Syndapins – also called PACSINs – are highly conserved Src-homology 3 (SH3)-domain-containing proteins that seem to exist in all multicellular eukaryotes. They interact with the large GTPase dynamin and several other proteins implicated in vesicle trafficking. Syndapin-dynamin complexes appear to play an important role in vesicle fission at different donor membranes, including the plasma membrane (endocytosis) and Golgi membranes. In addition, syndapins are implicated in later steps of vesicle cycling in neuronal and non-neuronal cells. Syndapins also interact with N-WASP, a potent activator of the Arp2/3 complex that forms a critical part of the actin polymerization machinery. Syndapin oligomers can thereby couple bursts of actin polymerization with the vesicle fission step involving dynamins. This allows newly formed vesicles to move away from the donor membrane driven by actin polymerization. Syndapins also engage in additional interactions with molecules involved in several signal transduction pathways, producing crosstalk at the interface between membrane trafficking and the cytoskeleton. Given the distinct expression patterns of the different syndapins and their splice forms, these proteins could have isoform-specific functions.
The formation and movement of vesicles, as well as the organization of different pools of vesicles within distinct compartments of cells, are thought to involve cytoskeletal elements; however, how the different molecular machineries involved are interconnected is mostly unclear (Qualmann et al., 2000b; Qualmann and Kessels, 2002; Engqvist-Goldstein and Drubin, 2003; Gundelfinger et al., 2003; Orth and McNiven, 2003). Recently, a handful of candidates for proteins that can act at this interface have been identified. Among these are members of the syndapin family, which are Src-homology 3 (SH3)-domain-containing proteins that exhibit several isoforms and splice variants. SH3 domains recognize proline-rich motifs of the PXXP type and their specificity relies mainly on the residues flanking such motifs. Syndapins belong to a growing class of accessory proteins functioning in membrane trafficking that interact with the proline-rich domain of the GTPase dynamin. Because the name `syndapins' (for synaptic dynamin-associated proteins) currently seems to reflect best what is known about the functions of these proteins in vivo, we refer to them as such here – avoiding use of other names (e.g. FAP52, SH3p14 and PACSIN) to keep the nomenclature as simple as possible.
Dynamin is an important player in endocytosis, a process that comprises several distinct steps. First, cell-surface receptors are bound by intracellular adaptors. A clathrin coat is then assembled on the underside of the membrane, which then invaginates. The developing clathrin-coated pits constrict at the neck, and finally they pinch off from the membrane. The newly formed vesicle is transported into the cytosol, where it is uncoated and can undergo further sorting. Dynamin is crucial for the fission step, in liberating newly formed vesicles from the donor membrane (Hinshaw, 2000; Sever et al., 2000).
All syndapins cloned and/or identified as DNA sequences show remarkably high conservation of both domain structure and amino acid sequence in species as diverse as worms, insects, fish, birds and mammals. Each is composed of an N-terminal region predicted to be almost exclusively α-helical and to engage in coiled-coil interactions, a flexible stretch that may contain up to three NPF motifs, and a C-terminal SH3 domain. The SH3 domain is responsible for interactions with dynamin and N-WASP, a potent activator of the Arp2/3 complex F-actin-nucleation machine. Syndapin complexes may thus link membrane trafficking and the actin cytoskeleton. Recent evanescent field-microscopy studies have demonstrated that actin, the Arp2/3 complex and N-WASP transiently accumulate at sites of endocytosis and that this is coordinated with dynamin-mediated vesicle fission (Merrifield et al., 2002; Merrifield et al., 2004). Here, we focus mainly on the roles of the actin cytoskeleton in vesicle formation, discuss how syndapins might work at the interface of actin and membrane trafficking, and highlight the molecular requirements and mechanisms involved. We also analyse the phylogenetic relationship between the different syndapin orthologs, isoforms and splice variants, which allows them to be organized into distinct subgroups.
Syndapin interactions and functions
Connecting the cytoskeleton with vesicle formation
Syndapins interact with N-WASP (a protein important for actin filament formation) as well as several molecules implicated in membrane trafficking: the GTPase dynamin (which controls endocytic vesicle formation) (Hinshaw, 2000; Sever et al., 2000), the phosphatidylinositol 5-phosphatase synaptojanin (a protein that plays a crucial role in the uncoating of clathrin-coated vesicles; Cremona et al., 1999) and synapsin I (a protein associated with the reserve pool of synaptic vesicles) (Hilfiker et al., 1999) (Fig. 1). These interactions first raised the possibility that syndapins have roles in both membrane trafficking and organization of the actin cytoskeleton (Qualmann et al., 1999), a hypothesis that was followed up by more-detailed studies of the interactions with dynamin and N-WASP.
The relevance of the interaction with dynamin is strongly supported by coimmunoprecipitation studies of the endogenous proteins (Qualmann et al., 1999; Qualmann and Kelly, 2000) and by the fact that a surplus of dynamin-binding syndapin SH3 domains inhibits receptor-mediated endocytosis in both permeabilized cell assays (Simpson et al., 1999) and intact cells (Qualmann and Kelly, 2000). This block of endocytosis occurs at the transition from invaginated clathrin-coated pits to closed endocytic membrane compartments (i.e. at the step at which dynamin is crucial) (Simpson et al., 1999). In common with other SH3-domain-containing dynamin-binding proteins, syndapins might influence the subcellular localization, GTP-binding and/or GTP hydrolysis rate of dynamin.
All syndapins also interact with the Arp2/3 complex activator N-WASP (Qualmann et al., 1999; Qualmann and Kelly, 2000; Modregger et al., 2000); they might thus connect the actin cytoskeleton with dynamin-mediated vesicle fission (Fig. 1). The in vivo relevance of this interaction is supported by studies showing coimmunoprecipitation of endogenous syndapin I and N-WASP (Qualmann et al., 1999) and by the fact that overexpression of syndapin I or syndapin II induces formation of numerous actin-rich filopodia. This requires activation of the Arp2/3 complex at the cell cortex (Qualmann and Kelly, 2000), and the phenotype is similar to that caused by activation of overexpressed N-WASP (Miki et al., 1998).
N-WASP exists in an autoinhibited state and needs to be opened up by effector molecules such as phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P2] and Cdc42 to associate with and activate the Arp2/3 complex (Kim et al., 2000; Higgs and Pollard, 2001; Welch and Mullins, 2002). As syndapins can recruit N-WASP to membranes and trigger local actin polymerization in vivo in an SH3-domain- and Arp2/3-complex-dependent manner (Kessels and Qualmann, 2002), syndapins seem to belong to the diverse set of N-WASP effectors that can trigger activation of the Arp2/3 complex and thereby actin polymerization. The cytoskeletal role of syndapins is also reflected by the fact that syndapins are enriched at sites of high actin turnover, such as lamellipodia (Qualmann and Kelly, 2000) and neuronal growth cones (Kessels and Qualmann, 2002).
Studies of the syndapin II isoform in chicken (FAP52) reinforce this connection with the actin cytoskeleton. FAP52 binds to the actin-crosslinking protein filamin/ABP-280 (Nikki et al., 2002a) (Fig. 1) and localizes to focal adhesions (Meriläinen et al., 1997). Note, however, that this has not been found in other mammalian systems or in Xenopus laevis (Ritter et al., 1999; Cousin et al., 2000) (M.M.K. and B.Q., unpublished). Moreover, filamin/ABP-280 is not a focal adhesion protein, and the distribution of the two proteins only partially overlaps at sites of contact between stress fibres and focal adhesions (Nikki et al., 2002a).
A link between the cytoskeletal and endocytic functions of syndapins was suggested by the observation that overexpression of N-WASP interferes with receptor-mediated endocytosis, and this depends solely on the syndapin-binding, central proline-rich domain of N-WASP. The phenotype can be rescued by syndapin co-overexpression (Kessels and Qualmann, 2002). The involvement of N-WASP interactions in endocytic vesicle formation is strongly supported by the observation that endocytosis is inhibited in cells in which endogenous N-WASP is confined to mitochondria or targeted by anti-N-WASP antibodies. One can rescue endocytosis by resupplying the cells with N-WASP (Kessels and Qualmann, 2002). Analysis of lymphocytes from mice lacking WASP also implicates WASP family members in endocytosis. These cells exhibit defects in T-cell receptor endocytosis in addition to defects in actin polymerization (Zhang et al., 1999).
What part might actin play in vesicle formation? The actin cytoskeleton might spatially organize the endocytic machinery. It might represent a barrier through which newly formed vesicles must be transported that needs to be removed by a local increase in actin turnover. It might also provide structural support for membrane topologies that facilitate vesicle formation, such as invaginated tubules, and/or promote vesicle formation by generating force through motor proteins and/or actin polymerization (Qualmann et al., 2000b). The latter could be achieved by activation of the Arp2/3 complex by N-WASP and syndapin, if actin polymerization is spatially restricted and occurs mainly at vesicle membrane areas facing the plasma membrane (Fig. 2). By contrast, F-actin formation at vesicle membrane areas facing the cytoplasm or within wide areas of the cortical cytoskeleton would create a barrier and thus instead be inhibitory.
The timing of local actin polymerization would need to be tightly controlled to correlate with the fission reaction (Fig. 2). Short-lived actin structures at sites of endocytosis whose appearance coincides with dynamin-mediated vesicle release can be observed by evanescent field microscopy (Merrifield et al., 2002). Additionally, both N-WASP and the Arp2/3 complex transiently appear at sites of endocytosis (Merrifield et al., 2004). The kinetics of Arp2/3 complex recruitment mirror those of formation of the actin structures – this is expected because the complex becomes incorporated into forming F-actin structures. By contrast, the catalytic Arp2/3 complex activator N-WASP appears transiently, being present at its highest levels during the initial phase of F-actin formation upon vesicle departure (Merrifield et al., 2004).
An attractive hypothesis is that the coincidence of actin nucleation and dynamin-mediated fission reflects the use of a common binding partner for both machineries, such as syndapin (Fig. 2). Dynamin forms a collar at the neck region of plasma membrane invaginations in synaptosomes incubated with GTPγS and in nerve terminals of shibire flies (Hinshaw, 2000). The interaction with the dynamin-associated syndapin could allow specific recruitment of N-WASP to the neck of coated pits and thereby produce polarity in the actin polymerization and directed movement of newly formed vesicles away from the plasma membrane (Fig. 2). Indeed, recent studies have revealed that syndapins can recruit N-WASP to intracellular membranes (Kessels and Qualmann, 2002). Such mechanisms might not only facilitate the departure of the vesicle from the donor membrane but might also create actin structures that have the appropriate localization, timing and polarity for moving detached vesicles away from the plasma membrane. Several studies showing actin tails attached to moving vesicles support this theory (Taunton, 2001). Interestingly, both dynamin and N-WASP have been detected in such actin tails, mainly at the actin-membrane interface (Taunton, 2001; Orth et al., 2002; Lee and De Camilli, 2002).
Proteins such as Abp1 and cortactin could have functions similar to those of syndapins in connecting vesicle fission with the cytoskeleton (Kessels and Qualmann, 2002; Orth and McNiven, 2003). Both bind to actin and dynamin through independent domains (Kessels et al., 2001; McNiven et al., 2000). By contrast, syndapins use their single SH3 domain for associations with both N-WASP and dynamin. They must therefore either switch between interacting with N-WASP and dynamin, or use bridging molecules or oligomerize to interact with the two simultaneously (see below).
In common with the plasma membrane, Golgi membranes are associated with a specialized actin-spectrin cytoskeleton (Beck and Nelson, 1998; De Matteis and Morrow, 2000; Stamnes, 2002) that seems to support membrane topology and organelle organization (Valderrama et al., 1998; di Campli et al., 1999) and might also be involved in membrane trafficking (Müsch et al., 2001; Valderrama et al., 2001; Fucini et al., 2002). Both dynamin and N-WASP localize to the trans-Golgi network (TGN) and play a role in vesicle budding at Golgi membranes (Jones et al., 1998; Luna et al., 2002), and the F-actin-binding Abp1 (Kessels and Qualmann, 2002) has also been reported to play a role in Golgi trafficking (Fucini et al., 2002). Recent data suggest that syndapins also associate with Golgi membranes. Interference with complexes of syndapin II and dynamin II by antibodies or dominant-negative constructs strongly inhibits budding from Golgi membranes (Kessels et al., 2003) (M.M.K. and B.Q., unpublished). The involvement of actin polymerization in vesicle formation might thus not be a speciality of the plasma membrane but a more general mechanism within the cell.
Syndapin oligomerization might physically link different syndapin interaction partners
All syndapin isoforms contain stretches of amino acids that are predicted to engage in coiled-coil interactions (Qualmann et al., 1999). Indeed, both homo-oligomers and heterooligomers composed of different syndapin isoforms can be observed, and the coiled-coil-domain-containing N-terminus is sufficient for syndapin-syndapin interactions in vitro and in vivo (Qualmann et al., 2000a) (M.M.K. and B.Q., unpublished). The hypothesis that syndapins oligomerize is furthermore supported by yeast two-hybrid studies showing that all isoforms can interact with each other (Modregger et al., 2000) and by in vitro work including gel filtration and surface plasmon resonance analyses of the syndapin-related chicken focal adhesion protein FAP52 (Nikki et al., 2002b). As the SH3 domain of syndapins is not involved, such oligomerization could create a multivalent platform to which different interaction partners of the syndapin SH3 domain are connected (Figs 1 and 2).
Crosstalk between syndapins and signalling pathways
Syndapins interact with several signalling molecules downstream of activated membrane receptors. These include the mammalian Sos (for `son-of-sevenless') protein (Qualmann et al., 2000a; Wasiak et al., 2001), which acts as a guanine nucleotide exchange factor (GEF) for the small GTPases Ras and Rac (Scita et al., 1999) (Fig. 1). Ras is involved in growth factor signalling, whereas the most prominent role of Rac is the regulation of actin cytoskeleton dynamics. The interaction between mSos and syndapin is direct and relies on an intact SH3 domain. Coimmunoprecipitation and colocalization studies indicate that syndapin and Sos interact in vivo (Qualmann et al., 2000a; Wasiak et al., 2001).
Syndapins also bind to the cytoplasmic tails of members of the ADAM family of metalloprotease disintegrins in vitro (Cousin et al., 2000; Howard et al., 1999; Mori et al., 2003). ADAM proteins are transmembrane proteins involved in cell-cell communication and proteolytic ectodomain shedding, which is required for a variety of developmental and maturation processes (Schlöndorff and Blobel, 1999; McFarlane, 2003) (Fig. 1). Developmental alterations induced by overexpression of ADAM13 can be rescued by cooverexpression of the syndapin II isoform X-PACSIN2 in Xenopus (Cousin et al., 2000). This suggests some form of negative regulation of ADAMs by syndapins. However, Mori et al. have shown that syndapin III overexpression increases the ectodomain shedding of heparin-binding epidermal growth factor-like growth factor (HB-EGF) and that knocking down syndapin III by RNA interference partially attenuates this proteolysis, which is thought to involve ADAM12 (Mori et al., 2003).
Much less is known about the interaction of the SH3 domain of syndapins with the proline-rich cytoplasmic portion of the CD95/Fas/Apo-1 ligand CD95L (Ghadimi et al., 2002). CD95L is a 40 kDa type II transmembrane receptor that belongs to the tumour necrosis factor (TNF) family of death factors and induces apoptosis through the cell death receptor CD95 but has also been described as costimulatory receptor for T-cell activation in mice in vivo (Janssen et al., 2003).
It is tempting to speculate not only that syndapins do interact with proteins involved in different signal transduction processes but also that syndapin function is controlled by signalling cascades, because syndapins contain phosphorylation sites for protein kinase C and casein kinase 2. Recombinant mouse syndapin I can be phosphorylated by these kinases in vitro (Plomann et al., 1998). Furthermore, an as-yet-uncharacterized signalling pathway regulated by inositol hexakisphosphate (InsP6) leads to the phosphorylation of syndapin I – a modification that seems to increase the association of glutathione-S-transferase (GST)-syndapin I with dynamin by a factor of 2-3 in vitro (Hilton et al., 2001).
The above observations represent promising starting points for studying the crosstalk of syndapins with different signalling pathways. It will be exciting to examine whether and to what extent the interaction of syndapins with mSos correlates with the endocytic and cytoskeletal functions of syndapins and to unravel the molecular details and the physiological relevance of their interactions with the signalling molecules mentioned.
Syndapin isoforms and their evolution
Database analyses reveal that syndapins only exist in multicellular animals. Plants do not seem to contain syndapins; neither do single-celled eukaryotes such as Dictyostelium discoideum and the different yeasts. On the basis of all currently available sequence information, it seems that the appearance of syndapins correlates with the arrival of coelomata, which are characterized by the presence of a body cavity (coelom) and a gut system that spans the body from one pole (mouth) to the other (anus). Other new structures that appeared at this stage were blood vessels, nephridiae and the brain. It remains unclear whether porifera, cnidaria and/or ctenophora contain syndapins because sequence data for these animals are generally not available; however, lower worms are known to possess a syndapin gene (Fig. 3). We have identified (partial) syndapin-related sequences in the most primitive plathelminthes, the turbellaria (not included in Fig. 3), as well as parasitic band worms (Echinococcus). We have also identified syndapins in roundworms, such as the nematode Caenorhabditis elegans (Fig. 3). Because the genomes of C. elegans and Drosophila melanogaster contain a single syndapin gene, this seems a general property of protostomia.
By contrast, deuterostomia, which ultimately gave rise to the vertebrates, possess several syndapin isoforms. The gene duplications must have occurred as much as 400-440 million years ago because lower vertebrates contain all three syndapin genes known in mammalia. Our phylogenetic analyses suggest the following evolutionary sequence: first, a syndapin I/II ancestor duplicated to give rise to syndapin III; then the former duplicated again (Fig. 3). In line with this, we have found as-yet-undescribed syndapin I, II and III sequences not only in mammals but also in the segregated branch of higher vertebrates formed by birds and reptiles. We have identified all three isoforms in Gallus gallus as well as in Xenopus laevis (Fig. 3).
Interestingly, in fugu and zebra fish, we have identified five syndapin genes. Sequences from other fish support the existence of the two additional groups of syndapin isoforms. The syndapin V group is clearly at the basis of all vertebrate syndapins. The data from our phylogenetic analysis could be interpreted as indicating loss of these most ancient syndapins during the evolution of higher chordata and of life on land (Fig. 3). The group of syndapin IV genes currently contains only two sequences suitable for phylogenetic analysis. We have termed the partial Danio rerio syndapin sequence put in isoform group I (shown in grey in Fig. 3) syndapin IV because alignments and phylogenetic analyses with the entire sequence suggest it to be a member of this subgroup. Further sequence data will be required for a firmer analysis of the nature and phylogenetic relationships between syndapin IV isoforms.
The picture is further complicated by the fact that not only do different syndapin isoforms exist but also they are alternatively spliced. As in the case of the rat syndapin II isoforms (Qualmann and Kelly, 2000), alternative splicing is also evident from database sequences of the other isoforms in all vertebrates. (Because this predominantly affects the flexible region preceding the SH3 domain, this region was excluded from the sequence alignments used to generate the phylogenetic analysis shown in Fig. 3.)
The evolution of different isoforms and splice variants might reflect a need for differential regulation and/or differential affinities for interacting molecules, but none of this has been studied in detail yet. Our current knowledge is therefore largely restricted to the differential distribution of syndapins in mammalian tissues (Table 1).
The syndapin III isoform is the least characterized and seems mainly to occur in skeletal muscle and heart (Table 1). In differentiated C2F3 myotubes, syndapin III has a cytosolic immunolabelling pattern (Modregger et al., 2000) (Fig. 4).
The syndapin II isoforms are expressed more ubiquitously (Table 1). Interestingly, the short and long splice variants display different tissue distributions (Qualmann and Kelly, 2000). Xenopus syndapin II proteins are observed as early as the two-cell stage of development. Immunostaining of sectioned embryos reveals expression in all cells of the three germ layers with varying intensities (Cousin et al., 2000).
Syndapin I is mainly restricted to the brain (Table 1). In common with other proteins involved in membrane trafficking, such as clathrin and dynamin, which are present at 10-50-fold higher concentrations in neuronal cells compared with non-neuronal cells (Morris and Schmid, 1995), syndapin I is detectable at high levels in the (adult) brain and accumulates in synaptic compartments (Plomann et al., 1998; Qualmann et al., 1999; Kessels and Qualmann, 2002; Modregger et al., 2002) (Fig. 4). This might reflect the need for high-capacity and high-speed recycling of synaptic vesicles in neurons, which might be facilitated by the coupling of membrane trafficking to the cytoskeleton by syndapins (Gundelfinger et al., 2003).
Such coupling might also be highly important in non-neuronal, regulated secretory cells. Lacrimal acini cells, for example, are the principal source of tear proteins that are released into nascent tear fluid at the apical plasma membrane. These cells perform massive exocytosis and compensatory endocytosis. Membrane trafficking processes in polarized cells like these must be tightly controlled in order to generate and maintain polarity. Endocytosis and exocytosis at the apical surface of many epithelial cells has to occur within an elaborate cortical actin network. Interference with syndapin interactions by introduction of the syndapin I or syndapin II SH3 domain significantly increases the F-actin content of these cells (da Costa et al., 2003) and blocks endocytosis at the stage of clathrin-coated pit formation. The result is a remarkable accumulation of components of the endocytic machinery at the apical plasma membrane and an increase in the number of clathrin-coated structures. Both phenotypes depend on the Arp2/3 complex, which suggests that the endocytosis block is caused by extensive actin polymerization elicited by the syndapin SH3 domains (da Costa et al., 2003). The syndapin isoform expressed in these specialized secretory cells is mainly the long version of syndapin II. Syndapin II but not syndapin I accumulates together with other endocytic components, such as clathrin and AP2, at the apical plasma membrane when these cells are stimulated by secretagogues, such as carbachol, or incubated with cytoskeletal toxins (da Costa et al., 2003) (Fig. 4). This suggests that the syndapin II isoform preferentially participates in apical endocytosis.
A further isoform-specific function might exist for the SH3-domain-mediated interaction with the huntingtin protein. This protein, which contains extended polyglutamine stretches in patients with Huntington's disease, binds directly to the brain-specific syndapin I isoform in vitro but not to the other two isoforms (Modregger et al., 2002). If this occurs in vivo, huntingtin would be the first SH3-domain-binding partner of syndapins that is specific for one isoform. Interestingly, the presence of an extended polyglutamine stretch in the huntingtin protein seems to enhance the binding of syndapin I in yeast two-hybrid analyses. Biochemical fractionation and immunocytochemical analysis has suggested that syndapin is relocalized in tissue from one Huntington's disease patient (Modregger et al., 2002). Huntingtin can associate with a plethora of factors involved in clathrin-mediated endocytosis, including α-adaptin, huntingtin-interacting protein 1 (HIP1), HIP1-related protein (HIP1R), endophilin and syndapin. All of these interactions are modulated by the length of the polyglutamine repeat (Harjes and Wanker, 2003). It thus seems possible that defects in membrane trafficking triggered by extended polyglutamine stretches participate in the pathology of Huntington's disease.
Finally, syndapins have very recently been shown to interact with EHD (eps15-homology domain) proteins (Braun et al., 2004), which are implicated in endocytic vesicle formation and/or recycling (Grant et al., 2001; Lin et al., 2001; Guilherme et al., 2004). The interaction is mediated by the syndapin NPF motifs and the highly conserved EH domain present in all four EHD isoforms (Braun et al., 2004). All syndapin III proteins known thus far (Fig. 3) lack NPFs; the interaction is therefore specific for the phylogenetically younger syndapin I and II isoforms.
Coordination of the cytoskeleton with membrane trafficking processes at various sites within cells is a complex task that is probably very important for cellular organization and for the function of individual cells within the context of complex tissues and organs. Studying syndapins as molecular components that can link the actin cytoskeleton with vesicle formation processes at the plasma membrane and at the Golgi apparatus will continue to provide us with a valuable research avenue that leads to a deeper understanding of the individual processes, their interconnection and the physiological processes within multicellular organisms that rely on them.
Syndapin functions are probably similar to those of other molecular links between actin and membrane trafficking, such as the F-actin-binding proteins Abp1 and cortactin. In common with syndapins, Abp1 and cortactin associate with dynamin and thus appear to play a role in endocytosis (Kessels et al., 2001; Mise-Omata et al., 2003; Cao et al., 2003). A plausible hypothesis is that the three proteins work together in sequential steps of a self-accelerating process of actin polymerization at sites of endocytosis. Syndapins seem to couple de novo actin nucleation at the vesicle membrane to the dynamin-mediated vesicle fission step by activating the Arp2/3 complex activator N-WASP, which has been localized to actin-membrane interfaces. By contrast, cortactin has the ability to activate the Arp2/3 complex directly (Olazabal and Machesky, 2001). As a starting point for filament polymerization, cortactin might use existing actin fibres, such as those created by syndapin, N-WASP and the Arp2/3 complex at the vesicle surface. This would create branched dynamic actin structures that could be coupled to the vesicle fission machinery by both cortactin and Abp1 (Orth and McNiven, 2003). Indeed, the endocytic role of Abp1 depends on its ability to bind to F-actin (Kessels et al., 2001). Furthermore, Abp1 and cortactin might help organize actin tails to propel vesicles.
To identify how syndapins coordinate actin nucleation with the fission event, it will be important to dissect syndapin regulation and to study the interactions with the signalling components in more detail. Furthermore, studies comparing various syndapins in terms of (different) sets of interacting proteins, their means of regulation and their functions in different cell systems should shed light on the functions of individual syndapin isoforms and splice variants. This might reveal how the cellular functions of syndapins can be fine-tuned and adapted to cope with the different needs of various cell types. Comparing the specialized roles of the individual syndapins in different cell types and studying gene-knockout models, particularly in worms and insects, which posses only a single syndapin gene, will also highlight common principles in vesicle trafficking from different cellular membranes such as the plasma membrane and Golgi membranes.
We thank Sarah Hamm-Alvarez and Markus Plomann for their immunofluorescence images, and we apologize to those whose work could not be cited and covered in more detail because of space limitations. This work was supported by fellowships from the Deutsche Forschungsgemeinschaft (Qu116/2-3; Qu116/3-1) and the Kultusministerium Land Sachsen-Anhalt (LSA 3451A/0502M).
- © The Company of Biologists Limited 2004