Isoprenylcysteine-O-carboxyl methyltransferase regulates aldosterone- sensitive Na⁺ reabsorption

The Xenopus laevis distal tubule epithelial cell line A6 was used as a model epithelia to study the role of isoprenylcysteine-O-carboxyl methyltransferase (pcMTase) in aldosterone-mediated stimulation of Na⁺ transport. Polyclonal antibodies raised against X. laevis pcMTase were immunoreactive with a 33-kDa protein in whole cell lysate. These antibodies were also reactive with a 33-kDa product from in vitro translation of the pcMTase cDNA. Aldosterone application increased pcMTase activity resulting in elevation of total protein methyl esterification in vivo, but pcMTase protein levels were not affected by steroid, suggesting that aldosterone increased activity independent of enzyme number. Inhibition of pcMTase resulted in a reduction of aldosterone-induced Na⁺ transport demonstrating the necessity of pcMTase-mediated transmethylation for steroid induced Na⁺ reabsorption. Transfection with an eukaryotic expression construct containing pcMTase cDNA increased pcMTase protein level and activity. This resulted in potentiation of the natriferic actions of aldosterone. However, overexpression did not change Na⁺ reabsorption in the absence of steroid, suggesting that pcMTase activity is not limiting Na⁺ transport in the absence of steroid, but that subsequent to aldosterone addition, pcMTase activity becomes limiting. These results suggest that a critical transmethylation is necessary for aldosterone-induction of Na⁺ transport. It is likely that the protein catalyzing this methylation is isoprenylcysteine-O-carboxyl methyltransferase and that aldosterone activates pcMTase without affecting transferase expression.