Cross-reactivity of antibodies against synthetic peptides.

Antiserum against the synthetic peptide Lys-Arg-Ser-Arg-His-Phe, corresponding to the carboxy terminus of polyoma virus medium tumor antigen (medium T antigen), immunoprecipitates a protein of 36,000 daltons from polyoma virus-infected and uninfected cell extracts treated with the sulfhydryl group reagent N-ethyl-maleimide. This protein appears to share an antigenic determinant with medium T antigen that is normally buried inside the protein or covered up by another protein or cellular structure. The two-dimensional tryptic fingerprints of the 36K protein and of medium T antigen are apparently unrelated to each other. Antiserum against the octapeptide Ac-Met-Asp-Lys-Val-Leu-Asn-Arg-Tyr, including the amino-terminal heptapeptide sequence of the simian virus 40 (SV40) large tumor (T) and small T antigens, cross-reacts with polyoma virus large T antigen, which has an identical amino-terminal heptapeptide sequence except that Lys is replaced by Arg and Asn by Ser. The problem of cross-reactivities of antipeptide sera is discussed.