PREFORMED GROES OLIGOMERS ARE NOT REQUIRED AS FUNCTIONAL COCHAPERONINS

We have previously shown that the C-terminal sequence of GroES is requ ired for oligomerization [Seale and Horowitz (1995), J. Biol. Chem. 27 0, 30268-30270]. In this report, we have generated a C-terminal deleti on mutant of GroES with a significantly destabilized oligomer and have investigated its function in the chaperonin-assisted protein folding cycle. Removal of the two C-terminal residues of GroES results in a co chaperonin [GroESD(96-97)] that is monomeric at concentrations where G roES function is assessed. Using equilibrium ultracentrifugation, we m easured the dissociation constant for the oligomer-monomer equilibrium to be 7.3 x 10(-34) M-6. The GroESD(96-97) is fully active as a cocha peronin. This mutant is able to inhibit the ATPase activity of GroEL t o levels comparable to wild-type GroES. It is also able to assist the refolding of urea-denatured rhodanese by GroEL. While GroESD(96-97) ca n function at levels comparable to wild-type GroES, higher concentrati ons of mutant are required to produce the same effect. These results s upport the idea that the preformed GroES heptamer is not required for function, but they suggest that the oligomeric cochaperonin is most ef ficient.