Microtubule-associated protein 1A is the fibroblast HMW MAP undergoing mitogen-stimulated serine phosphorylation.

A variety of antibodies to microtubule-associated protein (MAP) have been used to demonstrate that phosphorylation of a 350 kDa microtubule-associated protein is stimulated 2-3 fold by epidermal growth factor or serum in quiescent 3T3-L1 fibroblasts and by insulin in 3T3-L1 adipocytes. Phosphorylation occurs on serine residues, and is maximal by 15-20 min. The phosphoprotein has been identified as MAP1A by specific immunoprecipitation with a well-characterized monoclonal antibody.