Self-association of the molecular chaperone HSC70 as assessed by analytical ultracentrifugation.

The self-association properties of the molecular chaperone HSC 70 have been assessed by analytical ultracentrifugation. Sedimentation velocity analysis indicates the presence of three species, whose proportions were dependent on protein concentration, but whose sedimentation coefficients, s20, w, of 4.3 S, 6.6 S and 8.5 S did not vary with concentration, which is indicative of a slowly equilibrating system. Sedimentation equilibrium studies indicate a dissociation into monomers at low HSC 70 concentrations and an association into dimers and trimers at high concentrations. Multiple sets of sedimentation equilibrium data, obtained at various initial loading concentrations and rotor speeds, were adequately fitted to a single set of equilibrium constants by a monomerdimer-trimer association model in which the association constants for the monomer-dimer and dimer-trimer equilibrium. K1−2=1.1.·105 M−1 and K2−3=0.9·105 M−1 respectively, were nearly identical. Interestingly, na isodesmic, indefinite type of association describes the data almost equally well with a single constant of 1.2·105 M−1. These results might have important implications for the chaperone function of HSC 70. [ABSTRACT FROM AUTHOR]