FkpA Enhances Membrane Protein Folding using an Extensive Interaction Surface

Abstract

Abstract Outer membrane protein (OMP) biogenesis in gram-negative bacteria is managed by a network of periplasmic chaperones that includes SurA, Skp, and FkpA. These chaperones bind unfolded OMPs (uOMPs) in dynamic conformational ensembles to suppress uOMP aggregation, facilitate diffusion across the periplasm, and enhance OMP folding. FkpA primarily responds to heat-shock stress, but its mechanism is comparatively understudied. To determine FkpA chaperone function, we monitored the folding of a cognate client uOmpA 171 and found that FkpA increases the folded uOmpA 171 population but also slows the folding rate, dual functions distinct from the other periplasmic chaperones. The results indicate that FkpA behaves as a chaperone and not as a folding catalyst to directly influence the uOmpA 171 folding trajectory. We determine the binding affinity between FkpA and uOmpA 171 by globally fitting sedimentation velocity titrations and found it to be intermediate between the known affinities of Skp and SurA for uOMP clients. Notably, complex formation steeply depends on the urea concentration, suggestive of an extensive binding interface. Initial characterizations of the complex using photo-crosslinking indicates that the binding interface spans the inner surfaces of the entire FkpA molecule. In contrast to prior findings, folding and binding experiments performed using subdomain constructs of FkpA demonstrate that the full-length chaperone is required for full activity. Together these results support that FkpA has a distinct and direct effect on uOMP folding and that it achieves this by utilizing an extensive chaperone-client interface. Significance The periplasmic chaperone network is required for the survival and virulence of gram-negative bacteria. Here we find that the chaperone FkpA enhances outer membrane protein folding and tightly binds its clients with an extensive interaction interface. This modified holdase function of FkpA distinguishes it from other periplasmic chaperones and complements their functions to ensure robust outer membrane biogenesis.