Evidence for an extra-cellular function for protein kinase A.
In addition to its intra-cellular functions, cAMP-dependent protein kinase (PKA) may well have an extra-cellular regulatory role in blood. This suggestion is based on the following experimental findings: (a) Physiological stimulation of blood platelets brings about a specific release of PKA, together with its co-substrates ATP and Mg++; (b) In human serum, an endogenous phosphorylation of one protein (p75, M(r) 75 kDa) occurs; this phosphorylation is enhanced by addition of cAMP and blocked by the Walsh-Krebs specific PKA inhibitor; (c) No endogenous phosphorylation of p75 occurs in human plasma devoid of platelets, but the selective 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 platelet activation, and thus a protein whose function may require modulation for control; (e) The phosphorylation of vitronectin occurs at one site (Ser378) which, at physiological pH, is buried in its two-chain form (V65 + 10) but it becomes 'exposed' in the presence of glycosaminoglycans (GAGs) e.g. heparin or heparan sulfate. Such a transconformation may be used for targeting the PKA phosphorylation to vitronectin molecules bound to GAGs, for example in the extracellular matrix or on cell surfaces; (f) From the biochemical point of view (Km values and physiological concentrations) the phosphorylation of vitronectin can take place at the locus of a hemostatic event; (g) The phosphorylation of Ser378 in vitronectin alters its function, since it significantly reduces its ability to bind the inhibitor-1 of plasminogen activator(s) (PAI-1).(ABSTRACT TRUNCATED AT 250 WORDS)
Shaltiel
S
S
Department of Chemical Immunology, Weizmann Institute of Science, Rehovot, Israel.
Schvartz
I
I
Korc-Grodzicki
B
B
Kreizman
T
T
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D001792:Blood Platelets / Q000201:enzymology; D001798:Blood Proteins / Q000378:metabolism; D016384:Consensus Sequence; D017868:Cyclic AMP-Dependent Protein Kinases / Q000302:isolation & purification / Q000378:metabolism*; D005110:Extracellular Space / Q000502:physiology; D006023:Glycoproteins / Q000737:chemistry / Q000378:metabolism; D006025:Glycosaminoglycans / Q000494:pharmacology; D006493:Heparin / Q000494:pharmacology; D006497:Heparan Sulfate / Q000494:pharmacology; D006801:Humans; D008969:Molecular Sequence Data; D010766:Phosphorylation; D011487:Protein Conformation; D019096:Vitronectin
D001798:Blood Proteins; D017868:Cyclic AMP-Dependent Protein Kinases; D006023:Glycoproteins; D006025:Glycosaminoglycans; D006493:Heparin; D006497:Heparan Sulfate; D019096:Vitronectin