THE EPIDERMAL GROWTH-FACTOR RECEPTOR TYROSINE KINASE PHOSPHORYLATES CONNEXIN32

The epidermal growth factor (EGF) receptor purified by calmodulin-affi nity chromatography from solubilized rat liver plasma membranes phosph orylates connexin32 in gap junction plaques isolated from the same ori gin. Phosphorylation of connexin32 was stimulated by EGF and mainly oc curs at tyrosine residue(s), although phosphorylation of serine and th reonine residues was also detected. The kinetics parameters for the ph osphorylation of connexin32 parallel those for the transphosphorylatio n of the EGF receptor. m-Calpain proteolyzes phosphoconnexin32, and it s major 26 kDa proteolytic fragment only contains phosphotyrosine resi due(s). Calmodulin binds to connexin32 in the absence of calcium and p revents in great extent its phosphorylation by the EGF receptor tyrosi ne kinase.