Ca²⁺/Calmodulin-Dependent Protein Phosphorylation Associated with the Cytoskeleton of Quiescent Rat Fibroblast (3Y1) Cells

Abstract

Endogenous phosphorylation of the crude membrane fraction of cultured 3Y1 fibroblast cells was enhanced by the addition of Ca²⁺/calmodulin. Both Ca²⁺/calmodulin-dependent protein kinase activity and its substrate were present in a cytoskeletal fraction, obtained as a pellet after washing of the membrane fraction with 2mM EGTA, 0.6 M NaCl, and 1% Triton X-100. The phosphorylatable protein in the Triton X-insoluble fraction was identified by immunoblotting as vimentin. This endogenous phosphorylation induced by calmodulin was inhibited by the addition of KN-62, a specific Ca²⁺/calmodulin-dependent protein kinase II-inhibitor, in a dose-dependent manner. However, phosphorylation of the 59 kDa protein (vimentin) in this fraction was not stimulated by adding both phosphatidyl serine and cAMP, thereby suggesting the absence of protein kinase C or of cAMP-dependent protein kinase in this fraction. The protein kinase associated with the Triton X-insoluble fraction phosphorylated the Ca²⁺/calmodulindependent protein kinase II-specific site of synapsin I from the bovine cortex. Two-dimensional phosphopeptide maps of vimentin indicated that a major phosphopeptide phosphorylated by the endogenous calmodulin-dependent kinase also appears to be the same as a major phosphopeptide phosphorylated by the exogenous Ca²⁺/calmodulin-dependent protein kinase II. Our results suggest that cytoskeleton-associated Ca²⁺/calmodulin-dependent protein kinase II regulates dynamic cellular functions through the phosphorylation of cytoskeletal elements in non-neural cells.