Accessory factors in clathrin-dependent synaptic vesicle endocytosis

Protein-lipid interactions and phosphoinositide metabolism in membrane traffic: insights from vesicle recycling in nerve terminals

Great progress has been made in the elucidation of the function of proteins in membrane traffic. Less is known about the regulatory role of lipids in membrane dynamics. Studies of nerve terminals, compartments highly specialized for the recycling of synaptic vesicles, have converged with studies from other systems to reveal mechanisms in protein-lipid interactions that affect membrane shape as well as the fusion and fission of vesicles. Phosphoinositides have emerged as major regulators of the binding of cytosolic proteins to the bilayer. Phosphorylation on different positions of the inositol ring generates different isomers that are heterogeneously distributed on cell membranes and that together with membrane proteins generate a "dual keys" code for the recruitment of cytosolic proteins. This code helps controlling vectoriality of membrane transport. Powerful methods for the detection of lipids are rapidly advancing this field, thus complementing the broad range of information about biological systems that can be obtained from genomic and proteomic approaches.

Predicting pathway perturbations in Down syndrome

Comparative annotation of human chromosome 21 genomic sequence with homologous regions of mouse chromosomes 16, 17 and 10 has identified 170 orthologous gene pairs. Functional annotation of these genes, based on literature reports and computationally-derived predictions, shows that a broad range of cellular processes are represented. A goal of Down syndrome research is to determine which of these processes are perturbed by overexpression of chromosome 21 genes, and which may, therefore, contribute to the cognitive deficits that characterize Down syndrome. Eleven chromosome 21 genes are annotated to interact with or be affected by components of the MAP Kinase pathway and eight are involved in Ca2+/calcineurin signaling. Both pathways are critical for normal neurological function, and consequently their perturbations are proposed as candidates for phenotypic relevance. We present evidence suggesting that the MAP Kinase pathway is perturbed in the Ts65Dn mouse model of Down syndrome at 4-6 months of age. Analysis is complicated by the observation that overexpression of chromosome 21 genes in trisomy may be affected by method of detection, organism, tissue or brain region, and/or developmental age.

Inhibitory Role of Endophilin 3 in Receptor-mediated Endocytosis

Endophilin 1 (Endo1) participates in synaptic vesicle biogenesis through interactions of its Src homology 3 domain with the polyphosphoinositide phosphatase Synaptojanin and the GTPase Dynamin. Endo1 has also been reported to affect endocytosis by converting membrane curvature via its lysophosphatidic acid acyltransferase activity. Here we report that a closely related isoform of Endo1, Endo3, inhibits clathrin-mediated endocytosis. Mutational analyses showed that the variable region of Endo3 is important in regulating transferrin endocytosis. In the brain, Endo3 is co-localized with dopamine D2 receptor in olfactory nerve terminals and inhibits its clathrin-mediated endocytosis in COS-7 cells. Furthermore, overexpression of Endo3 in an olfactory epithelium-derived cell line suppressed dopamine D2 receptor-mediated endocytosis and therefore accelerated its dopamine-induced differentiation. These results indicate that Endo3 may act as a negative regulator of clathrin-mediated endocytosis in brain neurons.

Monitoring clathrin-mediated endocytosis during synaptic activity

To visualize clathrin redistribution during endocytosis in hippocampal boutons, we used a fusion protein of clathrin light chain with enhanced green fluorescent protein. Both high potassium and electric field stimulation lead after a stimulus-dependent delay to a transient increase of fluorescence in synapses, but a slight and transient decrease in adjacent axonal segments. We conclude that the rise and fall of the signal in boutons, with decay kinetics remarkably similar to previous estimates of the endocytic time course, reflects coat assembly and disassembly. Thus, we could selectively measure clathrin-mediated endocytosis and separate its kinetics from other modes of membrane retrieval in CNS synapses. A long-lasting delay preceding the fluorescent transients shows that endocytosis during the first few seconds of continuing stimulation cannot be mediated by newly formed clathrin-coated pits. Therefore, a fast mode of endocytosis is either clathrin-independent or involves preassembled (easily retrievable) clathrin lattices at sites of endocytosis.

Domains of the TGN: Coats, Tethers and G Proteins

The trans-Golgi network is the major sorting compartment of the secretory pathway for protein, lipid and membrane traffic. There is a constant flow of membrane and cargo to and from this compartment. Evidence is emerging that the trans-Golgi network has multiple biochemically and functionally distinct subdomains, each of which contributes to the combined sorting and transport requirements of this dynamic compartment. The recruitment of distinct arrays of protein complexes to trans-Golgi network membranes is likely to produce the diversity of structure and biochemistry observed amongst subdomains that serve to generate different carriers or maintain resident trans-Golgi network components. This review discusses how these subdomains may be formed and examines the molecular players involved, including G proteins, clathrin adaptors and golgin tethers. Diversity within these protein families is highlighted and shown to be critical for the functionality of the trans-Golgi network, as a mediator of protein sorting and membrane transport, and for the maintenance of Golgi structure.

Endocytic recycling

After endocytosis, most membrane proteins and lipids return to the cell surface, but some membrane components are delivered to late endosomes or the Golgi. We now understand that the pathways taken by internalized molecules that eventually recycle to the cell surface can be surprisingly complex and can involve a series of sorting events that occur in several organelles. The molecular basis for many of these sorting processes is only partly understood.

SNAP-23 functions in docking/fusion of granules at low Ca2+

Ca(2+)-triggered exocytosis of secretory granules mediates the release of hormones from endocrine cells and neurons. The plasma membrane protein synaptosome-associated protein of 25 kDa (SNAP-25) is thought to be a key component of the membrane fusion apparatus that mediates exocytosis in neurons. Recently, homologues of SNAP-25 have been identified, including SNAP-23, which is expressed in many tissues, albeit at different levels. At present, little is known concerning functional differences among members of this family of proteins. Using an in vitro assay, we show here that SNAP-25 and SNAP-23 mediate the docking of secretory granules with the plasma membrane at high (1 microM) and low (100 nM) Ca(2+) levels, respectively, by interacting with different members of the synaptotagmin family. In intact endocrine cells, expression of exogenous SNAP-23 leads to high levels of hormone secretion under basal conditions. Thus, the relative expression levels of SNAP-25 and SNAP-23 might control the mode (regulated vs. basal) of granule release by forming docking complexes at different Ca(2+) thresholds.

Identification and functional characterization of Arabidopsis AP180, a binding partner of plant alphaC-adaptin

Clathrin-mediated endocytosis is a well-studied uptake mechanism for nutrients and signalling receptors in mammalian cells that depends on the coordinated interaction of coat proteins and endocytic network proteins to perform the internalization. In this process AP180 promotes the assembly of clathrin triskelia into coated membrane patches at the plasma membrane, while alpha-adaptin interacts with various network proteins that are in turn required for the budding of the coated pits. The process of clathrin-mediated endocytosis in plants has not been dissected at the molecular level, nor have the members of an analogous uptake machinery been functionally described. In this respect, we have investigated the AP180 and alpha-adaptin orthologs from Arabidopsis thaliana: At-AP180 and At-alphaC-Ad. Both plant proteins display the same structural features as their mammalian counterparts and fulfill the same basic functions. To identify their interacting partners, the ear region of At-alphaC-Ad and the C-terminal region of At-AP180 were used as fusion proteins in pull-down experiments and plasmon-resonance measurements. At-alphaC-Ad binds several mammalian endocytic proteins, and its interaction with At-AP180 requires the DPF motif. At-AP180 functions as a clathrin assembly protein that promotes the formation of cages with an almost uniform size distribution. Deletion of the single DLL motif abolished the assembly activity of At-AP180 almost completely, but did not affect its binding to triskelia, suggesting the existence of additional binding determinants.

Internalization at glutamatergic synapses during development

Glutamate receptors are internalized from the cell membrane via clathrin-coated pits. However, little is known about where this occurs - whether at or near the synapse or at some distance from it. In this study we used immunogold localization in the rat brain (mainly hippocampus) to show that clathrin-coated pits are found both at the edge of the synaptic active zone and at further postsynaptic distances, including on the sides of the spine; we also localize these pits specifically to glutamatergic synapses. In addition, we show that clathrin-coated pits can internalize both N-methyl-d-aspartate (in vivo) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (in vitro data only) receptors at extrasynaptic sites not associated directly with synapses. Also, caveolin might be prevalent at excitatory synapses, although it is not known whether it is involved in receptor internalization, receptor stabilization, or some other function.

Tandem MS analysis of brain clathrin-coated vesicles reveals their critical involvement in synaptic vesicle recycling

Tandem MS has identified 209 proteins of clathrin-coated vesicles (CCVs) isolated from rat brain. An overwhelming abundance of peptides were assigned to the clathrin coat with a 1:1 stoichiometry observed for clathrin heavy and light chains and a 2:1 stoichiometry of clathrin heavy chain with clathrin adaptor protein heterotetramers. Thirty-two proteins representing many of the known components of synaptic vesicles (SVs) were identified, supporting that a main function for brain CCVs is to recapture SVs after exocytosis. A ratio of vesicle-N-ethylmaleimide-sensitive factor attachment protein receptors to target-N-ethylmaleimide-sensitive factor attachment protein receptors, similar to that previously detected on SVs, supports a single-step model for SV sorting during CCV-mediated recycling of SVs. The uncovering of eight previously undescribed proteins, four of which have to date been linked to clathrin-mediated trafficking, further attests to the value of the current organelle-based proteomics strategy.

Effects of wortmannin and latrunculin A on slow endocytosis at the frog neuromuscular junction

Phosphoinositides are key regulators of synaptic vesicle cycling and endocytic traffic; the actin cytoskeleton also seems to be involved in modulating these processes. We investigated the effects of perturbing phosphoinositide signalling and actin dynamics on vesicle cycling in frog motor nerve terminals, using fluorescence and electron microscopy, and electrophysiology. Antibody staining for beta-actin revealed that actin surrounds but does not overlap with synaptic vesicle clusters. Latrunculin A, which disrupts actin filaments by binding actin monomers, and wortmannin, an inhibitor of phosphatidyl inositol-3-kinase (PI3-kinase), each disrupted the pattern of presynaptic actin staining, but not vesicle clusters in resting terminals. Latrunculin A, but not wortmannin, also reduced vesicle mobilization and exocytosis. Both drugs inhibited the stimulation-induced uptake of the styryl dye FM1-43 and blocked vesicle reformation from internalized membrane objects after tetanic stimulation. These results are consistent with a role of PI3-kinase and the actin cytoskeleton in the slow pathway of vesicle endocytosis, used primarily by reserve pool vesicles.

Streamlined Synaptic Vesicle Cycle in Cone Photoreceptor Terminals

Cone photoreceptors tonically release neurotransmitter in the dark through a continuous cycle of exocytosis and endocytosis. Here, using the synaptic vesicle marker FM1-43, we elucidate specialized features of the vesicle cycle. Unlike retinal bipolar cell terminals, where stimulation triggers bulk membrane retrieval, cone terminals appear to exclusively endocytose small vesicles. These retain their integrity until exocytosis, without pooling their membranes in endosomes. Endocytosed vesicles rapidly disperse through the terminal and are reused with no apparent delay. Unlike other synapses where most vesicles are immobilized and held in reserve, only a small fraction (<15%) becomes immobilized in cones. Photobleaching experiments suggest that vesicles move by diffusion and not by molecular motors on the cytoskeleton and that vesicle movement is not rate limiting for release. The huge reservoir of vesicles that move rapidly throughout cone terminals and the lack of a reserve pool are unique features, providing cones with a steady supply for continuous release.

Analysis of clathrin-mediated endocytosis of epidermal growth factor receptor by RNA interference

To identify proteins that participate in clathrin-mediated endocytosis of the epidermal growth factor receptor (EGFR), 13 endocytic proteins were depleted in HeLa cells using highly efficient small interfering RNAs that were designed using a novel selection algorithm. The effects of small interfering RNAs on the ligand-induced endocytosis of EGFR were compared with those effects on the constitutive internalization of the transferrin receptor. The knock-downs of clathrin heavy chain and dynamin produced maximal inhibitory effects on the internalization of both receptors. Depletion of alpha, beta2, or micro2 subunits of AP-2 reduced EGF and transferrin internalization rates by 40-60%. Down-regulation of several accessory proteins individually had no effect on endocytosis but caused significant inhibition of EGF and transferrin endocytosis when the homologous proteins were depleted simultaneously. Surprisingly, knockdown of clathrin-assembly lymphoid myeloid leukemia protein, CALM, did not influence transferrin endocytosis but considerably affected EGFR internalization. Thus, CALM is the second protein besides Grb2 that appears to play a specific role in EGFR endocytosis. This study demonstrates that the efficient gene silencing by rationally designed small interfering RNA can be used as an approach to functionally analyze the entire cellular machineries, such as the clathrin-coated pits and vesicles.

Two distinct interaction motifs in amphiphysin bind two independent sites on the clathrin terminal domain β-propeller

During the assembly of clathrin-coated vesicles, many peripheral membrane proteins, including the amphiphysins, use LLDLD-type clathrin-box motifs to interact with the N-terminal beta-propeller domain (TD) of clathrin. The 2.3 A-resolution structure of the clathrin TD in complex with a TLPWDLWTT peptide from amphiphysin 1 delineates a second clathrin-binding motif, PWXXW (the W box), that binds at a site on the TD remote from the clathrin box-binding site. The presence of both sequence motifs within the unstructured region of the amphiphysins allows them to bind more tightly to free TDs than do other endocytic proteins that contain only clathrin-box motifs. This property, along with the propensity of the N-terminal BAR domain to bind curved membranes, will preferentially localize amphiphysin and its partner, dynamin, to the periphery of invaginated clathrin lattices.

The GGA proteins: adaptors on the move

The GGA proteins are a family of ubiquitously expressed, Arf-dependent clathrin adaptors that mediate the sorting of mannose-6-phosphate receptors between the trans-Golgi network and endosomes. Recent studies have elucidated the biochemical and structural bases for the interaction of the GGA proteins with many binding partners, and have shed light on the molecular and cellular mechanisms by which the GGA proteins participate in protein sorting.

Role of Amphiphysin II in Somatostatin Receptor Trafficking in Neuroendocrine Cells

Amphiphysins are SH3 domain-containing proteins thought to function in clathrin-mediated endocytosis. To investigate the potential role of amphiphysin II in cellular trafficking of G protein-coupled somatostatin (SRIF) receptors, we generated an AtT-20 cell line stably overexpressing amphiphysin IIb, a splice variant that does not bind clathrin. Endocytosis of (125)I-[d-Trp(8)]SRIF was not affected by amphiphysin IIb overexpression. However, the maximal binding capacity (B(max)) of the ligand on intact cells was significantly lower in amphiphysin IIb overexpressing than in non-transfected cells. This difference was no longer apparent when the experiments were performed on crude cell homogenates, suggesting that amphiphysin IIb overexpression interferes with SRIF receptor targeting to the cell surface and not with receptor synthesis. Accordingly, immunofluorescence experiments demonstrated that, in amphiphysin overexpressing cells, sst(2A) and sst(5) receptors were segregated in a juxtanuclear compartment identified as the trans-Golgi network. Amphiphysin IIb overexpression had no effect on corticotrophin-releasing factor 41-stimulated adrenocorticotropic hormone secretion, suggesting that it is not involved in the regulated secretory pathway. Taken together, these results suggest that amphiphysin II is not necessary for SRIF receptor endocytosis but is critical for its constitutive targeting to the plasma membrane. Therefore, amphiphysin IIb may be an important component of the constitutive secretory pathway.

Functional dissection of the interactions of stonin 2 with the adaptor complex AP-2 and synaptotagmin

Synaptic vesicle recycling is in part mediated by clathrin-mediated endocytosis. This process involves the coordinated assembly of clathrin and adaptor proteins and the concomitant selection of cargo proteins. Here, we demonstrate that the endocytotic protein stonin 2 localizes to axonal vesicle clusters through its micro-homology domain. Interaction of this domain with synaptotagmin I is sufficient to recruit stonin 2 to the plasmalemma. The N-terminal domain of stonin 2 harbors multiple AP-2-interaction motifs that bind to the clathrin adaptor complex AP-2. These motifs with the consensus sequence WVxF are capable of binding to the alpha-adaptin ear domain and to micro2. Mutation of the tyrosine motif-binding pocket of micro2 abolishes recognition of the WVxF peptide, suggesting that association with stonin 2 renders AP-2 incompetent to sort tyrosine motif-containing cargo proteins. We hypothesize that stonin 2 may function as an AP-2-dependent sorting adaptor for synaptic vesicle recycling.

Endophilin is required for synaptic vesicle endocytosis by localizing synaptojanin

Endophilin is a membrane-associated protein required for endocytosis of synaptic vesicles. Two models have been proposed for endophilin: that it alters lipid composition in order to shape membranes during endocytosis, or that it binds the polyphosphoinositide phosphatase synaptojanin and recruits this phosphatase to membranes. In this study, we demonstrate that the unc-57 gene encodes the Caenorhabditis elegans ortholog of endophilin A. We demonstrate that endophilin is required in C. elegans for synaptic vesicle recycling. Furthermore, the defects observed in endophilin mutants closely resemble those observed in synaptojanin mutants. The electrophysiological phenotype of endophilin and synaptojanin double mutants are virtually identical to the single mutants, demonstrating that endophilin and synaptojanin function in the same pathway. Finally, endophilin is required to stabilize expression of synaptojanin at the synapse. These data suggest that endophilin is an adaptor protein required to localize and stabilize synaptojanin at membranes during synaptic vesicle recycling.

Synaptojanin is recruited by endophilin to promote synaptic vesicle uncoating

We describe the isolation and characterization of Drosophila synaptojanin (synj) mutants. synj encodes a phosphatidylinositol phosphatase involved in clathrin-mediated endocytosis. We show that Synj is specifically localized to presynaptic terminals and is associated with synaptic vesicles. The electrophysiological and ultrastructural defects observed in synj mutants are strikingly similar to those found in endophilin mutants, and Synj and Endo colocalize and interact biochemically. Moreover, synj; endo double mutant synaptic terminals exhibit properties that are very similar to terminals of each single mutant, and overexpression of Endophilin can partially rescue the functional defects in partial loss-of-function synj mutants. Interestingly, Synj is mislocalized and destabilized at synapses devoid of Endophilin, suggesting that Endophilin recruits and stabilizes Synj on newly formed vesicles to promote vesicle uncoating. Our data also provide further evidence that kiss-and-run is able to maintain neurotransmitter release when synapses are not extensively challenged.

Endophilin and synaptojanin hook up to promote synaptic vesicle endocytosis

Clathrin-mediated endocytosis of synaptic vesicles requires molecular rearrangements of proteins as well as lipids. In this issue of Neuron, Schuske et al. and Verstreken et al. show that the lipid-modifying enzyme endophilin recruits and stabilizes the polyphosphoinositide phosphatase synaptojanin at nerve terminals. This remarkable pairing of two enzymatic activities promotes multiple steps of clathrin-mediated endocytosis of synaptic vesicles.

Cardiac steroids induce changes in recycling of the plasma membrane in human NT2 cells

Cardiac steroids (CSs) are specific inhibitors of Na+, K(+)-ATPase activity. Although the presence of CS-like compounds in animal tissues has been established, their physiological role is not evident. In the present study, treatment of human NT2 cells with physiological concentrations (nanomolar) of CSs caused the accumulation of large vesicles adjacent to the nucleus. Experiments using N-(3-triethylammonium propyl)-4-(dibutilamino)styryl-pyrodinum dibromide, transferrin, low-density lipoprotein, and selected anti-transferrin receptor and Rab protein antibodies revealed that CSs induced changes in endocytosis-dependent membrane traffic. Our data indicate that the CS-induced accumulation of cytoplasmic membrane components is a result of inhibited recycling within the late endocytic pathway. Furthermore, our results support the notion that the CS-induced changes in membrane traffic is mediated by the Na+, K(+)-ATPase. These phenomena were apparent in NT2 cells at nanomolar concentrations of CSs and were observed also in other human cell lines, pointing to the generality of this phenomenon. Based on these observations, we propose that the endogenous CS-like compounds are physiological regulators of recycling of endocytosed membrane proteins and cargo.

Regulation of synaptojanin 1 by cyclin-dependent kinase 5 at synapses

Synaptojanin 1 is a polyphosphoinositide phosphatase concentrated in presynaptic nerve terminals, where it dephosphorylates a pool of phosphatidylinositol 4,5-bisphosphate implicated in synaptic vesicle recycling. Like other proteins with a role in endocytosis, synaptojanin 1 undergoes constitutive phosphorylation in resting synapses and stimulation-dependent dephosphorylation by calcineurin. Here, we show that cyclin-dependent kinase 5 (Cdk5) phosphorylates synaptojanin 1 and regulates its function both in vitro and in intact synaptosomes. Cdk5 phosphorylation inhibited the inositol 5-phosphatase activity of synaptojanin 1, whereas dephosphorylation by calcineurin stimulated such activity. The activity of synaptojanin 1 was also stimulated by its interaction with endophilin 1, its major binding partner at the synapse. Notably, Cdk5 phosphorylated serine 1144, which is adjacent to the endophilin binding site. Mutation of serine 1144 to aspartic acid to mimic phosphorylation by Cdk5 inhibited the interaction of synaptojanin 1 with endophilin 1. These results suggest that Cdk5 and calcineurin may have an antagonistic role in the regulation of synaptojanin 1 recruitment and activity, and therefore in the regulation of phosphatidylinositol 4,5-bisphosphate turnover at synapses.

Regulation of Endocytosis by Phosphatidylinositol 4,5-Bisphosphate and ENTH Proteins

Clathrin-mediated endocytosis starts by a recruitment of endocytic proteins to the plasma membrane to induce invagination of lipid bilayer and subsequent vesicule formation. The recruitment of these components requires PtdIns(4,5)P2, a phosphoinositide on the plasma membrane. Although it is well known that the synthesis as well as the disruption of this lipid is important, recent studies have revealed the indispensable roles of direct interaction between PtdIns(4,5)P2 and the endocytic machinery. The ENTH domain is a newly found PtdIns(4,5)P2 binding unit conserved among endocytic proteins like epsins, AP180, and the Hip1/Sla2 family. This review focuses on the essential roles of PtdIns(4,5)P2 and its specific binding partner, the ENTH domain, in clathrin-mediated endocytosis.

N-Glycosylation Is Essential for Vesicular Targeting of Synaptotagmin 1

Synaptotagmins 1 and 7 are candidate Ca(2+) sensors for exocytosis localized to synaptic vesicles and plasma membranes, respectively. We now show that the N-terminal intraluminal sequence of synaptotagmin 1, when transplanted onto synaptotagmin 7, redirects synaptotagmin 7 from the plasma membrane to secretory vesicles. Conversely, mutation of the N-terminal N-glycosylation site of synaptotagmin 1 redirects synaptotagmin 1 from vesicles to the plasma membrane. In cultured hippocampal neurons, the plasma membrane-localized mutant of synaptotagmin 1 suppressed the readily releasable pool of synaptic vesicles, whereas wild-type synaptotagmin 1 did not. In addition to the intraluminal N-glycosylation site, the cytoplasmic C(2) domains of synaptotagmin 1 were required for correct targeting but could be functionally replaced by the C(2) domains of synaptotagmin 7. Our data suggest that the intravesicular N-glycosylation site of synaptotagmin 1 collaborates with its cytoplasmic C(2) domains in directing synaptotagmin 1 to synaptic vesicles via a novel N-glycosylation-dependent mechanism.

ENTH/ANTH proteins and clathrin-mediated membrane budding

The epsin N-terminal homology (ENTH) domain is an evolutionarily conserved protein module found primarily in proteins that participate in clathrin-mediated endocytosis. Structural analyses and ligand-binding studies have shown that a set of proteins previously designated as harboring an ENTH domain in fact contain a highly similar, yet unique module referred to as an AP180 N-terminal homology (ANTH) domain. ENTH and ANTH (E/ANTH) domains bind both inositol phospholipids and proteins and contribute to the nucleation and formation of clathrin coats on membranes. ENTH domains also function in the development of membrane curvature through lipid remodeling during the formation of clathrin-coated vesicles. E/ANTH-bearing proteins have recently been shown to function with adaptor protein-1 and GGA adaptors at the trans-Golgi network, which suggests that E/ANTH domains are universal components of the machinery for clathrin-mediated membrane budding.

Drosophila amphiphysin functions during synaptic Fasciclin II membrane cycling

Recent studies have revealed that endocytosis and exocytosis of postsynaptic receptors play a major role in the regulation of synaptic function, particularly during long-term potentiation and long-term depression. Interestingly, many of the proteins implicated in exocytosis and endocytosis of synaptic vesicles are also involved in postsynaptic protein cycling. In vertebrates, Amphiphysin is postulated to function during endocytosis in nerve terminals; however, several recent reports using a Drosophila amphiphysin (damph) null mutant have failed to substantiate such a role at fly synapses. In addition, Damph is surprisingly enriched at the postsynapse. Here we used the glutamatergic larval neuromuscular junction to study the synaptic role of Damph. By selectively labeling internal and external pools of the cell adhesion molecule Fasciclin II (FasII), and by using a novel in vivo surface FasII immunocapture protocol, we show that the level of external FasII is decreased in damph mutants although the total level of FasII remains constant. In vivo FasII internalization assays indicate that the reincorporation of FasII molecules into the cell surface is severely inhibited in damph mutants. Moreover, we show that blocking soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) function in postsynaptic muscle cells interferes with FasII exocytosis. These experiments suggest that in Drosophila, Damph functions during SNARE-dependent postsynaptic FasII membrane cycling. This study challenges the notion that synaptic Amphiphysin is involved exclusively in endocytosis and suggests a novel role for this protein in postsynaptic exocytosis.

Endophilin promotes a late step in endocytosis at glial invaginations in Drosophila photoreceptor terminals

Retrieval of synaptic vesicles from the membrane of neurons is crucial to maintain normal rates of neurotransmitter release. Photoreceptor terminals of the fly's eye release neurotransmitter in a tonic manner. They therefore rely heavily on vesicle regeneration. Null mutations in endophilin (endo) block clathrin-mediated endocytosis at the Drosophila neuromuscular junction, where previous analysis of hypomorphic mutations has suggested a function for Endophilin (Endo) before vesicle fission, during membrane bending. Here, at fly photoreceptor synapses, we show that Endo is localized to synaptic vesicles at sites of endocytosis that are glial invaginations called capitate projections, and that when the photoreceptor synapses lack Endo they are impaired in their ability to release neurotransmitter. Detailed ultrastructural analysis of endo null mutant photoreceptor synapses fails to reveal a defect at early stages of vesicle reformation but, instead, reveals an accumulation of clusters of electron-dense, apparently nonfunctional, late endocytotic vesicles. Using dynamin;endo double-mutant photoreceptors, we provide further evidence that ultimately the function of Endophilin is required late in endocytosis, allowing vesicles to progress through the synaptic vesicle cycle.

Signals for sorting of transmembrane proteins to endosomes and lysosomes

Sorting of transmembrane proteins to endosomes and lysosomes is mediated by signals present within the cytosolic domains of the proteins. Most signals consist of short, linear sequences of amino acid residues. Some signals are referred to as tyrosine-based sorting signals and conform to the NPXY or YXXO consensus motifs. Other signals known as dileucine-based signals fit [DE]XXXL[LI] or DXXLL consensus motifs. All of these signals are recognized by components of protein coats peripherally associated with the cytosolic face of membranes. YXXO and [DE]XXXL[LI] signals are recognized with characteristic fine specificity by the adaptor protein (AP) complexes AP-1, AP-2, AP-3, and AP-4, whereas DXXLL signals are recognized by another family of adaptors known as GGAs. Several proteins, including clathrin, AP-2, and Dab2, have been proposed to function as recognition proteins for NPXY signals. YXXO and DXXLL signals bind in an extended conformation to the mu2 subunit of AP-2 and the VHS domain of the GGAs, respectively. Phosphorylation events regulate signal recognition. In addition to peptide motifs, ubiquitination of cytosolic lysine residues also serves as a signal for sorting at various stages of the endosomal-lysosomal system. Conjugated ubiquitin is recognized by UIM, UBA, or UBC domains present within many components of the internalization and lysosomal targeting machinery. This complex array of signals and recognition proteins ensures the dynamic but accurate distribution of transmembrane proteins to different compartments of the endosomal-lysosomal system.

Definition of the consensus motif recognized by gamma-adaptin ear domains

The heterotetrameric adaptor complex 1 (AP-1) and the monomeric Golgi-localized, gamma ear-containing, Arf-binding (GGA) proteins are components of clathrin coats associated with the trans-Golgi network and endosomes. The carboxyl-terminal ear domains (or gamma-adaptin ear (GAE) domains) of two gamma-adaptin subunit isoforms of AP-1 and of the GGAs are structurally similar and bind to a common set of accessory proteins. In this study, we have systematically defined a core tetrapeptide motif PsiG(P/D/E)(Psi/L/M) (where Psi is an aromatic residue), which is responsible for the interactions of accessory proteins with GAE domains. The definition of this motif has allowed us to identify novel GAE-binding partners named NECAP and aftiphilin, which also contain clathrin-binding motifs. These findings shed light on the mechanism of accessory protein recruitment to trans-Golgi network and endosomal clathrin coats.

Heterogeneity of endocytic proteins: distribution of clathrin adaptor proteins in neurons and glia

Clathrin adaptor protein (AP)180 is a synaptic protein that regulates the assembly of clathrin-coated vesicles. Several endocytic proteins including AP2, CALM, and epsin 1 have functions or molecular structures similar to AP180. We determined if AP180 associates with functional synapses in cultured hippocampal neurons. We also compared the expression pattern of AP180 with the other endocytic proteins. The distribution of AP180 corresponded with the synaptic vesicle-associated protein synapsin I, and with functional presynaptic terminals labeled with the styryl dye FM1-43. Synaptic AP2 colocalized with AP180, but the distribution of AP2 was not limited to synapses of neurons and it was also expressed in glia. CLAM and epsin 1 immunoreactivities were also detected in both neurons and glia. Unlike AP180, the neuronal immunoreactivity of CALM was not intense in the synaptic puncta. Epsin 1 immunoreactivity was found in both synaptic and extrasynaptic sites, and its synaptic distribution only partially overlapped with that of AP180. These results support roles for AP180 in synaptic function in neurons. The findings also provide information on the distribution of AP2, CALM, and epsin 1 in cells of the nervous system that suggest different roles for these endocytic proteins in the biology of these cells.

Sorting it out: AP-2 and alternate clathrin adaptors in endocytic cargo selection

The AP-2 adaptor complex is widely viewed as a linchpin molecule in clathrin-mediated endocytosis, simultaneously binding both clathrin and receptors. This dual interaction couples cargo capture with clathrin coat assembly, but it has now been discovered that the association with cargo is tightly regulated. Remarkably, AP-2 is not obligatory for all clathrin-mediated uptake, and several alternate adaptors appear to perform similar sorting and assembly functions at the clathrin bud site.

Platelet-derived growth factor receptor-mediated signal transduction from endosomes

Although accumulated evidence supports the concept of endosomal signaling of receptor tyrosine kinases, most results are generated from studies of epidermal growth factor receptor (EGFR). It is not clear whether the concept of endosomal signaling could be generally applied to the other receptor tyrosine kinases. For example, platelet-derived growth factor receptor (PDGFR) is very similar to EGFR in terms of both signaling and trafficking; however, little is known about the endosomal signaling of PDGFR. In this research, we applied the same approaches from our recent studies regarding EGFR endosomal signaling to investigate the endosomal signaling of PDGFR. We showed in this communication that we are able to establish a system that allows the specific activation of endosome-associated PDGFR without the activation of the plasma membrane-associated PDGFR and without disrupting the overall endocytosis pathway. By using this system, we showed that endosomal activation of PDGFR recruits various signaling proteins including Grb2, SHC, phospholipase C-gamma1, and the p85alpha subunit of phosphatidylinositol 3-kinase into endosomes and forms signaling complexes with PDGFR. We also showed that endosomal PDGFR signaling is sufficient to activate the major signaling pathways implicated in cell proliferation and survival. Moreover, we demonstrate that endosomal PDGFR signaling is sufficient to generate physiological output including cell proliferation and cell survival.

Rapid Ca2+-dependent decrease of protein ubiquitination at synapses

Protein ubiquitination has been implicated in the regulation of axonal growth and synaptic plasticity as well as in the pathogenesis of neurodegenerative diseases. Here we show that depolarization-dependent Ca2+ influx into synaptosomes produces a global, rapid (range of seconds), and reversible decrease of the ubiquitinated state of proteins, which correlates with the Ca2+-dependent dephosphorylation of several synaptic proteins. A similar general decrease in protein ubiquitination was observed in nonneuronal cells on Ca2+ entry induced by ionomycin. Both in synaptosomes and in nonneuronal cells, this decrease was blocked by FK506 (a calcineurin antagonist). Proteins whose ubiquitinated state was decreased include epsin 1, a substrate for the deubiquitinating enzyme fat facets/FAM, which we show here to be concentrated at synapses. These results reveal a fast regulated turnover of protein ubiquitination. In nerve terminals, protein ubiquitination may play a role both in the regulation of synaptic function, including vesicle traffic, and in the coordination of protein turnover with synaptic use.

DEC-205lo Langerinlo neonatal Langerhans' cells preferentially utilize a wortmannin-sensitive, fluid-phase pathway to internalize exogenous antigen

Antigen treatment of neonatal epidermis results in antigen-specific immune suppression. Compared with adult counterparts, neonatal Langerhans' cells (LC) demonstrate an impaired ability to transport antigen to the lymph node (LN). As it is possible that neonatal LC have a reduced ability to endocytose antigen, we evaluated the acquisition of endocytic function, the expression of uptake receptors and the internalization of soluble and small particulate antigens in neonatal, juvenile and adult mice. Although LC from 4-day-old mice were weakly positive for the mannose-type receptor, Langerin, they were capable of internalizing fluorescein isothiocyanate (FITC)-dextran, but to a lesser extent than LC from 6-week-old mice. However, when ratio data were calculated to account for variations in fluorescence intensity at 4 degrees, it was demonstrated that neonatal LC continued to internalize antigen over a longer period of time than adult mice and, as the ratios were much higher, that neonatal cells were also relatively more efficient in antigen uptake. When receptors for mannan and mannose were competitively blocked, LC from neonatal mice, but not adult mice, could still efficiently internalize FITC-dextran. Consequently, the uptake of FITC-dextran, in part, occurred via alternative receptors or a receptor-independent fluid-phase pathway. A feasible pathway is macropinocytosis, as LC from 4-day-old mice demonstrated a reduction in FITC-dextran internalization by the macropinocytosis inhibitor, wortmannin. Evidence of a functional macropinocytosis pathway in neonatal LC was further supported by internalization of the soluble tracer Lucifer Yellow (LY). We conclude that neonatal LC preferentially utilize a wortmannin-sensitive, fluid-phase pathway, rather than receptor-mediated endocytosis, to internalize antigen. As neonatal LC are capable of sampling their environment without inducing immunity, this may serve to avoid inappropriate immune responses during the neonatal period.

Dsl1p, an Essential Component of the Golgi-Endoplasmic Reticulum Retrieval System in Yeast, Uses the Same Sequence Motif to Interact with Different Subunits of the COPI Vesicle Coat

Dsl1p is required for Golgi-endoplasmic reticulum (ER) retrograde transport in yeast. It interacts with the ER resident protein Tip20p and with delta-COP, a subunit of coatomer, the coat complex of COPI vesicles. To test the significance of these interactions, we mapped the different binding sites and created mutant versions of Dsl1p and delta-COP, which are unable to bind directly to each other. Three domains were identified in Dsl1p: a Tip20p binding region within the N-terminal 200 residues, a highly acidic region in the center of Dsl1p containing crucial tryptophan residues that is required for binding to delta-COP and essential for viability, and an evolutionarily well conserved domain at the C terminus. Most importantly, Dsl1p uses the same central acidic domain to interact not only with delta-COP but also with alpha-COP. Strong interaction with alpha-COP requires the presence of comparable amounts of epsilon-COP or beta' -COP. Thus, the binding characteristics of Dsl1p resemble those of many accessory factors of the clathrin coat. They interact with different layers of the vesicle coat by using tandemly arranged sequence motifs, some of which have dual specificity.

Endocytosis at the synaptic terminal

Exocytosis of neurotransmitter from a synaptic vesicle is followed by efficient retrieval of its constituent membrane and proteins. Real-time measurements indicate that fast and slow modes of retrieval operate in parallel at a number of presynaptic terminals. Two mechanisms can be distinguished by electron microscopy: clathrin-mediated retrieval of small vesicles and bulk retrieval of large cisternae. Methods that investigate the behaviour of individual vesicles have recently demonstrated a third route of retrieval: the rapid reversal of a pore-like connection between the vesicle and surface ('kiss-and-run'). Key aims for the future are to identify the molecules underlying different mechanisms of endocytosis at the synapse and the signals that select between them.

Specific localization and timing in neuronal signal transduction mediated by protein-lipid interactions

A large number of signaling proteins translocate from the cytosol to the plasma membrane in response to receptor and electrical stimuli. The site of translocation to the plasma membrane and the "on" and "off" rates of the translocation process are critical for defining the specificity of the signaling response. In addition to targeting mechanisms based on protein-protein interactions, signaling proteins have evolved a large repertoire of covalent lipid modifications and lipid binding protein modules that regulate reversible membrane association. The time constants of these membrane interactions range from milliseconds to several hours. Here we discuss how diversity in lipid-based membrane anchoring and targeting motifs contributes to plasticity in neuronal signaling by providing local and regional control mechanisms as well as a means to transduce and integrate signals over a broad range of different time scales.

Sorting nexin 9 participates in clathrin-mediated endocytosis through interactions with the core components

Sorting nexin 9 (SNX9) belongs to a family of proteins, the sorting nexins, that are characterized by the presence of a subclass of the phosphoinositide-binding phox domain. SNX9 has in its amino terminus a Src homology 3 domain and a region with predicted low complexity followed by a carboxyl-terminal part containing the phox domain. We previously found that SNX9 is one of the major proteins in hematopoietic cells that binds to the alpha and beta2-appendages of adaptor protein complex 2 (AP-2), a protein with a critical role in the formation of clathrin-coated vesicles at the plasma membrane. In the present study we show that clathrin and dynamin-2, two other essential molecules in the endocytic process, also interact with SNX9. We found that both AP-2 and clathrin bind to the low complexity region in SNX9 in a cooperative manner, whereas dynamin-2 binds to the Src homology 3 domain. In the cytosol, SNX9 is present in a 14.5 S complex containing dynamin-2 and an unidentified 41-kDa protein. In HeLa cells, SNX9 co-localized with both AP-2 and dynamin-2 at the plasma membrane or on vesicular structures derived from it but not with the early endosomal marker EEA1 or with AP-1. The results suggest that SNX9 may be recruited together with dynamin-2 and become co-assembled with AP-2 and clathrin at the plasma membrane. Overexpression in both K562 and HeLa cells of truncated forms of SNX9 interfered with the uptake of transferrin, consistent with a role of SNX9 in endocytosis.

Characterization of a γ-adaptin ear-binding motif in enthoprotin

Enthoprotin, a newly identified component of clathrin-coated vesicles, interacts with the trans-Golgi network (TGN) clathrin adapters AP-1 and GGA2. Here we perform a multi-faceted analysis of the site in enthoprotin that is responsible for the binding to the gamma-adaptin ear (gamma-ear) domain of AP-1. Alanine scan mutagenesis and nuclear magnetic resonance (NMR) studies reveal the full extent of the site as well as critical residues for this interaction. NMR studies of the gamma-ear in complex with a synthetic peptide from enthoprotin provide structural details of the binding site for TGN accessory proteins within the gamma-ear.

A putative role for intramolecular regulatory mechanisms in the adaptor function of amphiphysin in endocytosis

Amphiphysin 1 is a brain-specific protein enriched at the synapse and a major binding partner of several components of the clathrin-mediated endocytic machinery (Proc Natl Acad Sci USA 93 (1996) 331). It interacts with clathrin-coat proteins, dynamin, and membranes (Nat Cell Biol 1 (1999) 33; JBC). A role of amphiphysin in synaptic vesicle recycling is supported by both acute and chronic perturbation studies (Science 276 (1997) 259; Neuron 33 (2002) 789). Here we show that amphiphysin directly stimulates clathrin recruitment onto liposomes in an in vitro assay. Amphiphysin-dependent clathrin-coat recruitment is enhanced by the interaction of amphiphysin with dynamin. We also show that the amphiphysin SH3 domain binds full-length amphiphysin, likely via an internal poly-proline region, and that clathrin recruitment onto liposomes by amphiphysin is enhanced in the presence of the isolated amphiphysin SH3 domain. Expression of a mutant amphiphysin harboring two amino acid substitutions in the SH3 domain, and therefore unable to bind proline-containing motifs, induces an accumulation of large intracellular aggregates including amphiphysin, clathrin, AP-2, and other endocytic proteins, as well as a concomitant block of transferrin endocytosis. Thus, putative intramolecular interactions between the amphiphysin COOH-terminal SH3 domain and its internal poly-proline region may regulate clathrin recruitment onto membranes.

Impairing actin filament or syndapin functions promotes accumulation of clathrin-coated vesicles at the apical plasma membrane of acinar epithelial cells

In this article, we investigate the contributions of actin filaments and accessory proteins to apical clathrin-mediated endocytosis in primary rabbit lacrimal acini. Confocal fluorescence and electron microscopy revealed that cytochalasin D promoted apical accumulation of clathrin, alpha-adaptin, dynamin, and F-actin and increased the amounts of coated pits and vesicles at the apical plasma membrane. Sorbitol density gradient analysis of membrane compartments showed that cytochalasin D increased [14C]dextran association with apical membranes from stimulated acini, consistent with functional inhibition of apical endocytosis. Recombinant syndapin SH3 domains interacted with lacrimal acinar dynamin, neuronal Wiskott-Aldrich Syndrome protein (N-WASP), and synaptojanin; their introduction by electroporation elicited remarkable accumulation of clathrin, accessory proteins, and coated pits at the apical plasma membrane. These SH3 domains also significantly (p </= 0.05) increased F-actin, with substantial colocalization of dynamin and N-WASP with the additional filaments. Coelectroporation with the VCA domain of N-WASP blocked the increase in F-actin and reversed the morphological changes indicative of impaired apical endocytosis. We suggest that transient modulation of actin polymerization by syndapins through activation of the Arp2/3 complex via N-WASP coordinates dynamin-mediated vesicle fission at the apical plasma membrane of acinar epithelia. Trapping of assembled F-actin intermediates during this process by cytochalasin D or syndapin SH3 domains impairs endocytosis.

A novel AP-2 adaptor interaction motif initially identified in the long-splice isoform of synaptojanin 1, SJ170

Phosphoinositides play a fundamental role in clathrin-coat assembly at the cell surface. Several endocytic components and accessory factors contain independently folded phosphoinositide-binding modules that facilitate, in part, membrane placement at the bud site. As the clathrin-coat assembly process progresses toward deeply invaginated buds, focally synthesized phosphoinositides are dephosphorylated, principally through the action of the phosphoinositide polyphosphatase synaptojanin 1. Failure to catabolize polyphosphoinositides retards the fission process and endocytic activity. The long-splice isoform of synaptojanin 1, termed SJ170, contains a carboxyl-terminal extension that harbors interaction motifs for engaging several components of the endocytic machinery. Here, we demonstrate that in addition to DPF and FXDXF sequences, the SJ170 carboxyl terminus contains a novel AP-2 binding sequence, the WXXF motif. The WXXF sequence engages the independently folded alpha-subunit appendage that projects off the heterotetrameric AP-2 adaptor core. The endocytic protein kinases AAK1 and GAK also contain functional WXX(FW) motifs in addition to two DPF repeats, whereas stonin 2 harbors three tandem WXXF repeats. Each of the discrete SJ170 adaptor-interaction motifs bind to appendages relatively weakly but, as tandemly arrayed within the SJ170 extension, can cooperate to bind bivalent AP-2 with good apparent affinity. These interactions likely contribute to the appropriate targeting of certain endocytic components to clathrin bud sites assembling at the cell surface.

Identification of a family of endocytic proteins that define a new alpha-adaptin ear-binding motif

Endocytosis by clathrin-coated vesicles (CCVs) is an important mechanism mediating protein internalization. Here, we show that two proteins identified through a proteomics analysis of CCVs are new components of the endocytic machinery. The proteins, named NECAP (adaptin-ear-binding coat-associated protein) 1 and 2, are paralogues that display no sequence similarity or common domains with any known protein. Both are enriched in CCV coats, and further analysis of the brain-enriched isoform, NECAP 1, shows its partial localization to clathrin-coated pits and direct binding to the globular ear domain of the alpha-adaptin subunit (alpha-ear) of the adaptor protein 2 (AP-2) complex. Intriguingly, this interaction is mediated by a new motif, WVQF, that uses a distinct alpha-ear interface relative to known alpha-ear-binding partners. Disruption of this interaction blocks clathrin-mediated endocytosis. Together, our studies identify a new family of endocytic proteins that define a unique AP-2-binding motif.

Identification of a family of endocytic proteins that define a new alpha-adaptin ear-binding motif

Endocytosis by clathrin-coated vesicles (CCVs) is an important mechanism mediating protein internalization. Here, we show that two proteins identified through a proteomics analysis of CCVs are new components of the endocytic machinery. The proteins, named NECAP (adaptin-ear-binding coat-associated protein) 1 and 2, are paralogues that display no sequence similarity or common domains with any known protein. Both are enriched in CCV coats, and further analysis of the brain-enriched isoform, NECAP 1, shows its partial localization to clathrin-coated pits and direct binding to the globular ear domain of the alpha-adaptin subunit (alpha-ear) of the adaptor protein 2 (AP-2) complex. Intriguingly, this interaction is mediated by a new motif, WVQF, that uses a distinct alpha-ear interface relative to known alpha-ear-binding partners. Disruption of this interaction blocks clathrin-mediated endocytosis. Together, our studies identify a new family of endocytic proteins that define a unique AP-2-binding motif.

The AP-2 complex is excluded from the dynamic population of plasma membrane-associated clathrin

Numerous biologically relevant substrates are selectively internalized via clathrin-mediated endocytosis. At the plasma membrane the AP-2 complex plays a major role in clathrin coat formation, interacting with both cargo and clathrin. Utilizing simultaneous dual-channel total internal reflection fluorescence microscopy we have analyzed components of the AP-2 complex (alpha- and beta 2-adaptin) during clathrin-mediated endocytosis. Although in static images enhanced green fluorescent protein-tagged AP-2 markers significantly co-localized with clathrin and other components of clathrin-coated pits, AP-2 did not seem to be present in clathrin spots that appeared to undergo internalization or motility parallel to the plane of the plasma membrane. Two populations of clathrin at the plasma membrane seem to exist, the dynamic and the static, and AP-2 appears to be only found within the latter. These results suggest that colocalized clathrin/AP-2 puncta may represent loci for coated pit production and that previous models that assumed AP-2 was retained within clathrin coats during endocytosis may need to be re-evaluated.

Formation of an Endophilin-Ca2+ Channel Complex Is Critical for Clathrin-Mediated Synaptic Vesicle Endocytosis

A tight balance between synaptic vesicle exocytosis and endocytosis is fundamental to maintaining synaptic structure and function. Calcium influx through voltage-gated Ca2+ channels is crucial in regulating synaptic vesicle exocytosis. However, much less is known about how Ca2+ regulates vesicle endocytosis or how the endocytic machinery becomes enriched at the nerve terminal. We report here a direct interaction between voltage-gated Ca2+ channels and endophilin, a key regulator of clathrin-mediated synaptic vesicle endocytosis. Formation of the endophlin-Ca2+ channel complex is Ca2+ dependent. The primary Ca2+ binding domain resides within endophilin and regulates both endophilin-Ca2+ channel and endophilin-dynamin complexes. Introduction into hippocampal neurons of a dominant-negative endophilin construct, which constitutively binds to Ca2+ channels, significantly reduces endocytosis-mediated uptake of FM 4-64 dye without abolishing exocytosis. These results suggest an important role for Ca2+ channels in coordinating synaptic vesicle recycling by directly coupling to both exocytotic and endocytic machineries.

Tuba, a novel protein containing bin/amphiphysin/Rvs and Dbl homology domains, links dynamin to regulation of the actin cytoskeleton

Tuba is a novel scaffold protein that functions to bring together dynamin with actin regulatory proteins. It is concentrated at synapses in brain and binds dynamin selectively through four N-terminal Src homology-3 (SH3) domains. Tuba binds a variety of actin regulatory proteins, including N-WASP, CR16, WAVE1, WIRE, PIR121, NAP1, and Ena/VASP proteins, via a C-terminal SH3 domain. Direct binding partners include N-WASP and Ena/VASP proteins. Forced targeting of the C-terminal SH3 domain to the mitochondrial surface can promote accumulation of F-actin around mitochondria. A Dbl homology domain present in the middle of Tuba upstream of a Bin/amphiphysin/Rvs (BAR) domain activates Cdc42, but not Rac and Rho, and may thus cooperate with the C terminus of the protein in regulating actin assembly. The BAR domain, a lipid-binding module, may functionally replace the pleckstrin homology domain that typically follows a Dbl homology domain. The properties of Tuba provide new evidence for a close functional link between dynamin, Rho GTPase signaling, and the actin cytoskeleton.