Total internal reflection fluorescence microscopy (TIR-FM) has become a powerful tool for studying clathrin-mediated endocytosis. However, due to difficulties in tracking and quantifying their heterogeneous dynamic behavior, detailed analyses have been restricted to a limited number of selected clathrin-coated pits (CCPs). To identify intermediates in the formation of clathrin-coated vesicles and factors that regulate progression through these stages, we used particle-tracking software and statistical methods to establish an unbiased and complete inventory of all visible CCP trajectories. We identified three dynamically distinct CCP subpopulations: two short-lived subpopulations corresponding to aborted intermediates, and one longer-lived productive subpopulation. In a manner dependent on AP2 adaptor complexes, increasing cargo concentration significantly enhances the maturation efficiency of productive CCPs, but has only minor effects on their lifetimes. In contrast, small interfering RNA (siRNA) depletion of dynamin-2 GTPase and reintroduction of wild-type or mutant dynamin-1 revealed dynamin's role in controlling the turnover of abortive intermediates and the rate of CCP maturation. From these data, we infer the existence of an endocytic restriction or checkpoint, responsive to cargo and regulated by dynamin.

Clathrin-mediated endocytosis in mammalian cells is critical for a variety of cellular processes including nutrient uptake and cell surface receptor down-regulation. Despite the findings that numerous endocytic accessory proteins directly or indirectly regulate actin dynamics and that actin assembly is spatially and temporally coordinated with endocytosis, direct functional evidence for a role of actin during clathrin-coated vesicle formation is lacking. Here, we take parallel biochemical and microscopic approaches to address the contribution of actin polymerization/depolymerization dynamics to clathrin-mediated endocytosis. When measured using live-cell fluorescence microscopy, disruption of the F-actin assembly and disassembly cycle with latrunculin A or jasplakinolide results in near complete cessation of all aspects of clathrin-coated structure (CCS) dynamics. Stage-specific biochemical assays and quantitative fluorescence and electron microscopic analyses establish that F-actin dynamics are required for multiple distinct stages of clathrin-coated vesicle formation, including coated pit formation, constriction, and internalization. In addition, F-actin dynamics are required for observed diverse CCS behaviors, including splitting of CCSs from larger CCSs, merging of CCSs, and lateral mobility on the cell surface. Our results demonstrate a key role for actin during clathrin-mediated endocytosis in mammalian cells. INTRODUCTIONClathrin-mediated endocytosis 1 (CME) is critical for a variety of biological processes such as nutrient uptake, cell surface receptor down-regulation, and synaptic vesicle recycling. CME involves the spatial and temporal coordination of numerous lipid components and multiple accessory proteins (Slepnev and De Camilli, 2000). Structural proteins, including clathrin and adaptor proteins, are recruited from the cell cytosol and assemble on the plasma membrane forming a ϳ200-nm clathrin-coated pit (CCP). The CCP becomes invaginated and narrows at the neck, forming a constricted coated pit which subsequently pinches off from the plasma membrane, generating a nascent clathrin-coated vesicle (CCV). Finally, the cycle is completed by disassembly of the clathrin coat, which is utilized for subsequent rounds of endocytosis.Growing evidence has suggested a role for the actin cytoskeleton during clathrin-mediated endocytosis (Qualmann et al., 2000;Merrifield, 2004). In budding yeast, actin assembly and disassembly dynamics are tightly coupled to and essential for endocytosis (Engqvist-Goldstein and Drubin, 2003). Thus, disruption of the yeast cortical actin cytoskeleton by genetic or chemical means inhibits endocytosis. Similarly, several lines of evidence have suggested a close association between the endocytic machinery in mammalian cells and the actin cytoskeleton. First, the GTPase dynamin, a central player in CME, and several dynaminbinding partners, including cortactin, syndapin, mAbp1, intersectin, and profilin, have been shown to directly or indirectly regulate F-actin dynamics (Qualmann et ...

Multiple modes of endocytosis require actin-dependent remodeling of the plasma membrane; however, neither the factors linking these processes nor their mechanisms of action are understood. The sorting nexin, SNX9, localizes to clathrin-coated pits where it interacts with dynamin and functions in clathrin-mediated endocytosis. Here, we demonstrate that SNX9 also localizes to actin-rich structures implicated in fluid-phase uptake, including tubular membranes containing GPI-anchored proteins and dorsal membrane ruffles. Moreover, we show that SNX9 is critical for dorsal ruffle formation and for clathrin-independent, actin-dependent fluid-phase endocytosis. In vitro, SNX9 directly associates with N-WASP, an Arp2/3 complex activator, and stimulates N-WASP/Arp2/3-mediated actin assembly. SNX9-stimulated actin polymerization is greatly enhanced by PI(4,5)P(2)-containing liposomes, due in part to PI(4,5)P(2)-induced SNX9 oligomerization. These results suggest a mechanism for the spatial and temporal regulation of N-WASP-dependent actin assembly and implicate SNX9 in directly coupling actin dynamics to membrane remodeling during multiple modes of endocytosis.

Dynamin, a central player in clathrin-mediated endocytosis, interacts with several functionally diverse SH3 domaincontaining proteins. However, the role of these interactions with regard to dynamin function is poorly defined. We have investigated a recently identified protein partner of dynamin, SNX9, sorting nexin 9. SNX9 binds directly to both dynamin-1 and dynamin-2. Moreover by stimulating dynamin assembly, SNX9 stimulates dynamin's basal GTPase activity and potentiates assembly-stimulated GTPase activity on liposomes. In fixed cells, we observe that SNX9 partially localizes to clathrin-coated pits. Using total internal reflection fluorescence microscopy in living cells, we detect a transient burst of EGFP-SNX9 recruitment to clathrin-coated pits that occurs during the late stages of vesicle formation and coincides spatially and temporally with a burst of dynamin-mRFP fluorescence. Transferrin internalization is inhibited in HeLa cells after siRNA-mediated knockdown of SNX9. Thus, our results establish that SNX9 is required for efficient clathrin-mediated endocytosis and suggest that it functions to regulate dynamin activity. INTRODUCTIONThe GTPase dynamin plays a critical role in clathrin-mediated endocytosis (CME; Hinshaw, 2000). Dynamin self-assembles into a collar-like structure around the necks of deeply invaginated clathrin-coated pits (CCPs) where it is believed to directly mediate membrane fission and vesicle release. Dynamin is also associated with newly formed coated pits (Damke et al., 1994;Evergren et al., 2004) where it may function to regulate early stages of coated pit maturation (Sever et al., 2000a;Song and Schmid, 2003;Song et al., 2004).Although only one isoform of dynamin exists in Drosophila and in Caenorhabditis elegans, mammals express at least three isoforms, which are ϳ70% identical to each other. Dynamin-1, the first identified protein in the dynamin family, is expressed exclusively in neuronal cells where it functions in synaptic vesicle recycling (Shpetner and Vallee, 1989;Nakata et al., 1991;Sontag et al., 1994). Dynamin-2 is ubiquitously expressed (Cook et al., 1994;Sontag et al., 1994) and localizes to endocytic CCPs where it functions, like dynamin-1, in endocytosis (Damke et al., 1994;Altschuler et al., 1998). However, dynamin-2 may also be involved in vesicle formation at the Golgi Kreitzer et al., 2000), in regulating actin dynamics (Schafer, 2004), in cell signaling (Kranenburg et al., 1999;Fish et al., 2000), and has recently been localized to the centriole (Thompson et al., 2004). Dynamin-3, which is most highly expressed in testis but also detectable in neurons (Gray et al., 2003) and in lung (Nakata et al., 1993), has been less well studied.Dynamins are atypical GTPases, distinguished by their large size, low affinity for GTP, and high intrinsic rates of GTP hydrolysis (Sever et al., 2000b;Song and Schmid, 2003). Moreover, dynamin can self-assemble in solution (Hinshaw and Schmid, 1995) or on liposome templates into rings and spiral-like structures (Stowell et al., 199...

We report the development and characterization of an in vitro system for the formation of filopodia-like bundles. Beads coated with actin-related protein 2/3 (Arp2/3)–activating proteins can induce two distinct types of actin organization in cytoplasmic extracts: (1) comet tails or clouds displaying a dendritic array of actin filaments and (2) stars with filament bundles radiating from the bead. Actin filaments in these bundles, like those in filopodia, are long, unbranched, aligned, uniformly polar, and grow at the barbed end. Like filopodia, star bundles are enriched in fascin and lack Arp2/3 complex and capping protein. Transition from dendritic to bundled organization was induced by depletion of capping protein, and add-back of this protein restored the dendritic mode. Depletion experiments demonstrated that star formation is dependent on Arp2/3 complex. This poses the paradox of how Arp2/3 complex can be involved in the formation of both branched (lamellipodia-like) and unbranched (filopodia-like) actin structures. Using purified proteins, we showed that a small number of components are sufficient for the assembly of filopodia-like bundles: Wiskott-Aldrich syndrome protein (WASP)–coated beads, actin, Arp2/3 complex, and fascin. We propose a model for filopodial formation in which actin filaments of a preexisting dendritic network are elongated by inhibition of capping and subsequently cross-linked into bundles by fascin.