Biology subjects

Roskopf, M. A.

Publications and source records attributed to Roskopf, M. A..

2 recordsLinked to original sources

bioRxiv2022

FkpA Enhances Membrane Protein Folding using an Extensive Interaction Surface

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 uOmpA171 and found that FkpA increases the folded uOmpA171population 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 uOmpA171folding trajectory. We determine the binding affinity between FkpA and uOmpA171 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. SignificanceThe 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.

biophysics↗

bioRxiv2019

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

The periplasmic chaperone network ensures the biogenesis of bacterial outer membrane proteins (OMPs) and has recently been identified as a promising target for antibiotics. SurA is the most important member of this network both due to its genetic interaction with the {beta}-barrel assembly machinery complex as well as its ability to prevent unfolded OMP (uOMP) aggregation. Using only binding energy, the mechanism by which SurA carries out these two functions is not well understood. Here we use a combination of photo-crosslinking, mass spectrometry, solution scattering, and molecular modeling techniques to elucidate the key structural features that define how SurA solubilizes uOMPs. Our experimental data support a model in which SurA binds uOMPs in a groove formed between the core and P1 domains. This binding event results in a drastic expansion of the rest of the uOMP, which has many biological implications. Using these experimental data as restraints, we adopted an integrative modeling approach to create a sparse ensemble of models of a SurA*uOMP complex. We validated key structural features of the SurA*uOMP ensemble using independent scattering and chemical crosslinking data. Our data suggest that SurA utilizes three distinct binding modes to interact with uOMPs and that more than one SurA can bind a uOMP at a time. This work demonstrates that SurA operates in a distinct fashion compared to other chaperones in the OMP biogenesis network. Significance StatementOuter membrane proteins play critical roles in bacterial physiology and increasingly are exploited as antibiotic targets. SurA is the most important chaperone in the OMP biogenesis network and is thought to initiate their folding through an interaction with the BAM complex. We observe an unprecedented expansion of unfolded outer membrane proteins when bound to SurA. This expansion suggests a potential mechanism by which SurA can deliver uOMPs to the BAM complex. In addition, this study highlights the use of an integrative/hybrid structural biology approach and emerging methods to map highly heterogeneous structural ensembles such as that of an unfolded protein bound to a chaperone.

biophysics↗