Protein carboxylmethyltransferase activity in intact, differentiated neuroblastoma cells: quantitation by S-[3H]adenosylmethionine prelabeling.

Protein carboxylmethyltransferase has been proposed to play a role in the regulation of neuroblastoma differentiation (Kloog et al., 1983). When we investigated this hypothesis further, different results for methyl ester formation were obtained when measured in acid-precipitated proteins and in proteins separated by acidic polyacrylamide gel electrophoresis, following the incubation of intact neuroblastoma cells with [3H]methionine. These unexpected findings led to the development of a modified assay using S-[3H]-adenosylmethionine as the radiolabeled precursor for quantitating carboxyl methylation in intact cells. Data obtained from either acid-precipitated proteins or those separated on an electrophoresis gel following S-[3H]adenosylmethionine incubation directly correlated with data obtained from proteins separated by electrophoresis following [3H]methionine incubation. Using each of the three methods, an approximately twofold increase in the carboxyl methylation of cellular proteins was detected in neuroblastoma cells differentiated by reducing the serum concentration from 10 to 0.5%, but not in those cells differentiated with either 5 mM hexamethylene bisacetamide or 2% dimethyl sulfoxide. The finding that all detectable methyl acceptor proteins are increasingly methylated following 0.5% serum treatment and that this modification is substoichiometric suggests that protein carboxyl methylation is not an essential component of the differentiation process in neuroblastoma cells.