Caveolin-1, a constitutive protein of the caveolae, is implicated in processes of vesicular transport during caveolae-mediated endocytosis. However, the molecular mechanisms of caveolae-mediated endocytosis are not yet clearly defined. Here, we show the physiological role of the Rab5-caveolin-1 interaction during caveolae-mediated endocytosis. Rab5 was found in caveolae-enriched fractions and Rab5 directly bound to caveolin-1. Furthermore, binding sites of Rab5 to caveolin-1 were identified in the scaffold (SD), transmembrane (TM), and C-terminus (CC) domains, and the Rab5 binding domain of caveolin-1 was required for CTXB uptake. Subsequently, we performed a GST-R5BD pull-down assay to determine whether the Rab5 binding domain of caveolin-1 is involved in Rab5 activity or not. The results showed that overexpression of the Rab5 binding domain of caveolin-1 increase the amount of Rab5-GTP in Cos-1 cells. These findings imply that caveolin-1 controls the Rab5 activity during the caveolae-mediated endocytosis. (C) 2008 Elsevier Inc. All rights reserved.