Experimental Evidence for Conformational Spread in the Bacterial Switch Complex

Abstract

The allosteric regulation of proteins has classically been understood in terms of the Monod-Wyman-Changeux (MWC) or Koshland-Nemethy-Filmer (KNF) models. These are recognized as limiting cases of a general allosteric scheme that has recently been described in a model of conformational spread. A candidate proposed for testing the model is the bacterial switch complex, an ultrasensitive multimeric protein ring responsible for controlling the direction of rotation of the bacterial flagellar motor. The complex is too large for MWC-type interactions to be applicable and cooperative binding studies have ruled out the KNF model. Here we use high-resolution back-focal-plane interferometry to resolve intermediate states of the complex predicted by conformational spread, and demonstrate detailed quantitative agreement between our measurements and simulations. Individual switch events are not instantaneous, but follow a broad distribution of switch times with mean ∼ 20 ms, incomplete switches occur at a bias-dependent frequency and intervals between switches are exponentially distributed at all values of bias.