Side-chain specificity at three temperature-sensitive folding mutation sites of P22 tailspike protein.

The phage P22 tailspike protein is one of the few proteins for which both in vivo and in vitro folding pathways have been thoroughly characterized. Many temperature-sensitive folding (tsf) mutations that cause the mutant tailspike polypeptides not to be folded at high restrictive temperatures have been identified. One-third of the tsf mutation sites are located in one domain called the dorsal fin domain (residues 197-259), which protrudes on the solvent-exposed side of the main beta helix. In the present study, we introduced various amino acid substitutions at three tsf mutation sites (residue numbers 235, 238, and 244) in this domain to elucidate the mechanism of these tsf mutations in detail. The side-chain specificity at these tsf sites, together with structural examination in the tertiary fold, strongly suggests that destabilization of folding intermediates by loss of specific interactions is likely to be the major cause of the tsf defect in the dorsal fin domain.