Novel structural and functional insights into the MoxR family of AAA+ ATPases

The MoxR family of AAA+ ATPases is widespread among bacteria and archaea, although their cellular functions are not well characterized. Based on recent studies, MoxR ATPases are proposed to have chaperone-like function for the maturation of specific protein complexes or for the insertion of cofactors into proteins. MoxR proteins have been found to be important modulators of multiple stress response pathways in different organisms. For example, the respective MoxR proteins have been found to play important roles in the cell envelope stress response in Rhizobium leguminosarum, in the oxidative stress, acid stress, and heat stress responses in Francisella tularensis, in the acid stress and stringent responses in Escherichia coli, in viral tail formation in the crenarchaeal Acidianus two-tailed virus, and in the utilization of carbon monoxide as the sole carbon source by the Gram-negative chemolithoautotrophe Oligotropha carboxidovorans. Recent structural studies on the MoxR proteins from E. coli and Cytophaga hutchinsonii show the unique spatial arrangement of the αβα and all-α subdomains of the AAA+ domain in these proteins compared to the typical arrangement found in canonical AAA+ proteins such as HslU. The spatial organization of the subdomains in the AAA+ domain of MoxR proteins is similar to that found in the ATPase component of the magnesium chelatase complexes, possibly suggesting a similar mechanism of function. In this review, we provide an overview of the newly identified functions and the newly obtained structures of MoxR AAA+ ATPases.