J Biol Chem 1993 Dec 15;26835:26613-7.

Aldosterone-induced and GTP-stimulated methylation of a 90-kDa polypeptide in the apical membrane of A6 epithelia.

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Aldosterone treatment of A6 cultured renal epithelial cells methylates the apical membrane, and we examined the aldosterone-induced carboxymethylation of the apical membrane of these cells to determine the targeted polypeptides. Methionine-deprived A6 cells were incubated with aldosterone and [3H]methionine. Homogenates and apical membranes were solubilized and analyzed by SDS-polyacrylamide gel electrophoresis. Label incorporation in a 90-kDa polypeptide was more intense (4-fold) in membranes after aldosterone compared to control. For in vitro methylation, membranes were isolated, incubated with S-adenosyl-L-[methyl-3H]methionine, and analyzed for 3H-methyl uptake. Label incorporation was low in control membranes but markedly stimulated (4-fold) in membrane preparations from aldosterone-treated cells. Guanosine 5'-O-(3-thiotriphosphate) increased in vitro methylation of a 90-kDa polypeptide 5-fold in control membranes but after aldosterone, where methylation was already stimulated, little change was observed. We conclude that aldosterone induces methylation of an apical membrane 90-kDa polypeptide, possibly a subunit of the epithelial Na+ channel, in a GTP-dependent manner, and this may be one of the final steps in a cascade of reactions leading to the natriferic action of this hormone.

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