EVIDENCE FOR AN EXTRACELLULAR FUNCTION FOR PROTEIN-KINASE-A

In addition to its intra-cellular functions, cAMP-dependent protein ki nase (PKA) may well have an extra-cellular regulatory role in blood. T his suggestion is based on the following experimental findings: (a) Ph ysiological stimulation of blood platelets brings about a specific rel ease of PKA, together with its co-substrates ATP and Mg; (b) In human serum, an endogenous phosphorylation of one protein (p75, Mr 75 kDa) o ccurs; this phosphorylation is enhanced by addition of cAMP and blocke d by the Walsh-Krebs specific PKA inhibitor; (c) No endogenous phospho rylation of p75 occurs in human plasma devoid of platelets, but the se lective labeling of p75 can be reproduced by adding to plasma the pure catalytic subunit of PKA; (d) p75 was shown to be vitronectin (V), a multifunctional protein implicated in processes associated with platel et activation, and thus a protein whose function may require modulatio n for control; (e) The phosphorylation of vitronectin occurs at one si te (Ser(378)) which, at physiological pH, is buried in its two-chain f orm (V-65+10) but becomes 'exposed' in the presence of glycosaminoglyc ans (GAGs) e.g. heparin or heparan sulfate. Such a transconformation m ay be used for targeting the PKA phosphorylation to vitronectin molecu les bound to GAGs, for example in the extracellular matrix or on cell surfaces; (f) From the biochemical point of view (Km values and physio logical concentrations) the phosphorylation of vitronectin can take pl ace at the locus of a hemostatic event; (g) The phosphorylation of Ser (378) in vitronectin alters its function, since it significantly reduc es its ability to bind the inhibitor-1 of plasminogen activator(s) (PA I-1). Physiologically, this functional modulation may be involved in ' unleashing' PAI-1, allowing its translocation to control the inhibitor y function of PAI-1 and, through it, regulating the conversion of plas minogen to active plasmin. (Mel Cell Biochem 127/128: 283-291, 1993)