Caveolin Interacts with the Angiotensin II Type 1 Receptor during Exocytic Transport but Not at the Plasma Membrane

The mechanisms involved in angiotensin II type 1 receptor (AT 1 -R) trafficking and membrane localization are largely unknown. In this study, we examined the role of caveolin in these processes. Electron microscopy of plasma membrane sheets shows that the AT 1 -R is not concentrated in caveolae but is clustered in cholesterol-independent microdomains; upon activation, it partially redistributes to lipid rafts. Despite the lack of AT 1 -R in caveolae, AT 1 -R·caveolin complexes are readily detectable in cells co-expressing both proteins. This interaction requires an intact caveolin scaffolding domain because mutant caveolins that lack a functional caveolin scaffolding domain do not interact with AT 1 -R. Expression of an N-terminally truncated caveolin-3, CavDGV, that localizes to lipid bodies, or a point mutant, Cav3-P104L, that accumulates in the Golgi mislocalizes AT 1 -R to lipid bodies and Golgi, respectively. Mislocalization results in aberrant maturation and surface expression of AT 1 -R, effects that are not reversed by supplementing cells with cholesterol. Similarly mutation of aromatic residues in the caveolin-binding site abrogates AT 1 -R cell surface expression. In cells lacking caveolin-1 or caveolin-3, AT 1 -R does not traffic to the cell surface unless caveolin is ectopically expressed. This observation is recapitulated in caveolin-1 null mice that have a 55% reduction in renal AT 1 -R levels compared with controls. Taken together our results indicate that a direct interaction with caveolin is required to traffic the AT 1 -R through the exocytic pathway, but this does not result in AT 1 -R sequestration in caveolae. Caveolin therefore acts as a molecular chaperone rather than a plasma membrane scaffold for AT 1 -R.