P22 TAILSPIKE FOLDING MUTANTS REVISITED - EFFECTS ON THE THERMODYNAMIC STABILITY OF THE ISOLATED BETA-HELIX DOMAIN

The folding of the trimeric phage P22 tailspike protein is influenced by amino acid substitutions of two types, virtually all of which affec t residues in the central domain, a large parallel beta-helix. Tempera ture sensitive folding (tsf) mutations lead to drastically decreased f olding yields at elevated temperature. Their phenotype can be alleviat ed by global suppressor (su) mutations. Both types of mutations appear ed to have no influence on the stability of the native protein at the time of their first isolation and were thus suggested to carry informa tion needed for the folding pathway exclusively. The monomeric beta-he lix of tailspike, expressed as an isolated domain, exhibits freely rev ersible unfolding and refolding transitions, allowing us to analyse th e effects of two well-characterised tsf and all four known su mutation s on its thermodynamic stability. We find a marked decrease in stabili ty for the tsf mutants and a striking Increase in stability for all su mutants. This leads to the conception that the isolated beta-helix do main, although active in receptor-binding and native-like in its spect roscopic properties, is close in conformation to a crucial monomeric f olding intermediate whose thermolability is responsible for the kineti c partitioning between productive folding and irreversible aggregation during the maturation process of P22 tailspike protein. (C) 1998 Acad emic Press.