Millisecond-timescale motions contribute to the function of the bacterial response regulator protein Spo0F.

Protein backbones and side chains display varying degrees of flexibility, which allows many slightly different but related conformational substates to occur. Such fluctuations are known to differ in both timescale and magnitude, from rotation of methyl groups (nanoseconds) to the flipping of buried tyrosine rings (seconds). Because many mechanisms for protein function require conformational change, it has been proposed that some of these ground-state fluctuations are related to protein function. But exactly which aspects of motion are functionally relevant remains to be determined. Only a few examples so far exist where function can be correlated to structural fluctuations with known magnitude and timescale. As part of an investigation of the mechanism of action of the Bacillus subtilis response regulator SpoOF, we have explored the relationship between the motional characteristics and protein-protein interactions. Here we use a set of nuclear magnetic resonance 15N relaxation measurements to determine the relative timescales of SpoOF backbone fluctuations on the picosecond-to-millisecond timescale. We show that regions having motion on the millisecond timescale correlate with residues and surfaces that are known to be critical for protein-protein interactions.