Cells of mouse
neuroblastoma clone N1E-115 in the confluent phase of growth can catalyze the formation of endogenous
protein carboxyl methyl
esters, using a
protein carboxyl methylase and membrane-bound methyl acceptor
proteins. The
enzyme is localized predominantly in the cytosol of the cells and has a molecular weight of about 20,000 daltons. Treatment of the cells with
dimethylsulfoxide (
DMSO) or
hexamethylene-bisacetamide (
HMBA), agents that induce morphological and electrophysiological differentiation, results in a marked increase in
protein carboxyl methylase activity. Maximal levels are reached 6-7 days after exposure to the agents, a time course that closely parallels the development of electrical excitability mechanisms in these cells. Serum deprivation also causes neurite outgrowth but does not enhance electrical excitability or
enzyme activity. The capacity of membrane-bound
neuroblastoma protein(s) to be carboxyl methylated is increased by the differentiation procedures that have been examined. However, the increase in methyl acceptor
proteins induced by
DMSO or
HMBA is the largest, and its time course parallels electrophysiological differentiation. In contrast, serum deprivation induced a small increase that reached maximal levels within 24 h. The data suggest that increased
protein carboxyl methylation is a developmentally regulated property of
neuroblastoma cells and that at least two groups of methyl acceptor
proteins are induced during differentiation: a minor group related to morphological differentiation, and a major group that may be related to ionic permeability mechanisms of the excitable membrane.