N-terminal processing and modifications of caveolin-1 in caveolae from human adipocytes

Abstract

Caveolin, the principal structural protein of caveolae membrane domains, has a cytosol-exposed N-terminal part that was cleaved off by trypsin treatment of caveolae vesicles isolated from primary human adipocytes. Sequencing of the released tryptic peptides by nanospray quadrupole time-of-flight mass spectrometry revealed that both caveolin-1α and caveolin-1β were processed by excision of the starting methionines. The N-terminus of the mature caveolin-1α was acetylated, while caveolin-1β was found in acetylated as well as in non-acetylated forms. Fractional phosphorylation of serine-36 in the mature caveolin-1α and of the homologous serine-5 in caveolin-1β was identified. This is the first experimental evidence for in vivo phosphorylation of caveolin-1 at the consensus site for phosphorylation by protein kinase C. The phosphorylation was found in both the acetylated and non-acetylated variants of caveolin-1β. This variability in modifications is consistent with critical involvement of the N-terminal domain of caveolin in the regulation of caveolae.