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1. Introduction The enzyme S-adenosylmethionine:protein car- boxyl-U-methyl transferase (EC 2.1 .1.24; protein methylase II) methyl esterifies the aspartyl and/or glutamyl residues of preformed proteins, using S-adenosylmethionine as the methyl donor [ 141. The protein carboxyl methyl esters formed are un- stable and undergo rapid hydrolysis at physiological pH and temperature yielding methanol [5,6]. Several membrane proteins were shown to be the endogenous substrates for the enzyme in both prokaryotic and eukaryotic cells [ 1,7--91 and a number of studies were done to determine the physiological roles of this selec- tive, post-translational, protein methylation in mem- brane functions. The high activity of PM II and the large amounts of its membrane-bound endogenous substrates in nervous and endocrine tissues suggest a role of this reaction in the synaptic function [lo] and neurosecretory processes [ 1 O-l 21. In