A polyprotein is a polypeptide chain that supplies several protein products through proteolysis. One translation product can therefore contain the sequences of multiple eventual products. This differs from a protein complex assembled by association of independently made chains.
The relationship can be tested by mapping product sequences back onto a precursor. In 1981, investigators determined the poliovirus RNA sequence and identified a long open reading frame. Amino acid sequence analysis mapped twelve viral polypeptides to cleavage products of the encoded polyprotein. Agreement between the sequence and the isolated products supported a common precursor, rather than merely grouping similar-sized bands together.[ref]
Primary structure, gene organization and polypeptide expression of poliovirus RNA by N. Kitamura and others. Original Abstract: open reading frame and mapping of proteolytic products
For an original accounting example, suppose a hypothetical precursor consists of consecutive regions of 200, 150 and 250 amino acid residues. Two cuts at the boundaries can yield three chains containing 600 residues altogether. Cleavage increases the number of molecules without synthesizing additional amino acid residues. Partial processing would also produce intermediates containing more than one of the regions.
The precursor and the final products need not have identical functions or lifetimes. An assay recognizing a sequence retained in several intermediates can detect more than one molecular species. Identifying a polyprotein thus requires following both its protein primary structure and its processing state.
