1. Institut für Molekularbiologie und Biophysik, Eidgenössische Technische Hochschule Zürich, CH-8093 Zürich, Switzerland
2. Howard Hughes Medical Institute and Department of Genetics, Yale School of Medicine, New Haven, Connecticut 06510, USA

Correspondence to: Arthur L. Horwich²Kurt Wüthrich¹ Correspondence and requests for materials should be addressed to K.W. (e-mail: Email: wuthrich@mol.biol.ethz.ch) or A.L.H. (e-mail: Email: horwich@csbmet.csb.yale.edu).

Abstract

Biomacromolecular structures with a relative molecular mass (M_r) of 50,000 to 100,000 (50K–100K) have been generally considered to be inaccessible to analysis by solution NMR spectroscopy. Here we report spectra recorded from bacterial chaperonin complexes ten times this size limit (up to M_r 900K) using the techniques of transverse relaxation-optimized spectroscopy and cross-correlated relaxation-enhanced polarization transfer^{1, 2, 3, 4, 5}. These techniques prevent deterioration of the NMR spectra by the rapid transverse relaxation of the magnetization to which large, slowly tumbling molecules are otherwise subject. We tested the resolving power of these techniques by examining the isotope-labelled homoheptameric co-chaperonin GroES (M_r 72K), either free in solution or in complex with the homotetradecameric chaperonin GroEL (M_r 800K) or with the single-ring GroEL variant SR1 (M_r 400K). Most amino acids of GroES show the same resonances whether free in solution or in complex with chaperonin; however, residues 17–32 show large chemical shift changes on binding. These amino acids belong to a mobile loop region of GroES that forms contacts with GroEL^{6, 7, 8, 9, 10}. This establishes the utility of these techniques for solution NMR studies that should permit the exploration of structure, dynamics and interactions in large macromolecular complexes.