Topology of the TRiC/CCT Chaperonin

The eukaryotic Tcp1-Ring Complex (TRiC, also called CCT) is an essential ≈ 1MDa molecular chaperone complex. It is composed of two rings of 8 subunits, which are assembled in a back-to-back arrangement. The structure of the individual subunits is similar to subunits from other chaperonins like GroEL or the Thermosome. However, high resolution data for the assembled chaperonin is missing, leading to uncertainty in the topological arrangement.
Here we describe a mass spectrometry based approach to derive distance constraints for structural modeling of large molecular assemblies. The purified complex is subjected to crosslinking and analyzed by mass spectrometry to detect inter-molecular crosslinks revealing relative spatial positions.
When this procedure was applied to TRiC from S. cerevisiae we found a topology different from earlier models. Corresponding data on bovine TRiC agreed with the new arrangement (L. Joachimiak, J. Frydman personal communication). This method is generally applicable to macromolecular complexes for which high-resolution structural information is missing.