Abstract. Dynarnin is the mammalian homologue to the Drosophila shibire gene product. Mutations in this 100-kD GTPase cause a pleiotropic defect in endocytosis. To further investigate its role, we generated stable HeLa cell lines expressing either wild-type dynamin or a mutant defective in GTP binding and hydrolysis driven by a tightly controlled, tetracyclineinducible promoter. Overexpression of wild-type dynamin had no effect. In contrast, coated pits failed to become constricted and coated vesicles failed to bud in cells overexpressing mutant dynamin so that endocytosis via both transferrin (Tfn) and EGF receptors was potently inhibited. Coated pit assembly, invagination, and the recruitment of receptors into coated pits were unaffected. Other vesicular transport pathways, including Tfn receptor recycling, Tfn receptor biosynthesis, and cathepsin D transport to lysosomes via Golgiderived coated vesicles, were unaffected. Bulk fluidphase uptake also continued at the same initial rates as wild type. EM immunolocalization showed that membrane-bound dynamin was specifically associated with clathrin-coated pits on the plasma membrane. Dynamin was also associated with isolated coated vesicles, suggesting that it plays a role in vesicle budding. Like the Drosophila shibire mutant, HeLa cells overexpressing mutant dynamin accumulated long tubules, many of which remained connected to the plasma membrane. We conclude that dynamin is specifically required for endocytic coated vesicle formarion, and that its GTP binding and hydrolysis activities are required to form constricted coated pits and, subsequently, for coated vesicle budding.

Abstract. The role of human dynamin in receptormediated endocytosis was investigated by transient expression of GTP-binding domain mutants in mammalian cells. Using assays which detect intermediates in coated vesicle formation, the dynamin mutants were found to block endocytosis at a stage after the initiation of coat assembly and preceding the sequestration of ligands into deeply invaginated coated pits. Membrane transport from the ER to the Golgi complex was unaffected indicating that dynamin mutants specifically block early events in endocytosis. These results demonstrate that mutations in the GTP-binding domain of dynamin block Tfn-endocytosis in mammalian cells and suggest that a functional dynamin GTPase is required for receptor-mediated endocytosis via clathrincoated pits.ECEPTOR-mediated endocytosis occurs via the formation of clathrin-coated vesicles and involves coat assembly, receptor recruitment, coated pit invagination, and finally vesicle budding (Smythe and Warren, 1991;Robinson, 1992;Schmid, 1993). The major structural proteins of the coat, clathrin, and AP complexes have been well characterized (Pearse and Robinson, 1990;Keen, 1990;Brodsky, 1988). While proteins that regulate coated vesicle formation have not been identified (Schmid, 1992), a cellfree assay which reconstitutes reeeptor-mediated endocytosis has recently revealed a requirement for multiple GTPbinding proteins (Carter et al., 1993).Dynamin, a GTP-binding protein first isolated from bovine brain tissue (Shpetner and Vallee, 1989;Obar et al., 1990;Scaife and Margolis, 1990;Nakata et al., 1991) is a strong candidate for a GTPase that acts early in endocytosis. Dynamin is a member of a subfamily of structurally related GTP-binding proteins which share considerable sequence homology (43-66 %) in the amino-terminal GTPase domain, but more limited homology (<10-30%) in their COOHterminal regions. Other members of this family (for review see Collins, 1991) include the mammalian Mx-1 related, interferon-inducible proteins which confer viral resistance (Arnheiter and Meier, 1990), MGM1 which plays a role in yeast mitochondrial DNA maintenance (Jones and Fangman, 1992) and VPSlp, a protein involved in vacuolar sort-

Abstract. A stable HeLa cell line expressing a dynamin mutant, dyn ts, exhibits a temperature-sensitive defect in endocytic clathrin-coated vesicle formation. Dyn ts carries a point mutation, G273D, corresponding to the Drosophila shibire tsl allele. The ts-defect in receptormediated endocytosis shows a rapid onset (<5 min) and is readily reversible. At the nonpermissive temperature (38°C) HRP uptake is only partially inhibited. Moreover, when cells are held at the nonpermissive temperature, fluid phase uptake fully recovers to wildtype levels within 30 min, while receptor-mediated endocytosis remains inhibited. The residual HRP uptake early after shift to the nonpermissive temperature and the induced HRP uptake that occurs after recovery are insensitive to cytosol acidification under conditions that potently inhibit receptor-mediated endocytosis of Tfn. Together, these results suggest that a dynamin-and clathrin-independent mechanism contributes to the total constitutive pinocytosis in HeLa cells and that dyn ts cells rapidly and completely compensate for the loss of clathrin-dependent endocytosis by inducing an alternate endocytic pathway.

The GTPase dynamin is essential for receptor-mediated endocytosis, but its function remains controversial. A domain of dynamin, termed the GTPase effector domain (GED), controls dynamin's high stimulated rates of GTP hydrolysis by functioning as an assembly-dependent GAP. Dyn(K694A) and dyn(R725A) carry point mutations within GED resulting in reduced assembly stimulated GTPase activity. Biotinylated transferrin is more rapidly sequestered from avidin in cells transiently overexpressing either of these two activating mutants (Sever, S., A.B. Muhlberg, and S.L. Schmid. 1999. Nature. 398:481–486), suggesting that early events in receptor-mediated endocytosis are accelerated. Using stage-specific assays and morphological analyses of stably transformed cells, we have identified which events in clathrin-coated vesicle formation are accelerated by the overexpression of dyn(K694A) and dyn(R725A). Both mutants accelerate the formation of constricted coated pits, which we identify as the rate limiting step in endocytosis. Surprisingly, overexpression of dyn(R725A), whose primary defect is in stimulated GTP hydrolysis, but not dyn(K694A), whose primary defect is in self-assembly, inhibited membrane fission leading to coated vesicle release. Together, our data support a model in which dynamin functions like a classical GTPase as a key regulator of clathrin-mediated endocytosis.

The GTPase dynamin is essential for clathrin-mediated endocytosis. Numerous new and exciting discoveries regarding dynamin function in vivo and in vitro have led to various models in which dynamin functions directly in membrane fission and the release of clathrin-coated vesicles from the plasma membrane. This would make dynamin unique among GTPases in its ability to act as a mechanochemical enzyme. Here we review the various models and their supporting data. We then discuss new findings that raise doubts as to whether dynamin breaks the paradigm that governs regulatory GTPases.

Abundant evidence has shown that the GTPase dynamin is required for receptor-mediated endocytosis, but its exact role in endocytic clathrin-coated vesicle formation remains to be established. Whereas dynamin GTPase domain mutants that are defective in GTP binding and hydrolysis are potent dominant-negative inhibitors of receptor-mediated endocytosis, overexpression of dynamin GTPase effector domain (GED) mutants that are selectively defective in assembly-stimulated GTPase-activating protein activity can stimulate the formation of constricted coated pits and receptor-mediated endocytosis. These apparently conflicting results suggest that a complex relationship exists between dynamin's GTPase cycle of binding and hydrolysis and its role in endocytic coated vesicle formation. We sought to explore this complex relationship by generating dynamin GTPase mutants predicted to be defective at distinct stages of its GTPase cycle and examining the structural intermediates that accumulate in cells overexpressing these mutants. We report that the effects of nucleotide-binding domain mutants on dynamin's GTPase cycle in vitro are not as predicted by comparison to other GTPase superfamily members. Specifically, GTP and GDP association was destabilized for each of the GTPase domain mutants we analyzed. Nonetheless, we find that overexpression of dynamin mutants with subtle differences in their GTPase properties can lead to the accumulation of distinct intermediates in endocytic coated vesicle formation.

The GTPase dynamin controls a variety of endocytic pathways, participates in the formation of phagosomes, podosomal adhesions, and invadopodia, and in regulation of the cytoskeleton and apoptosis. Rac, a member of the Rho family of small GTPases, controls formation of lamellipodia and focal complexes, which are critical in cell migration and phagocytosis. We now show that disruption of dynamin ؊2 function alters Rac localization and inhibits cell spreading and lamellipodia formation even though Rac is activated. Dominant-negative K44A dynamin ؊2 inhibited cell spreading and lamellipodia formation on fibronectin without blocking cell adhesion; dynamin ؊2 depletion by specific small interfering RNA inhibited lamellipodia in a similar manner. Dyn2(K44A) induced Rac mislocalization away from cell edges, into abnormal dorsal ruffles, and led to increased total Rac activity. Fluorescence resonance energy transfer imaging of Rac activity confirmed its predominant localization to aberrant dorsal ruffles in the presence of dominant-negative dyn2(K44A). Dyn2(K44A) induced the accumulation of tubulated structures bearing membrane-bound Rac-GFP. Constitutively active but not wild-type GFP-Rac was found on macropinosomes and Rac-dependent, platelet-derived growth factor-induced macropinocytosis was abolished by Dyn2(K44A) expression. These data suggest an indispensable role of dynamin in Rac trafficking to allow for lamellipodia formation and cell spreading.

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