Serine 23 and 36 Phosphorylation of Caveolin-2 Is Differentially Regulated by Targeting to Lipid Raft/Caveolae and in Mitotic Endothelial Cells

Abstract

In the present study, using a combination of reconstituted systems and endothelial cells endogenously expressing caveolins, we show that phosphorylation of caveolin-2 at serines 23 and 36 can be differentially regulated by caveolin-1 mediated subcellular targeting to lipid raft/caveolae and in endothelial cells synchronized in mitosis. Detergent insolubility and sucrose flotation gradient experiments revealed that serine 23 phosphorylation of caveolin-2 preferably occurs in detergent-resistant membranes (DRMs), while serine 36 phosphorylation takes place in non-DRMs. Furthermore, immunofluorescence microscopy studies determined that in the presence of caveolin-1, serine 23-phosphorylated caveolin-2 mostly localizes to plasma membrane, while serine 36-phosphorylated caveolin-2 primarily resides in intracellular compartments. To directly address the role of caveolin-1 in regulating phosphorylation of endogenous caveolin-2, we have used the siRNA approach. The specific knockdown of caveolin-1 in endothelial cells decreases caveolin-2 phosphorylation at serine 23 but not at serine 36. Thus, upregulation of serine 23 phosphorylation of caveolin-2 depends on caveolin-1-driven targeting to plasma membrane lipid rafts and caveolae. Interestingly, although serine 36 phosphorylation does not seem to be regulated in endothelial cells by caveolin-1, it can be selectively upregulated in endothelial cells synchronized in mitosis. The latter data suggests a possible involvement of serine 36-phosphorylated caveolin-2 in modulating mitosis.