Tb. Topping et al., Direct demonstration that homotetrameric chaperone SecB undergoes a dynamic dimer-tetramer equilibrium, J BIOL CHEM, 276(10), 2001, pp. 7437-7441
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.