Comparison of Ca2+ -dependent phosphorylation in viable dispersed brain cells with calmodulin-dependent protein kinase activity in cell-free preparations of rat brain.
Using two depolarizing agents, veratrine and high concentrations of extracellular KCl, we studied depolarization-stimulated phosphorylations in 32P-labelled dispersed brain tissue in order to identify phosphoprotein substrates for Ca2+ - and calmodulin-dependent protein kinase activity at the cellular level, for comparison with findings in cell-free preparations. In intact brain cells, the only prominent depolarization-stimulated phosphorylation was a 77 kDa protein separated on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. This phosphorylation was dependent on external Ca2+, since chelation of Ca2+ in media with 6 mM-EGTA or the presence of verapamil (a Ca2+ -channel blocker) in the incubation media inhibited depolarization-stimulated phosphorylation of the 77 kDa protein. Phosphorylation of the 77 kDa protein also appeared to be dependent on calmodulin, because depolarization-stimulated phosphorylation was significantly decreased (P less than 0.05) when 100 microM-trifluoperazine was present in the incubation media. Polymyxin B, an inhibitor of Ca2+- and phospholipid-dependent phosphorylation, and 12-O-tetradecanoylphorbol 13-acetate, the phorbol ester enhancing Ca2+- and phospholipid-dependent phosphorylation, had no effect on the phosphorylation of the 77 kDa protein. The 77 kDa phosphoprotein was identified as a protein previously named synapsin I [Ueda, Maeno & Greengard (1973) J. Biol. Chem 248, 8295-8305] on the basis of similar migration of native and proteolytic fragments of the 77 kDa protein with those of authentic synapsin I on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Whereas several studies with cell-free preparations showed that 57 kDa and 54 kDa endogenous phosphoproteins were the most prominent species phosphorylated in a Ca2+ and calmodulin-dependent manner, these results indicate that synapsin is the most prominent Ca2+-and calmodulin-dependent phosphorylation in intact cells. The phosphorylations of 54 kDa and 57 kDa proteins may not be as important in vivo, but instead occur as a result of the disruption of cellular integrity inherent in preparation of cell-free subfractions of brain tissue.
D000818:Animals; D001921:Brain / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology*; D002147:Calmodulin / Q000494:pharmacology*; D002474:Cell-Free System / Q000187:drug effects; D004591:Electrophoresis, Polyacrylamide Gel; D066298:In Vitro Techniques; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D011188:Potassium / Q000494:pharmacology; D011494:Protein Kinases / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D014702:Veratrine / Q000494:pharmacology
D002147:Calmodulin; D010750:Phosphoproteins; D011494:Protein Kinases; D014702:Veratrine; D011188:Potassium; D002118:Calcium