Brownian ratchet mechanisms for carboxysome positioning in bacteria

Abstract

ParA-type ATPases position multiple cellular cargos in bacteria. DNA partitioning by ParA is proposed to occur via a Brownian ratchet mechanism. It is unclear whether this mechanism accounts for the distribution of other ParA-positioned cargos. Among these cargos are bacterial microcompartments (BMCs), protein-based organelles that encapsulate metabolic components in bacteria. The most widely studied BMC is the carboxysome, which sequesters carbon fixation machinery and contributes significantly to global carbon fixation. Carboxysomes are spatially regulated by the Maintenance of Carboxysome Distribution (Mcd) system. Similarities between the McdA protein and the ParA ATPase family proteins have led to the proposal of a Brownian ratchet–based positioning mechanism for carboxysomes. Here, we describe ParA-type ATPases, the proposed variations of the Brownian ratchet mechanism, and how they may account for carboxysome organization.