Tyrosine-phosphorylated caveolin is a physiological substrate of the low M-r protein-tyrosine phosphatase

Low phosphotyrosine-protein phosphatase is involved in the regulation of se veral tyrosine kinase growth factor receptors. The best characterized actio n of this enzyme is on the signaling pathways activated by platelet-derived growth factor, where it plays multiple roles. In this study we identify ty rosine-phosphorylated caveolin as a new potential substrate for low M-r pho sphotyrosine-protein phosphatase. Caveolin is tyrosine-phosphorylated in vi vo by Src kinases, recruits into caveolae, and hence regulates the activiti es of several proteins involved in cellular signaling cascades. Our results demonstrate that caveolin and low M-r phosphotyrosine-protein phosphatase coimmunoprecipitate from cell lysates, and that a fraction of the enzyme lo calizes in caveolae. Furthermore, in a cell line sensitive to insulin, the overexpression of the C12S dominant negative mutant of low M-r phosphotyros ine-protein phosphatase (a form lacking activity but able to bind substrate s) causes the enhancement of tyrosine-phosphorylated caveolin. Insulin stim ulation of these cells induces a strong increase of caveolin phosphorylatio n. The localization of low M-r phosphotyrosine-protein phosphatase in caveo lae, the in vivo interaction between this enzyme and caveolin, and the capa city of this enzyme to rapidly dephosphorylate phosphocaveolin, all indicat e that tyrosine-phosphorylated caveolin is a relevant substrate for this ph osphatase.