Protein carboxylmethylation in intact rat posterior pituitary lobes in vitro

Methylation of proteins was studied in intact rat posterior pituitary lobes in vitro. Posterior pituitary lobes can catalyze the formation of protein carboxylmethyl esters using methionine as a precursor. Methylation inhibitors inhibited formation of protein carboxylmethyl esters, indicating that the reaction proceeded via an enzymatic methylation pathway. At least six proteins were methylated in the intact lobe. Among these are the neurophysins as identified by their characteristic properties: molecular weight, [35S]cysteine labeling, disappearance after salt loading or pituitary stalk section, and low levels in the homozygous Brattleboro rat. Other still unidentified proteins of molecular weight lower than 21,000 were also methylated. Their disappearance after pituitary section and salt loading indicates that, like the neurophysins, these proteins are localized within the hypothalamoneuro-hypophysial axon terminals.