Abstract
Phosphorylation of cellular proteins is known to play an important role in mediating the metabolic effects of insulin in target cells. Here we show that exposure of intact adipocytes to physiological concentrations of insulin results in phosphorylation of the calcium receptor protein, calmodulin. The identity of the phosphorylated protein as being calmodulin in intact cells was demonstrated by two-dimensional electrophoresis, N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W7)-affinity chromatography, and positive staining with the Ca2+ binding protein stain Stains All. Phosphorylation of calmodulin occurred at physiological insulin concentrations with maximum stimulation (608 +/- 114% over basal) at 50 microunits/ml (3.3 X 10(-10) M) insulin. The 32Pi incorporated into calmodulin was stable to base, indicating that phosphotyrosine was involved and thus implicating the insulin-receptor tyrosine kinase as being responsible for its phosphorylation. The phosphorylation of calmodulin may represent an important component of the mechanism for intracellular signaling not only for insulin, but potentially for other physiological regulators of cellular metabolism.
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