Carboxy-terminal analysis of poliovirus proteins: termination of poliovirus RNA translation and location of unique poliovirus polyprotein cleavage sites.

The carboxy-terminal amino acids of a number of poliovirus proteins were determined by carboxypeptidase A analysis. The nonstructural proteins P3-2, P3-4b and their precursor. P3-1b, were found to be coterminal with a sequence of -Ser-Phe-COOH. As these proteins are coded for at the extreme 3' end of the viral RNA, it is possible to establish the termination site of translation at nucleotide 7,361, 73 nucleotides before the start of the polyadenylic acid tract of the RNA. Two additional nonstructural proteins, P2-X and its precursor, P2-3b, were also found to be coterminal with a sequence of -Phe-Gln-COOH. This result confirms the existence of at least one Gln-Gly proteolytic cleavage site. These Gln-Gly cleavage sites are predicted from the nucleotide sequence to be ubiquitous throughout the poliovirus genome. The only exceptions are the cleavage sites at the carboxy termini of the structural protein VP4 and VP1. Carboxypeptidase A analysis of VP1 establishes a terminal sequence of -Thr-Tyr-COOH, and similar analysis of VP4 shows Asn to be the terminal amino acid residue, observations that prove the existence of the exceptional C-terminal amino acids. In none of the analyzed cases has C-terminal trimming after cleavage been observed.