Coated vesicles in plant cells

Seminars in Cell & Developmental Biology 18 (2007) 471–478

Review

Coated vesicles in plant cells Matthew J. Paul, Lorenzo Frigerio ∗ Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, United Kingdom Available online 10 July 2007

Abstract Coated vesicles represent vital transport intermediates in all eukaryotic cells. While the basic mechanisms of membrane exchange are conserved through the kingdoms, the unique topology of the plant endomembrane system is mirrored by several differences in the genesis, function and regulation of coated vesicles. Efforts to unravel the complex network of proteins underlying the behaviour of these vesicles have recently benefited from the application in planta of several molecular tools used in mammalian systems, as well as from advances in imaging technology and the ongoing analysis of the Arabidopsis genome. In this review, we provide an overview of the roles of coated vesicles in plant cells and highlight salient new developments in the field. © 2007 Elsevier Ltd. All rights reserved. Keywords: Plant secretory pathway; Protein trafficking; Endocytosis; Endoplasmic reticulum; Golgi; Vacuole; Coated vesicles; Clathrin; COPI; COPII; Retromer

Contents 1. 2. 3.

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Vesicular trafficking in plant cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Small GTP-binding proteins and associated factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plant vesicle coat families . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Clathrin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. COPI and COPII . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Retromer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Vesicular trafficking in plant cells The plant endomembrane system encompasses a series of compartments which provide specialised surfaces and segregated areas for the production and storage of biomolecules. An overview of the plant secretory pathway and the coated vesicles described in this review is shown in Fig. 1.

Abbreviations: CCV, clathrin-coated vesicles; CHC, clathrin heavy chain; CLC, clathrin light chain; ER, endoplasmic reticulum; ERES, endoplasmic reticulum export sites; GAP, GTPase activating protein; GEF, guanosine nucleotide exchange factor; LV, lytic vacuole; MVB, multivesicular body; PVC, prevacuolar compartment; PSV, protein storage vacuole; VSR, vacuolar sorting receptor; VTC, vesiculo-tubular compartment ∗ Corresponding author. Fax: +44 24 765 23701. E-mail address: [email protected] (L. Frigerio). 1084-9521/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.semcdb.2007.07.005

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The endoplasmic reticulum, a dynamic mesh-like structure traversing the cortical cytoplasm of plant cells, represents the entry compartment for nascent polypeptides destined for secretion [1–3]. From the ER, cargo molecules following the secretory pathway reach a stack of the fragmented, mobile Golgi apparatus [4] without passing through an intermediate compartment such as the mammalian vesiculo-tubular compartment (VTC [5]). Compelling evidence now exists to support an extremely close functional and physical relationship between plant ER and Golgi compartments ([6,7], reviewed in Ref. [8]). Nevertheless, the formation of coatomer (COPI and COPII) transport intermediates remains essential for the transfer of cargo and for the maintenance of membrane flux [6,9]. Anterograde transport routes from the Golgi stacks and retrograde (endocytic) routes from the cell surface converge at a poorly defined group of compartments, interchangeably termed

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Fig. 1. A model for the distribution of coated vesicles and receptor proteins throughout the plant endomembrane system. Coloured circles represent soluble cargo molecules within the secretory/endocytic pathway and black arrows denote presumptive membrane fusion events. Cargoes entering the pathway at the level of the ER proceed towards the Golgi apparatus via COPII-coated vesicles, possibly following interaction with receptor proteins of the p24 family [84–86] or after passive diffusion into an ERES [8]. It is also possible that anterograde transport by cisternal maturation may occur for certain cargoes. ER resident proteins (orange circles) are recycled from downstream compartments by the plant ERD2 receptor homologue, while a distinct subpopulation of COPI coats (COPIb; [12]) mediates intra-Golgi retention and recycling events. Cargoes progress from the Golgi towards vacuoles either in uncoated, ‘dense’ vesicles [20,21] or clathrin-coated vesicles [12,22] depending on the nature of the cargo and/or the presence of a cognate cargo receptor. Clathrin-coated vesicles are also involved in endocytosis, e.g. that of the auxin transporter PIN2 [11], and these coats may be distinguished by the presence of distinct sets of adaptor proteins [55,59]. Finally, retromer coats have been implicated in the retrieval of cargo receptors from the PVC [23,26]. Abbreviations: ER, endoplasmic reticulum, ERES, ER export site, CCV, clathrin-coated vesicle, DV, dense vesicle, PVC, prevacuolar compartment, MVB, multivesicular body, VSR, vacuolar sorting receptor.

prevacuolar compartments (PVC) or endosomes [10]. Very recently, clathrin-coated vesicles (CCV) have been convincingly shown to be implicated in endocytosis in the plant secretory pathway [11]. This is consistent with their observed distribution within the cell [12–14]. In mammalian and yeast cells, CCV are also involved in post-Golgi trafficking routes to a terminal intracellular compartment [15,16]. In plant cells, the precise role of clathrin in anterograde transport is still a matter of some debate as there exist at least two separate routes to functionally distinct vacuolar compartments [17–19]. In addition, some plants can handle post-Golgi sorting of storage proteins through electronopaque, ‘dense’ vesicles (DV) with no detectable protein coat [20,21]. However, a role for clathrin in vacuolar transport in plant cells is illustrated by the recent in situ detection of clathrin coats in restricted regions around the trans-Golgi cisternae of the Golgi apparatus [12,22]. The concept of multiple roles for clathrin coats in plant cells is supported by the identifica-

tion of a full range of key plant orthologues for proteins with established roles in clathrin-mediated transport in mammals and yeasts. Plant cells also contain functional orthologues of the retromer coat complex which was first identified and characterised in yeast [23,24]. In yeast cells, retromer is required for the recycling of cargo receptors from the pre-vacuolar compartment to the trans-Golgi [25]. Consistent with an equivalent role in plant cells, a component of the plant retromer complex is required for the correct sorting of vacuolar cargo in developing Arabidopsis seed [26]. The similarity between vesiculation events in plants and those of mammalian and yeast cells has provided invaluable clues into the function of the plant endomembrane system. However, the unique topology, compartments and cargoes present in the plant secretory pathway necessitate the close examination of each aspect of vesicular transport in these cells. Here, we review

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the latest findings concerning the roles of various vesicle coats in the selective recruitment of cargo at each step of this pathway and highlight plant-specific adaptations to the vesiculation machinery. 2. Small GTP-binding proteins and associated factors In common with mammals and yeasts, plant vesiculation events are governed by members of the Ras superfamily of small GTP binding proteins [27]. The Arf subfamily of small GTPases, which includes the Sar, ARF and ARF-like ARL GTPases, are involved in the formation and budding of vesicles throughout the plant endomembrane system [28,29]. ARF GTPases are associated with clathrin coats as well as COPI coated vesicles, whereas COPII coat nucleation is triggered by a GTPase of the Sar subfamily. Uniquely, the retromer coat does not appear to contain a GTPase in any organism, including plants [23,24]. Twelve AtARF (Arabidopsis thaliana ARF) sequences have been identified through a BLAST search of the Arabidopsis genome against consensus mammalian and yeast ARF sequences [30]. The number of ARF homologues in Arabidopsis exceeds that in the genomes of humans or fission yeast (6 and 3, respectively [31]), a diversity which may reflect both functional redundancy and the complexity of the plant endomembrane system. In addition to this diversity, AtARF homologues may play several distinct roles within the plant cell. AtARF1 has been shown to play a role in a vacuolar sorting pathway in plants [29] while chimeric AtARF1-fluorescent protein fusions have been localised to unknown compartments derived from the Golgi apparatus [32] as well as putative endocytic structures [33]. In common with all Ras superfamily GTP-binding proteins, AtARFs adopt an active effector-binding conformation when associated with GTP and, upon hydrolysis, an inactive conformation when bound to GDP. ARF GTPase activating proteins (ARF GAPs) were initially characterised by their ability to stimulate the intrinsic GTPase activity of Arf, while guanosine nucleotide exchange factors (GEFs) catalyse the reverse reaction. However, it has recently become apparent that these proteins also frequently possess other biological activities mediated through various protein–protein and protein–lipid domains [34]. The Arabidopsis genome contains a large number of ARFGAPs and GEFs [30], raising the possibility that these regulators act in response to diverse cellular signals. Arabidopsis AGD7, a member of the ArfGAP1 family, has recently been shown to activate AtARF1 in a phosphatidic acid-dependent manner [35]. Furthermore, the authors also show that the overexpression of AGD7 led to the disruption of protein transport between the ER and the Golgi. Another report indicates that the uptake of auxin at the plasma membrane may be impaired by the overexpression of OsAGAP (Oryza sativa AGAP) in rice [36]. In view of these results, the overexpression of the ARF GAPs appears to provide an informative tool to identify the multiple functions of individual ARF GTPases. Equally, different GEFs may also mediate alternative functions for GTPases within the plant cell [37].

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3. Plant vesicle coat families 3.1. Clathrin The clathrin coat was the first proteinaceous vesicle coat to be observed [38] and structurally characterised [39]. It remains the most fully understood coat complex in both plants and animals today. Three structural clathrin heavy chain molecules (CHC, Mr ∼180 kDa) come together through the interactions between domains near the C-termini of the heavy chains to form a triskelion, the basic structural unit of the clathrin coat. Purified CHC triskelia demonstrate self-assembly at a physiological pH [40] to form cages which have been the focus of detailed structural studies [41,42]. CHC is encoded by two genes in both mammals and Arabidopsis and displays a remarkable degree of conservation between the two organisms (∼75% similarity, [13]). Each heavy chain is accompanied by a regulatory clathrin light chain (CLC, Mr ∼30 kDa) which is less recognisable against its mammalian counterpart. Out of a series of potential CLC genes in plants, Arabidopsis CLC1 has now been experimentally validated by the demonstration of its ability to bind human CHC via a conserved central coiled-coil domain [43]. Two further proteins with similarity to mammalian CLC were identified in this study (AtCLC2 and AtCLC3), and all three contained the N-terminal acidic motif implicated in preventing the spurious assembly of clathrin heavy chain triskelia into lattices [43]. Intriguingly, this discovery raises the possibility of an unparalleled degree of functional specialisation amongst Arabidopsis CLC sequences. This is supported by gene expression data indicating that the transcription of AtCLC1 may be relatively reduced in pollen granules, while that of AtCLC2 is raised in seeds (AtGenExpress developmental dataset [44]). Recently, inhibition of clathrin triskelion formation by overexpression of the ‘hub’ domain of clathrin heavy chain [40,45] in Arabidopsis protoplasts was used to prove that clathrin-mediated endocytosis is the principal endocytic mechanism in these cells [11]. It is not yet clear whether the same strategy can be successfully applied to study the role of clathrin in anterograde transport routes. Clathrin-mediated vesiculation occurs as a response to an accumulation of peripheral and intrinsic membrane factors on the donor membrane. Clathrin adaptors are peripheral components of this network with the ability to interact with clathrin at the N-terminal domain of CHC as well as with other components of the network through well-characterised motifs and modules. Clathrin adaptors range from 300 to 3000 amino acids in size and, in general, there exists no similarity between them. Recruitment of clathrin by these peripheral membrane components permits the system to avoid the induction of vesicles by recycling integral membrane proteins such as cargo receptors when away from their main point of function. Clathrin adaptors are recruited from the cytoplasm by an activated small GTP-binding protein from the ARF family [46] often in conjunction with a short-lived phospholipid species [47,48]. The prototype clathrin adaptors are the heterotetrameric assembly proteins (APs). Four AP complexes each consisting of two large subunits (␤(1–4) and either of ␣, ␥, ␦, ␧ dependent on the complex), one medium

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subunit (␮1–4) and one small subunit (␴1–4) have been identified to date in mammalian cells. Structural orthologues for each component of the four AP complexes have been identified in the Arabidopsis genome [49]. Several of these components have been functionally characterised in plant cells: a ␥-adaptin, three ␤-adaptins [50], and a ␮-adaptin [51] all from Arabidopsis, along with two Arabidopsis ␴-adaptins and a ␴1 from a Chinese medical tree (Camptotheca acuminata) [52,53] and a ␴2 from maize [54]. Each AP complex is associated with specific membranes through interactions with specific membrane phosphoinositides (PtdInsP) mediated by one of the large subunits (␣, ␥, ␦ or ␧). In mammals and yeasts, AP1 is associated with intracellular membranes and primarily the Golgi apparatus, AP2 with the plasma membrane, AP3 with lysosomes while AP4 is present on endosomes [55] (reviewed in Ref. [56]). Plant cells share the enzymes necessary for the synthesis of the PtdInsP2 series of phosphoinositide isomers [57]. The recruitment of clathrin adaptors from the cytosol may also occur in response to the build-up of cargo molecules themselves. In this case, a transmembrane receptor is required to transduce the sorting signal of the luminal cargo protein or extracellular ligand to the cytoplasmic side of the membrane. Several classes of peptide motifs within the cytosolic tails of these receptor proteins are recognised by ␮AP subunits, whereas other classes interact via additional adaptor proteins (reviewed in Ref. [58]). Two families of cargo receptors with anterograde trafficking roles and several receptors involved in events analogous to mammalian receptor-mediated endocytosis have been identified in plants. Of these, the tyrosine-based sorting signals present in the VSR family of cargo receptors [51] and a leucine-rich receptor kinase [59] have been shown to recruit ␮-adaptin, and thus potentially traffic via clathrin-coated vesicles. Several families of monomeric clathrin adaptors bearing characteristic conserved N-terminal domains have been described in animal and yeast cells. Three Arabidopsis proteins contain a consensus phosphoinositide-binding epsin NH3 terminal homology (ENTH) domain and seven contain the related AP180 NH3 -terminal homology (ANTH) domain. In non-plant systems, each monomeric adaptor is associated with specific, diverse cargo including the glutamate receptor (HIP1) [60], ubiquitinated receptors (epsins) [61] and v-SNAREs (synaptobrevin [62], Vti1b and Vps27/Hrs [63]). Recently, a detailed sequence analysis of the Arabidopsis E/ANTH domain proteins was performed in order to establish potential roles for these adaptors in plants [64]. Outside of the characteristic Nterminal domain, clathrin binding motifs were identified in each homologue. Importantly however, no Arabidopsis E/ANTH domain protein contains the established actin or ubiquitin binding motifs which are intrinsic to the reported functions of many non-plant adaptors of this type. Furthermore, structural changes in the ␣-helices of the plant E/ANTH domains indicate an altered phosphoinositide specificity when compared with the domain archetypes [64]. These observations suggest that these plant adaptors may drive substantially different vesiculation events compared with their animal and yeast counterparts. Functional data have very recently been published for three monomeric clathrin adaptors in Arabidopsis, the ANTH-

domain-bearing AtAP180, [65] and the ENTH-based adaptors AtEPSINR1 and AtEPSIN2R [66,67]. AtAP180 was initially identified as a binding partner for the large subunit of the plant AP-2 complex, At-␣C-Ad, which is implicated in receptormediated endocytosis. In rats, AP180 is a neuronal adaptor of ∼90 kDa with the ability to induce the formation of coated vesicles with a remarkably narrow size distribution [68]. AtAP180 was found to retain this assembly activity based on a critical DLL clathrin-binding motif. Despite being unable to promote the assembly of clathrin, mutant AtAP180 lacking the classical DLL clathrin-box could still bind the heavy chain. It was concluded that additional, undescribed clathrin binding motifs must be present within the adaptor and that the assembly activity was likely to be mediated by a structural array of such motifs [65]. A similar arrangement of clathrin-binding motif has been proposed for AtEPSINR1, which has also been shown to bind a large AtAP-2 subunit [67]. However, a possible role in endocytosis for this adaptor is contrasted with strong evidence implicating AtEPSINR1 in the vacuolar trafficking of soluble cargo proteins at the TGN: AtEPSINR1 has been shown to interact with the vacuolar sorting receptor AtVSR1 and the TGN v-SNARE VTI11 with which it was observed to colocalise. Moreover, an AtEPSINR1 T-DNA insertional mutant line missorted 100% of the lytic vacuolar cargo sporamin:GFP to the apoplast. Consistent with a role at distinct membranes, AtEPSINR1 binds a putative AP-1 ␥-subunit with greater affinity than the ␣-subunit of AP-2 [67]. Recently published data on a second Arabidopsis epsin homologue, AtEPSINR2, indicate that this clathrin adaptor interacts with a plant AP subunit with the strongest degree of similarity to the ␦-subunit of AP-3, thereby providing a link between clathrin and AP3 in plant cells [66] that has not been observed in studies of yeast or mammalian AP3 [69,70]. AtEPSINR2 was found to localise with the TGN and, through interactions mediated by the adaptor ENTH domain, with a potentially novel post-Golgi compartment enriched in Ptd(3)InsP and the ␦-adaptin related protein. Intriguingly, the functional identity of this compartment may be compromised by the phosphoinositide-3-kinase inhibitor wortmannin, a fungal metabolite which has been previously shown to interfere with vacuolar trafficking [71,72]. Recent reports have also indicated that Ptd(3)InsP is transiently enriched in certain vesicular membranes during cytokinesis [73]. It seems likely that new roles for clathrin coats within the plant endomembrane system will be forthcoming from further functional studies of the adaptor protein complement. 3.2. COPI and COPII Vesicles associated with polymeric coatomer (COP) coat complexes are associated with trafficking events at the ER and Golgi apparatus. A strong body of evidence supports a role for COPII coats in the movement of cargo from the ER to the Golgi apparatus [74]. Plant orthologues for several components of the COPII coat have been isolated (Sar1 and Sec12 [75]; Sec24 [76]; Sec13 [77]). A further component, Sec16, has been implicated in the formation and function of mobile ER export sites (ERES) in yeasts and mammalian cells [78,79]. Although

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a plant Sec16 orthologue has not yet been identified, similar ERES structures are apparent in the plant ER [6,80]. Recent evidence suggests that plant COPII vesicles are formed at ERES de novo in response to the accumulation of cargo signals present in membrane proteins [80,81]. This model would appear to distinguish the formation of plant COP-coated vesicles from the constitutive formation of CCV at the plasma membrane [11]. It has been established that COPII vesicles are also involved in the trafficking of soluble proteins from the ER [6,9], although these cargoes alone do not induce the formation of ERES in plants. A passive ‘bulk-flow’ model has therefore been proposed for the trafficking of these soluble cargoes [9,82]. A family of type I transmembrane protein factors, the p24 proteins, was identified as potential cargo receptors for ER export and as structural components of coatomer coated vesicles in mammals, yeast and now plants [83–86]. A recent investigation into plant p24 proteins has revealed a synergy between two distinct sorting motifs in the cytosolic tail of these receptors which mediate binding in vitro to both Sec23 and subunits of the COPI coat complex [85,86]. The precise role of the COPI complex remains unclear; although it has been associated with vesicular transport within the Golgi apparatus and from the Golgi apparatus to the ER [87]. Plants contain a full complement of COPI components (ARF1, ␦-, ␧-COP [88], ␣-, ␤-, ␤ -, ␥-COP [89], Sec21p [76]). The binding of plant p24 proteins to both COPII and COPI coated vesicles provides a possible means for receptor recycling in planta and is consistent with the situation in mammalian cells [90]. This model of bidirectional cargo transport between the ER and the Golgi via coated vesicles has been supported by recently published EM observations of COPI-coated vesicles associated with the Golgi apparatus of Scherffelia dubia and Arabidopsis [12]. The authors identify a subpopulation of retrograde COPI vesicles (COPIa) which appear to be concentrated in the space between the ER and the Golgi along with COPII-coated vesicles. In yeast cells, the total ablation of the p24 family leads to the accumulation of the secreted enzymes invertase and Gas1p [91,92], as well as interfering with the activity of the H/KDEL receptor ERD2 which functions to retrieve ER-resident proteins from downstream compartments [93]. COPI-coated vesicles are also likely to be involved in the recycling of the H/KDEL receptor in plants. Stripped membranes from tobacco plants overexpressing an ER-retained reporter can be induced to form COPI vesicles containing detectable levels of the reporter and the soluble ER chaperone calreticulin [88]. However, no report highlighting the requirement for p24 proteins in the trafficking of the plant ERD2 receptor or of specific plant cargoes has yet been published.

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and Vps5p). Vps35p interacts with the cargo receptor Vps10p and the complex is anchored to the membrane via Vps26p and to the small subunit via Vps29p [97]. The two small subunit components of the yeast retromer complex can dimerise via BAR protein-protein interaction domains [98] and interact with Ptd(3)InsP-enriched membranes via Phox homology (PX) domains [99]. The Arabidopsis genome contains orthologues for each of the large retromer subunits (3 for VPS35p, 2 for VPS26p and 1 for VPS29p) and three for the small subunit Vps5p [23]. The plant vacuolar sorting receptor VSR1 [100], which plays an analogous role to yeast Vps10p, has been immunoprecipitated with antisera cognate for AtVPS35. Fluorescent reporter constructs based on these two proteins have been shown to co-localise under the microscope [23]. These observations suggest that plant retromer coats, like their yeast and mammalian counterparts, are involved in the retrieval of cargo receptors from downstream compartments. The interaction of VSR1 with both the retromer at the PVC and the clathrin-recruiting AP2␮ subunit at the TGN also suggests that specific vesiculation events occur in response to membrane-intrinsic factors and not cargo receptors themselves. Recently, an allele of AtVPS29 (maigo1-1) was identified in a screen for Arabidopsis mutants that accumulate abnormal levels of the precursors of the major seed storage proteins [26]. This striking phenotype is likely due to AtVPS29 being the only retromer component without further potential homologues in Arabidopsis. This result is important as it indicates that correct, retromer-mediated recycling of VSR1 between the Golgi and the PVC is necessary to guarantee the proper sorting of all vacuole-bound cargo. It is not yet clear, however, whether VSR1 participates directly in this sorting event [17]. The presence of additional VPS35 and VPS26 genes in plants is unusual and, assuming they are functional, may point to multiple cargo specificities for large retromer subunits in plants. In contrast, membrane-selective small retromer subunits are drawn from a family of phosphoinositide-binding proteins with roles in membrane trafficking, the sorting nexins (SNX). In mammalian cells, SNX1, 2, 5 and 6 have been described as components of retromer coats, while no Vps17p orthologue exists [101,102]. The three Vps5p/SNX orthologues identified to date in plant cells may therefore interact with each other or with further currently unidentified factors to form a complete small subunit with a unique membrane specificity. Given that no small Arf family GTPase has been implicated in retromer coat formation in mammalian or yeast systems, these interactions may be of pivotal importance to the function of these coat complexes in plant cells. 4. Conclusions and perspectives

3.3. Retromer In yeast cells, the vacuolar sorting receptor Vps10p is recycled from the prevacuolar compartment to the TGN through a vesiculation event mediated by a cytosolic protein complex known as the retromer [24,94–96]. The retromer coat consists of a cargo-selective trimeric large subunit (Vps35p, Vps29p and Vps26p) and a membrane-selective small subunit (Vps17p

In this brief review we have attempted to describe the currently known features of plant coated vesicles. An earlier review [21] outlined the high degree of similarity between plant and yeast or mammalian coat proteins. Almost a decade later, our knowledge of the molecular composition of such transport vesicles has dramatically increased. Whilst recent advances confirm the predicted, general degree of conservation in coat components

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