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The 14-3-3 protein as a novelregulator of ion channellocalisationMasaru Ishii and Yoshihisa Kurachi

Department of Pharmacology II,Graduate School of Medicine, OsakaUniversity, Suita, Osaka 565-0871,Japan

Email: ykurachi@pharma2.med.osaka-u.ac.jp

An exciting epoch is starting as we begin tograsp the variety of physiological functions ofthe protein 14-3-3. Moore & Perez firstdescribed 14-3-3 as a brain-enriched proteinin 1968. 14-3-3 was subsequently found torepresent a family of regulatory proteinswhich are ubiquitously expressed in eu-karyotic tissues (Fu et al. 2000). The unusualname is derived from the fraction number ofDEAEcellulose chromatography and itsmigration position in starchgel electro-phoresis. Today 14-3-3 is regarded as a multi-functional protein, like calmodulin, whichbinds to a variety of cellular proteins andmodulates their function. Biochemical andmolecular biological techniques haverevealed that more than 50 signallingmolecules can be regulated by 14-3-3.Through interaction with its effectors, 14-3-3is now considered to play a pivotal role in theregulation of diverse cell signalling pathways,including activation of protein kinases, cellcycle control, neural development and patho-genesis of bacteria and viruses. To date, sevenmammalian 14-3-3 isoforms have beenfound and these are distinguished by theGreek letters b, e, g, h, s, u and z. Thestructural basis of the specific interactionbetween a 14-3-3 protein and its targetpeptide has been elucidated (Yaffe et al.1997), but the physiological significance of14-3-3 in mammalian cells has remainedelusive.

This year a number of reports on the possiblephysiological functions of 14-3-3 inmammalian tissues have appeared. Both Niuet al. (2002) and Benzing et al. (2002)revealed that 14-3-3 controls the function ofprotein regulators of G protein signalling(RGS). RGS proteins are a family of proteinswhich accelerate the GTPase activity oftrimeric Gi or Gq protein a subunits and thusact as a negative regulator for a number ofG protein signalling pathways. If RGSproteins were active unrestrictedly, theywould completely suppress G protein-mediated signalling pathways. This is seen inexperiments with the over-expression of RGSproteins. Niu et al. (2002) and Benzing et al.(2002) both showed that 14-3-3 binds to andinactivates RGS proteins and that it thereforeacts as a physiological regulator of theG protein cycle. Kagan et al. (2002) recently

showed that 14-3-3 modulates ion channelfunction. 14-3-3 bound to N- and C-terminiof the HERG-K+ channel and stabilizedPKA-mediated phosphorylation, whichpotentiated the effects of cAMP/PKA. Thisstudy was of importance because it was thefirst to show an interaction between 14-3-3and an ion channel, though in this case themode of action of 14-3-3 was conventional asa modulator of kinase activity.

The study by Rajan et al. (2002) in this issueof The Journal of Physiology, on the otherhand, provides a novel protocol whereby 14-3-3 plays an essential role in the control ofmembrane localisation of ion channels. Theyshowed that the 14-3-3 protein is required forsurface localisation and functional expressionof the two-pore K+ channels TASK-1 andTASK-3 in Xenopus oocytes and HEK293cells, by interacting with their conserved C-terminal domains. Moreover, they showedthat 14-3-3 and TASK-1 or -3 could be co-immunoprecipitated from synaptic proteinextracts, suggesting their in vivo coupling innative brain tissues. This novel role for 14-3-3in membrane biology is sure to push the areaon to the next step.

14-3-3 is a relatively small protein (~30 kDa)with no additional functional domain, and ingeneral the effects of 14-3-3, includingactivation or deactivation of enzyme activityand conformation changes in target proteinstructure, seem to result simply from binding.In this case we should then ask how 14-3-3regulates membrane localisation (Rajan et al.2002). The control of membrane traffickingand localisation of proteins involves manyprocesses including the intracellular vesiculartransport system, actin- or tubulin-basedcytoskeleton systems, motor proteins, etc.The mechanism of 14-3-3-mediated regulationof trafficking and localisation of TASK-1 and-3 remains to be identified and furtherstudies are needed to clarify the whole pictureof the role of 14-3-3 proteins in the control ofmembrane excitability.

In addition, we would like to point out thatRajan et al. (2002) show that the putativePDZ-binding motif in the C-terminus ofTASK-1 (-SSV) is not responsible for themembrane localisation of the channel. Sincefirst revealed as a key regulator, PDZ(PSD-95, Disk-large, ZO-1) domain-containing anchoring proteins have been atthe centre of studies on the localisation ofmembrane proteins such as ion channels,transporters and receptors (Sheng & Sala,2001). If a membrane protein possesses aputative PDZ protein-binding motif at its C-terminal end, it has often been assumed tocouple with a PDZ protein. Such enthusiasmhas helped to identify a number of novel PDZproteins. Nevertheless, there may have beencases where these attempts have failed,

despite extensive searching with molecularbiological (e.g. yeast two-hybrid system) orbiochemical (e.g. co-immunoprecipitation)methods. Although the failures might bepartly due to lack of skill or luck, it is nowplausible that some putative PDZ-binding C-terminal motifs may be inherently unrelatedto PDZ proteins. We are only beginning tounderstand the profound wisdom of naturein her manner of arranging the localisation ofion channels to support different functions incellular electrophysiology.

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KAGAN, A., MELMAN, Y. F., KRUMERMAN, A. &MCDONALD, T. V. (2002). EMBO Journal 21,18891898.

MOORE, B. W., PEREZ, V. J. & GEHRING, M.(1968). Journal of Neurochemistry 15,265272.

NIU, J., SCHESCHONKA, A., DRUEY, K. M.,DAVIS, A., REED, E., KOLENKO, V., BODNAR, R.,VOYNO-YASENETSKAYA, T., DU, X., KEHRL, J. &DULIN, N. O. (2002). Biochemical Journal365, 677684.

RAJAN, S., PRESISIG-MLLER, R., WISCHMEYER,E., NEHRING, R., HANLEY, P. J., RENIGUNTA,V., MUSSET, B., SCHLICHTHRL, G., DERST, C.,KARSCHIN, A. & DAUT, J. (2002). Journal ofPhysiology 545, 1326.

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YAFFE, M. B., RITTINGER, K., VOLINIA, S.,CARON, P. R., AITKEN, A., LEFFERS, H.,GAMLIN, S. J., SMERDON, S. J. & CANTLEY,L. C. (1997). Cell 91, 961971.

PERSPECTIVES

Journal of Physiology (2002), 545.1, p. 2 DOI: 10.1113/jphysiol.2002.033886 The Physiological Society 2002 www.jphysiol.org