Characterization of a rat liver protein carboxyl methyltransferase involved in the maturation of proteins with the -CXXX C-terminal sequence motif.

We have utilized S-farnesyl-Leu-Ala-Arg-Tyr-Lys-Cys as a methyl-accepting substrate to characterize a membrane-bound C-terminal protein methyltransferase from rat liver. We have localized the activity to the microsomal fraction and show that the bulk of the enzyme fractionates by density gradient centrifugation with glucose-6-phosphatase, a marker of the endoplasmic reticulum, and not with 5'-nucleotidase, a marker of the plasma membrane, or galactosyl:N-acetylglucosamine transferase, a marker of the Golgi apparatus. This methyltransferase appears to form an integral part of the membrane structure. Its activity is markedly affected by a variety of detergents used to solubilize membrane proteins in their native form. All activity is lost when membranes are treated with seven different detergents at a concentration of 1% (w/v). The activity is inhibited by N-ethylmaleimide, although it can be protected against inactivation with its substrate S-adenosyl-L-methionine, or its product S-adenosyl-L-homocysteine. Finally, we find that 5'-methylthioadenosine, a substrate analogue reported to be an inhibitor of this activity in other studies, is not an effective inhibitor in vitro.