Compensatory changes in GroEL/Gp31 affinity as a mechanism for Allele-specific genetic interaction

Previous work has shown that the GroEL-GroES interaction is primarily media ted by the GroES mobile loop. In bacteriophage T4 infection, GroES is subst ituted by the gene 31-encoded cochaperonin, Gp31. Using a genetic selection scheme, we have identified a new set of mutations in gene 31 that affect i nteraction with GroEL; all mutations result in changes in the mobile loop o f Gp31, Biochemical analyses reveal that the mobile loop mutations alter th e affinity between Gp31 and GroEL, most likely by modulating the stability of the GroEL-bound hairpin conformation of the mobile loop. Surprisingly, m utations in groEL that display allele-specific interactions with mutations in gene 31 alter residues in the GroEL intermediate domain, distantly locat ed from the mobile loop binding site. The observed patterns of genetic and biochemical interaction between GroES or Gp31 and GroEL point to a mechanis m of genetic allele specificity based on compensatory changes in affinity o f the protein-protein interaction. Mutations studied in this work indirectl y alter affinity by modulating a folding transition in the Gp31 mobile loop or by modulating a hinged conformational change in GroEL.