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Using site-directed mutagenesis, we have introduced mutations encoding 17
different amino acids at codon 61 of the human rasH gene. Fifteen of these
substitutions increased rasH transforming activity. The remaining two mutants,
encoding proline and glutamic acid, displayed transforming activities similar to
the normal gene. Overall, these mutants vary over 1000-fold in transforming
potency. Increased levels of p21 expression were required for transformation by
weakly transforming mutants. The mutant proteins were unaltered in guanine
nucleotide binding properties. However, all 17 different mutant proteins
displayed equivalently reduced rates of GTP hydrolysis, 8- to 10-fold lower than
the normal protein. There was no quantitative correlation between reduction in
GTPase activity and transformation, indicating that reduced GTP hydrolysis is
not sufficient to activate ras transforming potential.
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