Characterization of a cytosolic heat-shock protein-caveolin chaperone complex. Involvement in cholesterol trafficking.

Caveolin is a 22-kDa protein that appears to play a critical role in regulating the cholesterol concentration of caveolae. Even though caveolin is thought to be a membrane protein, several reports suggest that this peculiar protein can traffic independently of membrane vesicles. We now present evidence that a cytosolic pool of caveolin is part of a heat-shock protein-immunophilin chaperone complex consisting of caveolin, heat-shock protein 56, cyclophilin 40, cyclophilin A, and cholesterol. Treatment of NIH 3T3 cells with 1 microM cyclosporin A or 100 nM rapamycin disrupted the putative transport complex and prevented rapid (10-20 min) transport of cholesterol to caveolae. The lymphoid cell line, L1210-JF, does not express caveolin, does not form an immunophilin-caveolin complex, and does not transport newly synthesized cholesterol to caveolae. Transfection of caveolin cDNA into L1210-JF cells allowed the assembly of a transport complex identical to that found in NIH 3T3 cells. In addition, newly synthesized cholesterol in transfected cells was rapidly (10-20 min) and specifically transported to caveolae. These data strongly suggest that a caveolin-chaperone complex is a mechanism by which newly synthesized cholesterol is transported from the endoplasmic reticulum through the cytoplasm to caveolae.