Single amino acid substitutions influencing the folding pathway of the phage P22 tail spike endorhamnosidase.

Temperature-sensitive mutations in the gene for the thermostable tail spike of phage P22 interyFere with the folding and subunit association pathway at the restrictive temperature but not with the activity or stability of the protein once matured. The local sites of these mutations and the mutant amino acid substitutions have been determined by DNA sequencing. Of 11 temperature-sensitive folding mutations, 3 were replacements of glycine residues by polar residues, and three were replacements of threonine residues by residues unable to form a side-chain H-bond. There were no proline replacements. Two of the temperature-sensitive sites in which threonine residues were replaced by isoleucine residues were homologous. These sequences probably maintain the correct local folding pathway at higher temperatures. The temperature-sensitive amino acid substitutions appear to destabilize a thermolabile intermediate in the wild-type folding pathway or to increase the rate of a competing off-pathway reaction.