The PKA phosphorylation of vitronectin: effect on conformation and function.

Vitronectin (Vn) stabilizes the inhibitory form of plasminogen activator inhibitor-1 (PAI-1), an important modulator of fibrinolysis. We have previously reported that Vn is specifically phosphorylated by PKA (at Ser378), a kinase we have shown to be released from platelets upon their physiological activation. Here we describe the molecular consequences of this phosphorylation and show (by circular dichroism, and by phosphorylation with casein kinase II) that it acts by modulating the conformation of Vn. The PKA phosphorylation of Vn is enhanced in the presence of either PAI-1, or heparin, or both. This enhanced phosphorylation occurs exclusively on Ser378 as shown with the Vn mutants Ser378Ala and Ser378Glu. The binding of PKA phosphorylated Vn to immobilized PAI-1 and to immobilized plasminogen is shown to be lower than that of Vn. The evidence compiled here suggests that this phosphorylation of Vn can modulate plasminogen activation and consequently control fibrinolysis.