Identification of sites influencing the folding and subunit assembly of the P22 tailspike polypeptide chain using nonsense mutations.

Amber mutations have been isolated and mapped to more than 60 sites in gene 9 of P22 encoding the thermostable phage tailspike protein. Gene 9 is the locus of over 30 sites of temperature sensitive folding (tsf) mutations, which affect intermediates in the chain folding and subunit association pathway. The phenotypes of the amber missense proteins produced on tRNA suppressor hosts inserting serine, glutamine, tryosine and leucine have been determined at different temperatures. Thirty-three of the sites are tolerant, producing functional proteins with any of the four amino acids inserted at the sites, independent of temperature. Tolerant sites are concentrated at the N-terminal end of the protein indicating that this region is not critical for conformation or function. Sixteen of the sites yield temperature sensitive missense proteins on at least one nonsense suppressing host. Most of the sites with ts phenotypes map to the central region of the gene which is also the region where most of the tsf mutations map. Mutations at 15 of the sites have a lethal phenotype on at least one tRNA suppressor host. For nine out of ten sites tested with at least one lethal phenotype, the primary defect was in the folding or subunit association of the missense polypeptide chain. This analysis of the tailspike missense proteins distinguishes three classes of amino acid sites in the polypeptide chain; residues whose side chains contribute little to folding, subunit assembly or function; residues critical for maintaining the folding and subunit assembly pathway at the high end of the temperature range of phage growth; and residues critical over the entire temperature range of growth.