Conformational Spread as a Mechanism for Cooperativity in the Bacterial Flagellar Switch

Abstract

Complex Cooperativity Cooperativity in multisubunit protein complexes is classically understood in terms of either a concerted model, in which all subunits switch conformation simultaneously, or a sequential model, in which a subunit switches conformation whenever a ligand binds. More recently, a “conformational spread” model has suggested that a conformational coupling between subunits and between subunit and ligand is probabilistic. Using high-resolution optical microscopy, Bai et al. (p. 685 ; see the Perspective by Hilser ) observed multistate switching of the bacterial flagellar switch complex that was previously understood in terms of a concerted allosteric model. The conformational spread model gives quantitative agreement with the data.