Syntaxin 16 controls the intracellular sequestration of GLUT4 in 3T3-L1 adipocytes.

The regulated delivery of Glut4-containing vesicles to the plasma membrane is a specialised example of regulated membrane trafficking. Present models favour the transporter trafficking through two inter-related endosomal cycles. The first is the proto-typical endosomal system. This is a fast trafficking event that, in the absence of insulin, serves to internalise Glut4 from the plasma membrane. Once in this pathway, Glut4 is further sorted into a slowly recycling pathway that operates between recycling endosomes, the trans Golgi network, and a population of vesicles often referred to as Glut4-storage vesicles. Little is known about the molecules that regulate these distinct sorting steps. Here, we have studied the role of Stx16 in Glut4 trafficking. Using two independent strategies, we show that Stx16 plays a crucial role in Glut4 traffic in 3T3-L1 adipocytes. Over-expression of a mutant form of Stx16 devoid of a transmembrane anchor was found to significantly slow the reversal of insulin-stimulated glucose transport. Depletion of Stx16 using antisense approaches profoundly reduced insulin-stimulated glucose transport but was without effect on cell surface transferrin receptor levels, and also reduced the extent of Glut4 translocation to the plasma membrane in response to insulin. These data support a model in which Stx16 is crucial in the sorting of Glut4 from the fast cycling to the slow cycling intracellular trafficking pathways in adipocytes.

Regulation of caveolar endocytosis by syntaxin 6-dependent delivery of membrane components to the cell surface.

Caveolar endocytosis has an important function in the cellular uptake of some bacterial toxins, viruses and circulating proteins. However, the molecular machinery involved in regulating caveolar uptake is poorly defined. Here, we demonstrate that caveolar endocytosis is regulated by syntaxin 6, a target membrane soluble N-ethylmaleimide attachment protein receptor (t-SNARE) involved in membrane fusion events along the secretory pathway. When syntaxin 6 function was inhibited, internalization through caveolae was dramatically reduced, whereas other endocytic mechanisms were unaffected. Syntaxin 6 inhibition also reduced the presence of caveolin-1 and caveolae at the plasma membrane. In addition, syntaxin 6 inhibition decreased the delivery of GM1 ganglioside (GM1) and glycosylphosphatidylinositol (GPI)-GFP (but not vesicular stomatitis virus-glycoprotein G; VSV-G) protein from the Golgi complex to the plasma membrane. Addition of GM1 to syntaxin 6-inhibited cells resulted in the reappearance of caveolin-1 and caveolae at the plasma membrane, and restored caveolar uptake. These results suggest that syntaxin 6 regulates the delivery of microdomain-associated lipids and proteins to the cell surface, which are required for caveolar endocytosis.