The protein products of yeast and mammalian ras genes are posttranslationally modified to give mature forms that are localized to the inner surface of the plasma membrane. We have previously demonstrated that the mature form of the Saccharomyces cerevisiae RAS2 gene product is methyl esterified at a modified C-terminal cysteine residue. Here we provide evidence that this residue is an S-farnesylcysteine α-carboxyl methyl ester. This result establishes common posttranslational modifica tions for RAS proteins and fungal sex factors. These polypeptides exhibit sequence similarities at their C-termini that appear to be the critical recognition elements for a common set of modification enzymes. In mammalian cells, proteins with analogous C-terminal sequences appear to be prenylated and carboxyl methylated by a similar mechanism.
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