Endocytotic Cycling of Endocytotic Cycling of PM ProteinsPM Proteins

Junxia An

Xihua Chen

Annu.Rev.plant.biol.2005:221-251

SummarizeSummarize

Comparison of plasma membrane localization in polarized animal and plant cells.

Types of endosytosis Endosomal Cycling In Animals: An Overvei

w Protein Components of Endocytotic Cycling

Mechanisms

Tight junction

N

Endosomes

LysosomeGolgi

Mitochondrum

Plastid

Vacu

ole

IntroductionIntroduction

Eukaryotic cell respond to external signals by altering( 改变 ) transcriptional( 转录 ) output and regulating the abundance( 含量 ) and distribution( 分布 ) of PM proteins that comprise the functional interface with the external environment .

Animals (Yeast)Plants

Animals (Yeast)Animals (Yeast)

PM protein removal and recompartmentalization is regulated by endocytosis and endocytotic cycling.

PlantsPlants

PM localization of the PIN1 auxin efflux carrier complex and the KAT1 potassium channel are dynamically regulated has intensified efforts to conclusively demonstrate endocytotic recycling in plants.

Endosomal Cycling In Animals:Endosomal Cycling In Animals:An OverveiwAn Overveiw Endocytosis: • Take up extracellular substances and/or internalize PM

proteins for transport to endosomes.• comprise Highly dynamic structures• Helps maintain homeostatic regulation

Endosomes• PM proteins Target into the lysosome/vacuole for degradati

on or recycled back to cell surface.(M6P)• Mammalian endocytosis regylates multiple physiological pr

ocesses

Types of endocytosisTypes of endocytosis

Receptor-mediated endocytosis (受体介导)

Clathrin-dependent endocytosis (网格蛋白介导)

Clathrin-independent endocytosis (非网格蛋白介导) Caveolae/lipid raft-mediated endocytosis ( 晶格镶嵌型脂筏 ) Fluid-phase endocytosis ( 液相内吞 ) Phagocytosis ( 吞噬 )

Receptor-mediated endocytosis RMEReceptor-mediated endocytosis RME ( ( 受体介导的内吞作用受体介导的内吞作用 ))

Ligand bind to a PM receptor (uptake proteins or protein complexes) from the cell surface, then internalized in the cell.

Clathrin-mediated RMEClathrin-mediated RME(( 网格蛋白介导的受体介导内吞作用网格蛋白介导的受体介导内吞作用 ))

Steps:1. Ligands bound to their receptors (A

P2 adaptor complex; AP-180; ARF6GTPase)

2. Clustering( 聚集 ) in clathrin-coat3. Destined for degradation in lysosomes (M6

p)

Clathrin-independent processeClathrin-independent processess Caveolae/lipid raft-mediated endocytosis Fluid-phase endocytosis Phagocytosis ( 吞噬 )

TGNL

Sorting endosome

MVB

PCR

MVB

L

Tf-R

焦磷酸酶

Tf-R(-Fe)

ERC

FM4-64GNOMGFP FM4-64

Early endosomes/Sorting endosomesEarly endosomes/Sorting endosomes早胞内体；分选内体早胞内体；分选内体 Localized at the peripherally( 边缘 ) of the cell Structures: tubular-vesicular The first organelles to receive PM-derived c

argo (LDL; transferrin Tf) ( 来自 PM 的 )

First junction( 分叉 ) in the REM, tow main destination: the PM and the late endosomes

The kinetics are fast with a half-life(t1/2) of about 2 minutes

Elements

ERC ERC 内体循环区室内体循环区室 Associated with microtubules and exhibits a

variable( 可变 ) distribution Do not contain ligands or receptors (lysoso

mal enzymes; LDL) Marked by the presence of LDL-R and iron-f

ree Tf bound to the Tf-R The main intracellular cholesterol repository

( 胆固醇贮藏 )

ERCERC 内体循环区室内体循环区室 Tow kinetics( 活动 ) have been described for

ERC-to-PM recycling: A fast rate which recycle almost completely

back to the PM(t1/2=10 minutes)

A slower rate (t1/2=30 minutes); be sorted by lipid microdomains( 微结构域 )

Relatively long-lived organelle (longer than late endosomes)

ERCERC 内体循环区室内体循环区室 Insulin-regulated trafficking of the glucose-tr

ansporter GLUT4( 葡萄糖转移酶 )

Response to increased insulin( 胰岛素 ) concentration( 浓缩 ), resulting in higher uptake of glucose in fat and muscle cells.

IRAP

Sorting endosome

ERC ERC

Insulin-responsive compartment

SE

GLUT4

Protein Components of EndocytoProtein Components of Endocytotic Cycling Mechanismstic Cycling Mechanisms Adaptins and Adaptor Complexes ARFs/ARF-GEF Dynamins Rab GTPase SNAREs Other Cytoskeletal Interaction Other Proteins Contributing to PM Protein C

ycling

Adaptins and Adaptor ComplexesAdaptins and Adaptor Complexes

Mediate the selection of cargo molecules for inclusion( 包括 ) into coated vesicles( 衣被小泡 ) in the late secretory and endocytotic pathway.

One group consist of cargo adaptors are monomeric( 单体 ) proteins (AP180; β-arrestins; GGAs)

Another group consists of heterotetrameric( 异四聚体 ) AP complexes composed of adaptin subunits

Adaptins and Adaptor ComplexesAdaptins and Adaptor Complexes

Little is known about adaptin function in plants. Only two adaptins and one monomeric adaptor homolog( 同系物 ) from Arabidopsis( 拟南芥 ) have been functionally characterized on the molecular level.

ARFs/ARF-GEFARFs/ARF-GEF

AFRs are rasrelated GTP-binding proteins that maintain organellar integrity( 完整 ) and regulate intracellular( 内 ) transport.

Six types of ARFs.

ARFs/ARF-GEFARFs/ARF-GEF

Under normal condition , cytosolic( 胞液 ) GDP-bound ARFs are inactive.

A vesicle-associated GTPase activating protein (GAP) hydrolyzes GTP and releases the ARF protein, resulting in coat dissociation( 游离 ) prior to vesicle.

Dynamins Dynamins 发动蛋白发动蛋白 (not dyneins(not dyneins 动力蛋白动力蛋白 ))

Dynamins constitute a family of GTP Phosphohydrolases (P- 水解酶 ) that carry out diverse roles in eukaryotie membrane cycling.

A coiled-coil region(CC domain) serves as the GTPases, effector domain. Dynamins participate in membrane scission( 切割 ) events by self-assembling into multimeric ring structures.

The mechanism hypothesis of Rabs reguThe mechanism hypothesis of Rabs regulating endocytosislating endocytosis

The GTP-GDP cycle of Rabs might act as a timer switch( 定时的开关） to regulate the functionality of a membrane domain.

Rabs act as GTP-activated switches that simply stabilize protein complexes required for transition events.

The ability of Rabs to link membranes to cytoskeletal motor proteins suggests that they can generate uniquely functional membrane subdiomains.

SNAREsSNAREs N-ethymaleimide-sensitive factor （ N- 乙基

马来酰亚胺敏感因子） adoptor protein receptors are essential components of vesicle-trafficking machinery where they play a central role in membrane fusion events.

Other Cytoskeletal InteractionOther Cytoskeletal Interaction

All endocytotic pathways of membrane protein internalization depend on interaction with the cytoskeleton.

Direct participation of F-actin in internalization events has been documented in animals and yeast.

Other Proteins Contributing to Other Proteins Contributing to PM Protein CyclingPM Protein Cycling

Rho GTPases

Immunophilins(抑免蛋白）

Fatty acyl CoA

Rho GTPasesRho GTPases

Rho GTPases are membranes of a small but diverse protein familly involved in signaling and regulation of edocytotic traffic.

Rho-GTPases are not found in plants,but plant Rho-related GTPases comprise a plant-specific subfamily.

Immunophilins(Immunophilins( 抑免蛋白）抑免蛋白） Immunophilin-like proteins are likely involve

d in fundamental cellular processes. Most immunophilins possess peptidyl prolyl

（脯胺酰基） cis/trans-isomerase activity required for protein folding and modification.

Fatty acyl CoAFatty acyl CoA

Fatty acyl CoA has been identified in cycling GLUT4 vesicles and has been implicated in membrane budding and fusion events.In another study,acyl CoA dehydrogenase( 脱氢酶） was found to mediate intracellular retention of GLUT4 vesicles via association with IRAP.

What do we know about What do we know about endocytendocytotic cyclingotic cycling in plants in plants

Endocytosis（内吞作用）

Endocytotic compartments

Endosomal Sorting and Redirection

EndocytosisEndocytosis Recent studies indicate that endocytosis medi

ates the internalization and recycling of PM molecules,including membrane proteins and sterols in plants.

At least four forms of endocytosis operate in plants: clathrin independent and dependent,

phagocytotic,fluid-phases and lipid raft-mediated endocytosis.

Endocytotic compartmentsEndocytotic compartments Molecules may be transported into or out of

the compartment through transport vesicles or, alternatively,the compartments itself may be modifyed and mature over time.

Plant endocytic compartment are not well characterizd and the term endosome is often used for any comparment containing an endocytosed material.

Evidence for endocytotic recyclinEvidence for endocytotic recycling in plant g in plant

Auxin Transport Proteins And GNOM The KAT1 Inward-Rectifying Potassiu

m Channel( 内流钾通道）

Other Examples

Auxin Transport Proteins And GNOMAuxin Transport Proteins And GNOM

Establishing plant polarity depends on the polar transport of the growth hormone auxin. The directionality of auxin transport is maintained by a polar transport apparatus that requires asymmetrically localized transporters and regulators. The PIN proteins are the best characterized of such asymmetrically localized auxin efflux regulators.

The KAT1 Inward-Rectifying PotassiThe KAT1 Inward-Rectifying Potassium Channelum Channel （内流钾通道）（内流钾通道） The dogma( 信号） that the high turgor of pl

ant cells precludes endocytosis was recently challenged by evidence that the KAT K+ inward-rectifying channel is constitutively endocytosed from the guard cell Pms against high turgor pressure.

CONCLUSIONCONCLUSION

Recent experimental evidence suggests that plant cells respond to developmental programming and environmental condition by regulating the endocytosis of PM proteins. This evidence also suggests that some of the mechanisms underlying endocytotic cycling are conserved between plants and animals.

Thank you