Epidermal growth factor (EGF) sensitive phosphorylation of calmodulin (CAM) in A431 cell membrane

A431, a transformed cell line, is known to contain a high concentration of EGF receptors (EGFR). Exogenous CAM, when combined with purified membrane from A431 is strongly phosphorylated in the presence of EGF. The EGF-dependent phosphorylation of CAM did not alter the normal profile of the A431 EGFR autophosphorylation, as demonstrated by SDS-PAGE and autoradiography. In addition to its EGF dependency, the presence of divalent cations is also critical for CAM phosphorylation. The presence of a divalent cation chelator, such as EGTA, caused a complete inhibition of CAM phosphorylation, which can be reversed with cations in the following order of effectiveness: Mg/sup + +/ > Mn/sup + +/ > Ca/sup + +/. Divalent cations also break up CAM into four co-migrating bands as indicated by Coomassie Blue stained gels and the corresponding autoradiograms. Double antibody precipitation followed by phospho-amino acid analysis revealed that the EGF-sensitive CAM phosphorylation occurs exclusively on the serine residue. Using radioimmunoassay, purified A431 membrane was shown to contain a significant amount of endogenous CAM. The implications of the EGF-sensitive CAM phosphorylation are currently under investigation.