Direct demonstration that homotetrameric chaperone SecB undergoes a dynamic dimer-tetramer equilibrium

We have shown here that the cytosolic bacterial chaperone SecB is a structu ral dimer of dimers that undergoes a dynamic equilibrium between dimer and tetramer in the native state. We demonstrated this equilibrium by mixing tw o tetrameric species of SecB that can be distinguished by size. We showed t hat the homotetrameric species exchanged dimers, because when the mixture w as analyzed both by size exclusion chromatography and native polyacrylamide gel electrophoresis a third hybrid tetrameric species was detected. Furthe rmore, treatment of SecB with 5,5'-dithiobis-(2-nitrobenzoic acid), which m odifies the sulfhydryl group on cysteines, caused irreversible dissociation to a dimer indicating that cysteine must be involved in the stabilizing in teractions at the dimer interface. It is clear that the two dimer-dimer int erfaces of the SecB tetramer are differentially stable. Dissociation at one interface allows for a dynamic dimer-tetramer equilibrium. Because only di mers were exchanged it is clear that the other interface between dimers is significantly more stable, otherwise oligomers should have formed with a ra ndom distribution of monomers.