Conformational plasticity of molecular chaperones involved in periplasmic and outer membrane protein folding

How proteins reach their native conformation and location has been a major question of biology during the last 50 years. To counterbalance protein misfolding and the accumulation of aggregation products, a complex network of chaperones and proteases takes care of protein quality control in the cell. Such a chaperone network is in place in the periplasm of Gram-negative bacteria, where it is necessary for the survival of the bacteria as well as for outer membrane biogenesis. First mechanistic insights into the periplasmic chaperones that comprise this system came from crystal structures of their apo states. While these crystal structures represent stable conformations of the proteins, they typically lack the information to understand the conformational changes that regulate the functional cycle and the mechanisms coordinating the dynamic adaptation of the chaperones to client proteins. During the past few years, the main actors of periplasmic and outer membrane protein folding have been extensively studied by a combination of experimental techniques. This review aims to give an overview of how recent structural biology developments have helped to achieve a better understanding of the functional cycles of the molecular chaperones Skp, SurA and BamA and how these cycles are regulated by dynamic conformational rearrangements.