SurA is a 'Groove-y' Chaperone That Expands Unfolded Outer Membrane Proteins

Chaperone proteins play a critical role in the biogenesis of many nascent polypeptides in vivo. In the periplasm of E. coli this role is partially fulfilled by SurA, which promotes the efficient assembly of unfolded outer membrane proteins (uOMPs) into the bacterial outer membrane, though the mechanism by which SurA interacts with uOMPs is not well understood. Here we identify multiple conformations of SurA in solution, one of which contains a cradle-like groove in which client uOMPs bind. Access to this binding groove by clients is gated by the intrinsic conformational dynamics of SurA. Crosslinking mass spectrometry experiments identify multiple regions of native client uOMPs that bind to SurA, providing insight into the molecular determinants of SurA-uOMP interactions. In contrast to other periplasmic chaperones that encapsulate uOMPs, small angle neutron scattering data demonstrate that SurA binding greatly expands client uOMPs. These data can explain the dual roles of SurA as both a holdase and a foldase. Using an integrative modeling approach that combines crosslinking, mass spectrometry, small angle neutron scattering, and simulation, we propose structural models of SurA in complex with an unfolded protein client. We further find that multiple SurA monomers are able to bind discrete sites on a single uOMP. The structural arrangement of SurA and uOMPs provides the basis for a possible mechanism by which SurA binds and expands clients in a manner that facilitates their folding into the outer membrane. Significance Statement Outer membrane proteins play critical roles in bacterial physiology and increasingly are being exploited as antibiotic targets. Their biogenesis requires chaperones in the bacterial periplasm to safely ferry them to their destination membrane. We used crosslinking, mass spectrometry, and small angle neutron scattering to propose an ensemble of structural models that explain how one chaperone, SurA, stabilizes client outer membrane proteins through expansion of their overall size, which positions them for delivery to the BAM complex. This study highlights the use of an hybrid integrative approach and emerging methods in structural biology to map highly heterogeneous structural ensembles like that of an unfolded protein bound to a chaperone.