TI - The epithelial/carcinoma antigen EGP-1 , recognized by monoclonal antibody RS7-3G11 , is phosphorylated on serine 303 . AB - RS7-3G11 is a murine monoclonal antibody ( MAb ) raised against human non-small cell lung carcinoma , and is under clinical evaluation . The epithelial/carcinoma antigen EGP-1 , defined by RS7-3G11 , was isolated and purified to homogeneity from a cervical carcinoma cell line , ME180 . EGP-1 is a glycoprotein with an average molecular mass of 47.8 kDa . Metabolic labeling of the antigen with 32P-orthophosphate and subsequent immunoprecipitation with RS7-3G11 showed that it is a phosphoprotein . Phosphoamino acid analysis of the in vivo phosphorylated EGP-1 revealed that the phosphorylation is on serine . In vitro analysis with purified antigen demonstrated that protein kinase C , and not protein kinase A , is involved in phosphorylating the antigen in vitro . In vitro analysis indicated a stoichiometry of phosphorylation of 0.54 mole of phosphate per mole of EGP-1 . Phosphoamino acid analysis and phosphopeptide mapping of the antigen phosphorylated in vitro by protein kinase C showed that phosphorylation occurred on a serine residue , specifically on serine 303 , located in the cytoplasmic domain of EGP-1 . Treatment of ME180 cells with phorbol ester increased the phosphorylation of EGP-1 . The biological function of EGP-1 remains to be elucidated . In this report we elucidate an involvement of protein kinase C in phosphorylating EGP-1 , which may signify a role for this antigen in signal transduction across the cell membrane .