The processing of ras and of other
GTP-binding proteins includes a final reversible step in which the carboxy terminal prenylated
cysteine is methylated by the
enzyme prenylated
protein methyltransferase (
PPMTase). The significance of this modification and of the role of
PPMTase in human
tumors has yet to be fully elucidated. Here we characterize the
PPMTase of human
endometrial carcinomas (
tumors in which the frequency of ras gene mutations is relatively high) and compare it to the
PPMTase of the normal endometrium. Our results show that in both types of tissues the
enzyme is bound to the membranes. It can utilize synthetic substrates such as N-acetyl-S-farnesyl-
L-cysteine (Km = 18-20 microM) and is blocked by the
PPMTase inhibitor S-
farnesylthioacetic acid (Ki = 2 microM). In vitro methylation assays and [alpha-32P]
GTP blot-overlay assays showed that the major endogenous
PPMTase substrates are small
GTP-binding proteins. Methylation of these
proteins in vitro is blocked by
farnesylthioacetic acid. The kinetic properties of
PPMTase from the
carcinomas and the normal tissues are very similar. However, levels of
PPMTase activity (but not of its endogenous substrates) are higher in the carcinomatous endometrium than in the normal one. The elevated
enzyme activity is restricted to the crude mitochondrial fraction (8.0 +/- 0.4 vs. 5.4 +/- 0.1 pmol N-acetyl
farnesylcysteine methyl ester formed/min/mg
protein by the
carcinoma and by the normal endometrial preparations, respectively). As this fraction is enriched in plasma membranes, it appears that the elevated
enzyme activity could be related to
ras protein methylation; if so, selective methylation blockers might inhibit the growth of
endometrial carcinomas.