SOLUTION SMALL-ANGLE X-RAY-SCATTERING STUDY OF THE MOLECULAR CHAPERONE HSC70 AND ITS SUBFRAGMENTS

Solution X-ray scattering experiments have been carried out on recombinant bovine Hsc70 (with 650 amino acid residues), a 60 kDa subfragment (residues 1-554) which has ATPase- and peptide-binding activities, a 44kDa subfragment (residues 1-386) which has only ATPase activity, and a peptide-binding fragment (residues 388-554). Modeling based on steady-state values of radii of gyration (R(g)'s) and P(r) functions shows that the 44 kDa and peptide-binding domains are oblate fragments while Hsc70 and the 60 kDa fragment are prolate and relatively elongated. R(g) values decrease significantly in the presence of MgATP relative to their values in the presence of MgADP (Delta R(g) similar to 4-5 Angstrom) for Hsc70 and the 60 kDa fragment; in contrast, they are essentially equal in the presence of either nucleotide for the 44 kDa ATPase fragment. The kinetics of the change of R(g) for Hsc70 and the 60 kDa fragment under single-ATPase cycle conditions show that the transition to the ATP-induced R(g) occurs significantly more rapidly than ATP hydrolysis while the reverse transition to the larger R(g) value does not occur before product release. Altogether, the solution scattering data support a model in which a conformational change in Hsc70 (presumably to the low-peptide-affinity state) is predicated on ATP binding while the reverse transition is predicated on product release.