Solution structures of GroEL and its complex with rhodanese from small-angle neutron scattering

Background: Molecular chaperonins 60 are cylindrical oligomeric complexes which bind to unfolded proteins and assist in their folding. Studies to identify the location of the protein substrate have produced contradictory results: some suggest that the substrate-binding site is buried within the interior of the complex, whereas others indicate an external (polar) location. Results: Small-angle neutron scattering (SANS) measurements were made on GroEL chaperonin and on a complex of GroEL with rhodanese. The radius of gyration and the molecular weight determined from SANS measurements of GroEL agree well with those from its crystal structure. The positions of residues which were unresolved in the crystal structure have been confirmed. In addition, through model fitting of the SANS data, conformational changes in solution have been assessed and the location of bound rhodanese has been determined. Conclusions: The overall structure of GroEL in solution is similar to the crystal structure. In GroEL the N-terminal and C-terminal residues are organized compactly near the equator of the cylinder and the apical domains are flared by about 5 degrees. The best fit of SANS data suggests the existence of an equilibrium between the complex and single rings and monomers. SANS data for the GroEL-rhodanese complex are consistent with a model wherein one rhodanese molecule binds across the opening to the chaperonin cavity, rather than within it.