The effects of protein modification on the antigenic determinants of p30 and gp7O of type C retroviruses were investigated by using solid-phase competition radioimmunoassays. Proteins were modified by reduction with 2-mercaptoethanol and subsequent carboxymethylation of SH groups with iodoacetamide or by amidination of a and e amino groups with methylacetimidate. The type-specific determinants of gp7O were found to be conformational in nature, as they were destroyed by these chemical modifications. Groupand interspecies-specific determinants of gp7O antigens, however, appear to be sequential and do not involve residues susceptible to these chemical reagents. Conformation-dependent typespecific determinants of p30 were affected only by methylacetimidate. Group-and interspecies-specific determinants of p30 are similar to those of gp7O in that they also appear to be sequential antigenic sites. Therefore, the broadly reactive groupand interspecies-specific determinants of gp7O and p30 can be followed into small peptides. Accordingly, a cyanogen bromide cleavage fragment derived from the carboxyl-terminal one-third of Rauscher leukemia virus p30 was found to carry group-specific determinants but no detectable interspecies-specific determinants. In contrast, a peptide obtained by limited trypsin cleavage of p30, which was derived from the NH2-terminal region of the protein, contained at least one of the interspecies determinants shared with feline leukemia virus p27.

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