Perturbation of dynamin II with an amphiphysin SH3 domain increases GLUT4 glucose transporters at the plasma membrane in 3T3-L1 adipocytes. Dynamin II participates in GLUT4 endocytosis

SNAP23 promotes insulin-dependent glucose uptake in 3T3-L1 adipocytes: possible interaction with cytoskeleton

The acute stimulation of glucose uptake by insulin in fat and muscle cells is primarily the result of translocation of facilitative glucose transporter 4 (GLUT-4) from an internal compartment to the plasma membrane. Here, we investigate the role of SNAP23 (a 23-kDa molecule resembling the 25-kDa synaptosome associated protein) in GLUT-4 translocation and glucose uptake in 3T3-L1 adipocytes. Microinjection of a polyclonal antibody directed to the carboxy terminus of SNAP23 inhibited GLUT-4 incorporation into the membrane in response to insulin, whereas microinjection of full-length recombinant SNAP23 enhanced the insulin effect. Introduction of recombinant SNAP23 into chemically permeabilized cells also enhanced insulin-stimulated glucose transport. These results indicate that SNAP23 is required for insulin-dependent, functional incorporation of GLUT-4 into the plasma membrane and that the carboxy terminus of the protein is essential for this process. SNAP23 is therefore likely to be a fusion catalyst along with syntaxin-4 and vesicle-associated membrane protein (VAMP)-2. Furthermore, the endogenous content of SNAP23 appears to be limiting for insulin-dependent GLUT-4 exposure at the cell surface. A measurable fraction of SNAP23 was sedimented with cytoskeletal elements when extracted with Triton X-100, unlike VAMP-2 and syntaxin-4, which were exclusively soluble in detergent. We hypothesize that SNAP23 and its interaction with the cytoskeleton may be targets for regulation of GLUT-4 traffic.

Dynamin spirals

Dynamin is an important component of membrane recycling at the plasma membrane and, potentially, within the cell. The role of dynamin in clathrin-mediated endocytosis has been based on numerous endocytosis assays, as well as on the discovery and gross characterization of the assembled spiral structure of dynamin. Recently, it has been shown that dynamin can also bind to liposomes and form helical tubes that constrict and vesiculate upon GTP addition. This suggests that dynamin is capable of and may be responsible for the pinching off of clathrin-coated vesicles from the plasma membrane during clathrin-mediated endocytosis.

Functional diversity in the dynamin family

The function of the GTPase dynamin has been discussed for several years. It clearly plays a role in vesicle budding, but, despite recent insights, precisely how it functions in this process is still a matter of debate. In addition, it is now clear that dynamin is a member of a large protein family, present in a variety of cellular locations where members apparently perform a range of functions. This article describes current understanding of the structure and function of the various dynamin family members.

Expression of a dominant interfering dynamin mutant in 3T3L1 adipocytes inhibits GLUT4 endocytosis without affecting insulin signaling

To examine the role of clathrin-coated vesicle endocytosis in insulin receptor signaling and GLUT4 trafficking, we used recombinant adenovirus to express a dominant interfering mutant of dynamin (K44A/dynamin) in 3T3L1 adipocytes. Functional expression of K44A/dynamin, as measured by inhibition of transferrin receptor internalization, did not affect insulin-stimulated insulin receptor autophosphorylation, Shc tyrosine phosphorylation, or mitogen-activated protein kinase activation. Although the tyrosine phosphorylation of insulin receptor substrate-1 was slightly reduced, correlating with a 25% decrease in insulin receptor substrate-1-associated phosphatidylinositol 3-kinase activity, insulin-stimulated Akt kinase activation was unaffected. In contrast, expression of K44A/dynamin resulted in the cell-surface accumulation of GLUT4 under basal conditions and an inhibition of GLUT4 endocytosis without affecting insulin-stimulated GLUT4 exocytosis. These data demonstrate that disruption of clathrin-mediated endocytosis does not significantly perturb insulin receptor signal transduction pathways. Furthermore, K44A/dynamin expression causes an accumulation of GLUT4 at the cell surface, suggesting that GLUT4 vesicles exist in at least two distinct intracellular compartments, one that undergoes continuous recycling and a second that is responsive to insulin.

Crystal structure of the amphiphysin-2 SH3 domain and its role in the prevention of dynamin ring formation

The amphiphysins are brain-enriched proteins, implicated in clathrin-mediated endocytosis, that interact with dynamin through their SH3 domains. To elucidate the nature of this interaction, we have solved the crystal structure of the amphiphysin-2 (Amph2) SH3 domain to 2.2 A. The structure possesses several notable features, including an extensive patch of negative electrostatic potential covering a large portion of its dynamin binding site. This patch accounts for the specific requirement of amphiphysin for two arginines in the proline-rich binding motif to which it binds on dynamin. We demonstrate that the interaction of dynamin with amphiphysin SH3 domains, unlike that with SH3 domains of Grb2 or spectrin, prevents dynamin self-assembly into rings. Deletion of a unique insert in the n-Src loop of Amph2 SH3, a loop adjacent to the dynamin binding site, significantly reduces this effect. Conversely, replacing the n-Src loop of the N-terminal SH3 domain of Grb2 with that of Amph2 causes it to favour dynamin ring disassembly. Transferrin uptake assays show that shortening the n-Src loop of Amph2 SH3 reduces the ability of this domain to inhibit endocytosis in vivo. Our data suggest that amphiphysin SH3 domains are important regulators of the multimerization cycle of dynamin in endocytosis.

Dynamin and its partners: a progress report

Dynamin's role in clathrin-mediated endocytosis is now well established. Here we review new evidence from the past two years for the function of dynamin and related GTPases in other Intracellular trafficking events. We then summarize current information on the domain structure and function of this multidomain GTPase. Finally, we describe dynamin partners and their function in the context of clathrin-mediated endocytosis.

Dynamin undergoes a GTP-dependent conformational change causing vesiculation

The dynamin family of GTPases is essential for receptor-mediated endocytosis and synaptic vesicle recycling, and it has recently been shown to play a role in vesicle formation from the trans-Golgi network. Dynamin is believed to assemble around the necks of clathrin-coated pits and assist in pinching vesicles from the plasma membrane. This role would make dynamin unique among GTPases in its ability to act as a mechanochemical enzyme. Data presented here demonstrate that purified recombinant dynamin binds to a lipid bilayer in a regular pattern to form helical tubes that constrict and vesiculate upon GTP addition. This suggests that dynamin alone is sufficient for the formation of constricted necks of coated pits and supports the hypothesis that dynamin is the force-generating molecule responsible for membrane fission.