WM bothin �B and �B medium (Fig. 5, C and D) A part of the GFPsignal in the round structures was co-localized with the endo-cytic dye FM4-64, indicating that the structures were enlargedprevacuolar compartment/LE/MVBs. In contrast,BOR1K590A-GFP was rarely observed in enlarged MVBs. Thisresult suggested that BOR1K590A-GFP is scarcely able to enterthe MVB pathway. Taken together, these data suggest thatLys-590 is not involved in the endocytosis of BOR1 from thePM but is essential for sorting in MVBs for subsequent vacuo-lar degradation (supplemental Fig. S6).

DISCUSSION

We showed that BOR1 is biochemically modified withmono- or diubiquitin in response to boron availability andthat a Lys-590 residue is required for both boron-inducedubiquitination and boron-induced endocytic degradation.Mutational analysis showed that a K590A mutant,BOR1K590A-GFP, was not degraded under �B conditions un-like BOR1-GFP (Fig. 3), indicating that Lys-590 is required forboron-induced endocytic degradation. Our most recent workrevealed the other BOR1 residues involved in endocytic deg-radation (14): three putative tyrosine-based sorting signals(YXXØ, where Y is tyrosine, X is any amino acid, and Ø is abulky hydrophobic residue) in the predicted largest cytosolicloop region. The tyrosine-based sorting signal is recognizedby the clathrin-adaptor protein complex for sorting trans-membrane proteins into clathrin-coated vesicles and is in-volved in many post-Golgi trafficking steps including endocy-tosis and polar PM trafficking (42). Mutations in all of thecorresponding tyrosines (Y373A/Y398A/Y405A) of BOR1prevent boron-induced degradation. Additionally, the tyro-sine mutations inhibit polar localization in the root tip cells.In contrast, the K590A mutation inhibits only endocyticdegradation and does not affect polar localization (Fig. 3),indicating that Lys-590 is specifically required for BOR1

FIGURE 4. Boron-dependent mono- or diubiquitination of BOR1 andinvolvement of Lys-590 in the ubiquitinations. The microsomal fractionswere prepared from the roots of transgenic plants expressing BOR1-GFP,BOR1K590A-GFP, and BOR1K648A,K689A-GFP, which were grown in �B mediumand then treated with a �B concentration for the indicated times. The GFP-tagged proteins were immunoprecipitated with anti-GFP antibody, andubiquitination was assessed by immunoblotting with anti-Ub antibody. Theletters m and d represent the presumed band sizes for monoubiquitinationand diubiquitination, respectively. The white arrowhead indicates the pre-sumed diubiquitination band detected during a long image exposure ofanti-GFP.

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vacuolar trafficking. We also showed that introducing anN590K or N590K/P591G mutation into the chimera 1-GFPand N596K or N596K/P597G into BOR4-GFP did not pro-mote boron-induced degradation in either chimera 1-GFPor BOR4-GFP (supplemental Fig. S3), indicating that Lys-590 is necessary but not sufficient for endocytic degrada-tion. Although BOR4 lacks two tyrosine residues corre-sponding to the putative tyrosine-based sorting signals ofBOR1, Tyr-373 and Tyr-405 (supplemental Fig. S5), chi-mera 1-GFP mutants contain all of the putative tyrosine-based sorting signals. This suggests the existence of extracomponent(s) other than the three tyrosine-based sortingsignals and indicates that Lys-590 functions in BOR1 endo-cytic degradation.Immunoprecipitation and immunoblotting analyses

showed that BOR1 is mono- or diubiquitinated in a �B-de-pendent manner and that Lys-590 is likely a principal ubiq-uitin-acceptor site (Fig. 4). Our data are consistent with stud-ies in yeast showing that several nutrient transporters areposttranslationally down-regulated by substrate-dependentmono- or multi-monoubiquitination (22, 34). Diverse formsof ubiquitination are involved in the distinct membrane pro-tein degradation pathways in yeast and mammalian systems(19); polyubiquitination mainly regulates proteasomal degra-dation, and mono- or multi-monoubiquitination is requiredfor the endocytic degradation pathway. In mammals, Lys-63-linked short-chain ubiquitination and di- or triubiquitinationat the Lys-270 lysine residue regulate endocytosis of the aqua-porin-2 water channel (43). As a PM localized protein inplants, PIN2 has been shown to be ubiquitinated (26). An-other report demonstrated that endocytically internalizedPIN2 is degraded mainly in the vacuole (36). However, forPIN2, signal-inducing ubiquitination, which is a direct linkbetween ubiquitination and turnover, the form of ubiquitina-tion, and the ubiquitination site are still unknown. Anotherexample is that down-regulation of PIP2, a PM aquaporin,is regulated via ubiquitination catalyzed by drought stress-induced Rma1, a homolog of a RING domain containing E3ligase in A. thaliana (44). In addition, A. thaliana iron-regulated transporter 1 (IRT1), a high affinity iron trans-porter, is also posttranslationally regulated in response toiron availability, and two lysine residues in the intercellularloop region are necessary for iron-induced turnover (45).However, IRT1 ubiquitination has not yet been reported.Our results show the ubiquitination form, an ubiquitina-tion site, and inducibility by substrate availability (Fig. 4),thus filling a gap in the current knowledge regarding post-translational turnover of PM proteins in plants. Our find-ings suggest that plants are able to use mono- or diubiq-

uitination as critical labeling to identify endocyticdegradation of PM proteins.Our previous report showed that after treatment with �B

for 30–60 min, BOR1-GFP is translocated into dot-like struc-tures corresponding to MVBs (18, 23). However, BOR1K590A-GFP was not observed as dot-like signals after �B treatment(Fig. 3). Furthermore, although BOR1-GFP was observed inenlarged MVBs following WM treatment even under a �Bcondition, BOR1K590A-GFP was much less frequently ob-served in enlarged MVBs. This observation suggests that apool of BOR1-GFP is transferred into MVBs even under a �Bcondition and that this MVB sorting is largely dependent onLys-590.In yeast, many PM proteins require mono- or multi-mo-

noubiquitination for the endocytic internalization step (22).In contrast, ubiquitination of the epidermal growth factorreceptor (EGFR) in animals is not necessary for endocytic in-ternalization from the PM but is required for lysosomal sort-ing (46). Similar to the EGF receptor, Lys-590 of BOR1 seemsnot to be involved in endocytosis from the PM but is essentialfor transport to the MVB pathway and then to the vacuole(Fig. 5 and supplemental Fig. S6). This result agrees with theobservation that BOR1K590A-GFP and wild-type BOR1-GFPshowed inward polarity (Figs. 2 and 3), because endocytic cy-cling is required to maintain polar localization of PM proteins(14, 42). Substrate-induced degradation of a number of yeastnutrient transporters is regulated at the endocytic internaliza-tion step (22). It is considered an efficient way because unicel-lular yeast does not require endocytically generated polarity ofnutrient transporters for efficient nutrient uptake. However,plants might require continuous endocytic recycling of theirnutrient transporters to maintain polar localization for direc-tional nutrient transport under nutrient-deficient conditions.One may reasonably assume that plants have independentregulatory mechanisms for endocytosis (internalization fromPM) and vacuolar sorting.In conclusion, the data presented here show that boron-

induced mono- or diubiquitination of BOR1 at Lys-590 is anessential transport step to MVBs and that ubiquitination doesnot control the rate of endocytosis from the PM (supplemen-tal Fig. S6). This mechanism is distinct from that of the yeastsystem and may be important in enabling polar localized plantnutrient transporters to achieve a balance between the polarlocalization and the vacuolar degradation systems. BOR1 is agood model for studying endocytic degradation of transmem-brane proteins in plants because degradation and ubiquitina-tion can be controlled by only one amino acid residue. Fur-ther studies may show how plants sense the concentration of

FIGURE 5. Effects of the K590A mutation on intracellular membrane trafficking of BOR1. A, intracellular distribution of BOR1-GFP and BOR1K590A-GFPafter treatment with BFA. The roots from transgenic plants grown on �B medium were incubated with 50 �M cycloheximide for 30 min and then with 25�M FM4-64 for 2 min followed by treatment with 50 �M cycloheximide and 50 �M BFA. The GFP and FM4-64 signals in the epidermal cells are shown ingreen and red, respectively. In the merged images, the GFP and FM4-64 overlapping signals appear in yellow. Dimethyl sulfoxide (DMSO) was used as a neg-ative control. Arrowheads indicate examples of BOR1-GFP and FM4-64 (top) and BOR1K590A-GFP and FM4-64 (bottom) co-localization. Bar, 5 �m. B, distribu-tion of GFP signals derived from BOR1-GFP or BOR1K590A-GFP in the epidermal cells of the root tips after treatment with �B or �B in the dark (2 h). Arrow-heads indicate GFP signals in the vacuole. Bar, 10 �m. C, distribution of BOR1-GFP and BOR1K590A-GFP after treatment with WM in the epidermal cells of theroot tips. The roots were incubated with �B medium containing 4 �M FM4-64 for 90 min and then with �B or �B medium containing 16.5 �M WM or0.0825% dimethyl sulfoxide for 90 min. Arrowheads indicate co-localization. Bar, 60 �m. D, magnified images of the boxed areas (a and b, respectively) in C.Bar, 10 �m.

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boron and regulate ubiquitination in a boron-dependentmanner.

Acknowledgments—We are grateful to Takehiro Kamiya, KentaroFuji, and Shimpei Uraguchi for helpful discussions and criticalreading of the manuscript.

REFERENCES1. Shorrocks, V. M. (1997) Plant Soil 193, 121–1482. Nable, R. O., Banuelos, G. S., and Paull, J. G. (1997) Plant Soil 193,

181–1983. O’Neill, M. A., Eberhard, S., Albersheim, P., and Darvill, A. G. (2001)

Science 294, 846–8494. Iwai, H., Hokura, A., Oishi, M., Chida, H., Ishii, T., Sakai, S., and Satoh,

S. (2006) Proc. Natl. Acad. Sci. U.S.A. 103, 16592–165975. Ruiz, J. M., Rivero, R. M., and Romero, L. (2003) Plant Sci. 165, 811–8176. Reid, R. J., Hayes, J. E., Post, A., Stangoulis, J. C., and Graham, R. D.

(2004) Plant Cell Environ. 27, 1405–14147. Nozawa, A., Miwa, K., Kobayashi, M., and Fujiwara, T. (2006) Biosci.

Biotechnol. Biochem. 70, 1724–17308. Nozawa, A., Takano, J., Kobayashi, M., von Wiren, N., and Fujiwara, T.

(2006) FEMS Microbiol. Lett. 262, 216–2229. Ochiai, K., Uemura, S., Shimizu, A., Okumoto, Y., and Matoh, T. (2008)

Theor. Appl. Genet. 117, 125–13310. Takano, J., Noguchi, K., Yasumori, M., Kobayashi, M., Gajdos, Z., Miwa,

K., Hayashi, H., Yoneyama, T., and Fujiwara, T. (2002) Nature 420,337–340

11. Takano, J., Wada, M., Ludewig, U., Schaaf, G., von Wiren, N., and Fuji-wara, T. (2006) Plant Cell 18, 1498–1509

12. Takano, J., Miwa, K., and Fujiwara, T. (2008) Trends Plant Sci. 13,451–457

13. Frommer, W. B., and von Wiren, N. (2002) Nature 420, 282–28314. Takano, J., Tanaka, M., Toyoda, A., Miwa, K., Kasai, K., Fuji, K., Onou-

chi, H., Naito, S., and Fujiwara, T. (2010) Proc. Natl. Acad. Sci. U.S.A.107, 5220–5225

15. Miwa, K., Takano, J., and Fujiwara, T. (2006) Plant J. 46, 1084–109116. Kato, Y., Miwa, K., Takano, J., Wada, M., and Fujiwara, T. (2009) Plant

Cell Physiol. 50, 58–6617. Miwa, K., Takano, J., Omori, H., Seki, M., Shinozaki, K., and Fujiwara, T.

(2007) Science 318, 141718. Takano, J., Miwa, K., Yuan, L., von Wiren, N., and Fujiwara, T. (2005)

Proc. Natl. Acad. Sci. U.S.A. 102, 12276–1228119. Mukhopadhyay, D., and Riezman, H. (2007) Science 315, 201–20520. Raiborg, C., and Stenmark, H. (2009) Nature 458, 445–45221. Saksena, S., Sun, J., Chu, T., and Emr, S. D. (2007) Trends Biochem. Sci.

32, 561–57322. Hicke, L., and Dunn, R. (2003) Annu. Rev. Cell Dev. Biol. 19, 141–17223. Viotti, C., Bubeck, J., Stierhof, Y. D., Krebs, M., Langhans, M., van den

Berg, W., van Dongen, W., Richter, S., Geldner, N., Takano, J., Jurgens,G., de Vries, S. C., Robinson, D. G., and Schumacher, K. (2010) PlantCell 22, 1344–1357

24. Katzmann, D. J., Babst, M., and Emr, S. D. (2001) Cell 106, 145–155

25. Spitzer, C., Reyes, F. C., Buono, R., Sliwinski, M. K., Haas, T. J., and Ote-gui, M. S. (2009) Plant Cell 21, 749–766

26. Abas, L., Benjamins, R., Malenica, N., Paciorek, T., Wisniewska, J., Mou-linier-Anzola, J. C., Sieberer, T., Friml, J., and Luschnig, C. (2006) Nat.Cell Biol. 8, 249–256

27. Higuchi, R., Krummel, B., and Saiki, R. K. (1988) Nucleic Acids Res. 16,7351–7367

28. Koncz, C., and Schell, J. (1986)Mol. Gen. Genet. 204, 383–39629. Clough, S. J., and Bent, A. F. (1998) Plant J. 16, 735–74330. Alonso, J. M., Stepanova, A. N., Leisse, T. J., Kim, C. J., Chen, H., Shinn,

P., Stevenson, D. K., Zimmerman, J., Barajas, P., Cheuk, R., Gadrinab, C.,Heller, C., Jeske, A., Koesema, E., Meyers, C. C., Parker, H., Prednis, L.,Ansari, Y., Choy, N., Deen, H., Geralt, M., Hazari, N., Hom, E., Karnes,M., Mulholland, C., Ndubaku, R., Schmidt, I., Guzman, P., Aguilar-Hen-onin, L., Schmid, M., Weigel, D., Carter, D. E., Marchand, T., Risseeuw,E., Brogden, D., Zeko, A., Crosby, W. L., Berry, C. C., and Ecker, J. R.(2003) Science 301, 653–657

31. Fujiwara, T., Hirai, M. Y., Chino, M., Komeda, Y., and Naito, S. (1992)Plant Physiol. 99, 263–268

32. Ito, K., Bassford, P. J., Jr., and Beckwith, J. (1981) Cell 24, 707–71733. Bolte, S., Talbot, C., Boutte, Y., Catrice, O., Read, N. D., and Satiat-Jeun-

emaitre, B. (2004) J. Microsc. 214, 159–17334. Rotin, D., Staub, O., and Haguenauer-Tsapis, R. (2000) J. Membr. Biol.

176, 1–1735. Robinson, D. G., Jiang, L., and Schumacher, K. (2008) Plant Physiol. 147,

1482–149236. Kleine-Vehn, J., Leitner, J., Zwiewka, M., Sauer, M., Abas, L., Luschnig,

C., and Friml, J. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 17812–1781737. Geldner, N., Hyman, D. L., Wang, X., Schumacher, K., and Chory, J.

(2007) Genes Dev. 21, 1598–160238. Tamura, K., Shimada, T., Ono, E., Tanaka, Y., Nagatani, A., Higashi, S. I.,

Watanabe, M., Nishimura, M., and Hara-Nishimura, I. (2003) Plant J.35, 545–555

39. Tse, Y. C., Mo, B., Hillmer, S., Zhao, M., Lo, S. W., Robinson, D. G., andJiang, L. (2004) Plant Cell 16, 672–693

40. Jaillais, Y., Fobis-Loisy, I., Miege, C., and Gaude, T. (2008) Plant J. 53,237–247

41. Wang, J., Cai, Y., Miao, Y., Lam, S. K., and Jiang, L. (2009) J. Exp. Bot. 60,3075–3083

42. Mellman, I., and Nelson, W. J. (2008) Nat. Rev. Mol. Cell Biol. 9,833–845

43. Kamsteeg, E. J., Hendriks, G., Boone, M., Konings, I. B., Oorschot, V.,van der Sluijs, P., Klumperman, J., and Deen, P. M. (2006) Proc. Natl.Acad. Sci. U.S.A. 103, 18344–18349

44. Lee, H. K., Cho, S. K., Son, O., Xu, Z., Hwang, I., and Kim, W. T. (2009)Plant Cell 21, 622–641

45. Kerkeb, L., Mukherjee, I., Chatterjee, I., Lahner, B., Salt, D. E., and Con-nolly, E. L. (2008) Plant Physiol. 146, 1964–1973

46. Duan, L., Miura, Y., Dimri, M., Majumder, B., Dodge, I. L., Reddi, A. L.,Ghosh, A., Fernandes, N., Zhou, P., Mullane-Robinson, K., Rao, N., Do-noghue, S., Rogers, R. A., Bowtell, D., Naramura, M., Gu, H., Band, V.,and Band, H. (2003) J. Biol. Chem. 278, 28950–28960

47. Arai, M., Mitsuke, H., Ikeda, M., Xia, J. X., Kikuchi, T., Satake, M., andShimizu, T. (2004) Nucleic Acids Res. 32,W390–393

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Koji Kasai, Junpei Takano, Kyoko Miwa, Atsushi Toyoda and Toru FujiwaraArabidopsis thalianaBorate Transporter in

High Boron-induced Ubiquitination Regulates Vacuolar Sorting of the BOR1

doi: 10.1074/jbc.M110.184929 originally published online December 9, 20102011, 286:6175-6183.J. Biol. Chem. 

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