CAP defines a second signalling pathway required for insulin-stimulated glucose transport

Insulin stimulates the transport of glucose into fat and muscle cells. Alth ough the precise molecular mechanisms involved in this process remain uncer tain, insulin initiates its actions by binding to its tyrosine kinase recep tor, leading to the phosphorylation of intracellular substrates. One such s ubstrate is the Cbl protooncogene product(1). Cbl is recruited to the insul in receptor by interaction with the adapter protein CAP, through one of thr ee adjacent SH3 domains in the carboxy terminus of CAP(2). Upon phosphoryla tion of Cbl, the CAP-Cbl complex dissociates from the insulin receptor and moves to a caveolin-enriched, triton-insoluble membrane fraction(3). Here, to identify a molecular mechanism underlying this subcellular redistributio n, we screened a yeast two-hybrid library using the amino-terminal region o f CAP and identified the caveolar protein flotillin. Flotillin forms a tern ary complex with CAP and Cbl, directing the localization of the CAP-Cbl com plex to a lipid raft subdomain of the plasma membrane. Expression of the N- terminal domain of CAP in 3T3-L1 adipocytes blocks the stimulation of gluco se transport by insulin, without affecting signalling events that depend on phosphatidylinositol-3-OH kinase. Thus, localization of the Cbl-CAP comple x to lipid rafts generates a pathway that is crucial in the regulation of g lucose uptake.