A protein modulator stimulates C kinase-dependent phosphorylation of a 90K substrate in synaptic membranes.
We have identified and partially purified an acidic, heat-stable, noncalmodulin protein from bovine brain cytosol that stimulates Ca2+-dependent phosphorylation of an Mr 90K substrate in crude rat brain synaptic membranes. We show that this modulator of phosphorylation (MOP) enhances Ca2+- and phospholipid-dependent protein kinase (C kinase) phosphorylation of this 90K substrate. The 90K substrate is a higher Mr form of an 87K substrate that is a major C kinase substrate in rat brain. The Ca2+-dependent phosphorylation of both substrates is inhibited by the Ca2+-binding proteins S-100 and calmodulin. Both substrates yield phosphopeptide fragments of Mr 9K and 13K after limited proteolysis with V8 protease. Two-dimensional polyacrylamide gel electrophoresis reveals that they have similar acidic isoelectric points (pI 5.0). MOP enhances Ca2+-dependent phosphorylation of the 90K substrate whereas the phosphorylation of 87K is diminished. This reciprocal relationship suggests that the mobility of the 87K substrate in sodium dodecyl sulfate-polyacrylamide gels is decreased to 90K with increasing phosphorylation. MOP may be a novel protein modulator of C kinase-mediated phosphorylation in the nervous system.
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D
Kligman
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J
Patel
D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002417:Cattle; D055598:Chemical Phenomena; D002621:Chemistry; D009419:Nerve Tissue Proteins / Q000302:isolation & purification / Q000494:pharmacology*; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013268:Stimulation, Chemical; D013570:Synaptic Membranes / Q000378:metabolism*
D009419:Nerve Tissue Proteins; D011493:Protein Kinase C; D002118:Calcium