Biophysical characterization of the influence of salt on tetrameric SecB

SecB is a tetrameric chaperone, with a monomeric molecular mass of 17 kDa, that is involved in protein translocation in Escherichia coli. It has been hypothesized that SecB undergoes a conformational change as a function of t he salt concentration. To gain more insight into the salt-dependent behavio r of SecB, we studied the protein in solution by dynamic light scattering, size exclusion chromatography, analytical ultracentrifugation, and small an gle neutron scattering. The results clearly demonstrate the large influence of the salt concentration on the behavior of SecB. At high salt concentrat ion, SecB is a non-spherical protein with a radius of gyration of 3.4 nm. A t low salt concentration the hydrodynamic radius of the protein is apparent ly decreased, whereas the ratio of the frictional coefficients is increased . The protein solution behaves in a non-ideal way at low salt concentration s, as was shown by the analytical ultracentrifugation data and a pronounced interparticle effect observed by small angle neutron scattering. A possibl e explanation is a change in surface charge distribution dependent on the s alt concentration in the solvent. We summarize our data in a model for the salt-dependent conformation of tetrameric SecB.