Microsecond Motion Modulates Ubiquitin Binding through an Allosteric Backbone/Side Chain Network

Motion is involved in a large number of protein functions. Relaxation dispersion (RD) NMR experiments sensitively probe microsecond to millisecond motions. We conducted an in-depth RD analysis of the backbone and side chain methyl groups of ubquitin. This survey showed a large number of atoms (>30) with microsecond fluctuations. These atoms are distributed throughout the structure. Strikingly, nearly all show the same exchange rate, which suggests that ubiquitin undergoes collective motion involving both the backbone and side chains. Furthermore, comparison of different methyl nuclei indicates that the nature of the side chain fluctuations is almost entirely due to changes in rotamer populations. Thus, collective microsecond backbone motion is coupled to redistribution of side chain rotamer populations through a mechanism we term “population shuffling”. We present a single collective mode of motion that yields a reaction coordinate corresponding to the relaxation dispersion data. The resulting model indicates that a localized conformational switch distant from the binding interface propagates changes throughout the structure. Analysis of crystal structures confirms this allosteric network and suggests that the microsecond motion modulates binding to particular interaction partners.