Detection of activated Mr 21,000 protein, the product of ras oncogenes, using antibodies with specificity for amino acid 12.

Antisera raised to a set of chemically synthesized peptides spanning position 12 of ras Mr 21,000 protein (p21) (residues 5 to 17) were able to distinguish between different forms of p21 according to the amino acid at the twelfth codon. The peptide immunogens differed in one amino acid corresponding to position 12 of the protein; the substitutions were valine, serine, arginine, aspartate, alanine, or cysteine at this position. Normal p21 contains glycine at position 12; the other amino acid substitutions are those which would result from a single base change in codon 12 and may therefore be the activating mutations most likely to occur in human tumors. The peptide antisera were evaluated by the Western immunoblot procedure for reactivity with v-ki-ras p21 expressed in Escherichia coli containing the corresponding position 12 mutations. Five of the antisera reacted with p21, and of these, anti-serine, -valine, -arginine, and -aspartate peptide antibodies were specific for their cognate protein. Similar analysis using mammalian cells as sources of position 12 variant forms of p21 demonstrated the ability of these antisera to distinguish among their oncogenic forms of p21 differing by single amino acid substitutions.