title: Direct demonstration that homotetrameric chaperone SecB undergoes a dynamic dimer-tetramer equilibrium.
abstract: We have shown here that the cytosolic bacterial chaperone SecB is a structural dimer of dimers that undergoes a dynamic equilibrium between dimer and tetramer in the native state. We demonstrated this equilibrium by mixing two tetrameric species of SecB that can be distinguished by size. We showed that the homotetrameric species exchanged dimers, because when the mixture was analyzed both by size exclusion chromatography and native polyacrylamide gel electrophoresis a third hybrid tetrameric species was detected. Furthermore, treatment of SecB with 5,5'-dithiobis-(2-nitrobenzoic acid), which modifies the sulfhydryl group on cysteines, caused irreversible dissociation to a dimer indicating that cysteine must be involved in the stabilizing interactions at the dimer interface. It is clear that the two dimer-dimer interfaces of the SecB tetramer are differentially stable. Dissociation at one interface allows for a dynamic dimer-tetramer equilibrium. Because only dimers were exchanged it is clear that the other interface between dimers is significantly more stable, otherwise oligomers should have formed with a random distribution of monomers.
authors: School of Molecular Biosciences, Washington State University, Pullman 99164-4660, USA. topping@wsu.edu
authors: T B
authors: TB
authors: Topping
authors: R L
authors: RL
authors: Woodbury
authors: D L
authors: DL
authors: Diamond
authors: S J
authors: SJ
authors: Hardy
authors: L L
authors: LL
authors: Randall
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D002845:Chromatography; D002851:Chromatography, High Pressure Liquid; D003545:Cysteine / Q000737:chemistry; D003600:Cytosol / Q000378:metabolism; D019281:Dimerization; D004228:Dithionitrobenzoic Acid / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D006863:Hydrogen-Ion Concentration; D008954:Models, Biological; D011485:Protein Binding; D017510:Protein Folding; D013439:Sulfhydryl Reagents / Q000494:pharmacology
chemical_list: D001426:Bacterial Proteins; C057842:SecB protein, Bacteria; D013439:Sulfhydryl Reagents; D004228:Dithionitrobenzoic Acid; D003545:Cysteine