Structural insight into the role of the periplasmic chaperone Skp in pathogenic Neisseria.

Structural insights into the outer membrane proteins PorA, OMP50 and Cj0034c from native Campylobacter jejuni membranes.

Bacterial outer membrane proteins (OMPs) are critical players in host-pathogen interactions and environmental adaptation. Here we describe the newly developed "Gradient Enrichment of Native Targets from Lipid Environments" (GENTLE) methodology and use this approach to elucidate the structures of Campylobacter jejuni OMPs directly from native, detergent-solubilized crude membranes. We identify and solve high-resolution cryo-EM structures of PorA, OMP50, and Cj0034c from C. jejuni membranes, all of which are required for Campylobacter invasion, adhesion, and initiation of host infection. Notably, our results provide the first structural information of OMP50, revealing a two-domain architecture constructed with an all β-stranded transmembrane domain and an all α-helical periplasmic domain. This structure depicts that all tyrosine residues, many of which are expected to be critical for phosphorylation and host-pathogen interaction, are localized to the outer membrane of C. jejuni. Our studies also led to the first structure of the full-length Cj0034c protein, which assembles as a nonamer with each protomer containing a single-spanning transmembrane helix and a large periplasmic domain. The nine protomers stack side-by-side to form a channel that spans the entire lipid bilayer. However, whether Cj0034c spans the outer membrane (OM) or inner membrane (IM) of C. jejuni must await further experimental studies. In addition, we observed that the surface-exposed extracellular loop L4 of PorA is very flexible, which may be critical for the virulence of this porin. Collectively, this work provides novel structural information for functionally important OMPs and sheds light on how they assemble in native bacterial membranes. These findings further demonstrate that it is possible to obtain high-resolution structural information for targeted membrane proteins from crude native membranes without their overexpression and purification.

Assembly and priming of the VPS4B AAA+ ATPase of the human ESCRT system

Vacuolar protein sorting-associated protein 4 (VPS4), which occurs in A and B isoforms in humans, is the only enzyme in the core endosomal sorting complex required for transport (ESCRT) machinery. Human VPS4 is considered a potential therapeutic target for activation in neurodegeneration, and for inhibition in cancer and HIV-1 infection. VPS4 assembles transiently into a hexamer, which then removes ESCRT-III subunits from polymeric assemblies by unfolding them and threading them through its central pore. The N-terminal MIT domain of VPS4 binds to C-terminal MIM motifs of ESCRT-III. Here, we determined the cryo-electron microscopy structure of the full-length human VPS4B hexamer in six-membered helical “spiral staircase” states. In one of these states, two of the six MIT domains are ordered and stabilize the hexamer by bridging the seam of the spiral staircase. Residues involved in seam-bridging contacts were found to be important for biochemical and cellular activity. The structures also revealed two modes for polypeptide occupancy of the central pore, one of which involves the MIT-AAA linker peptide. These observations suggest a mechanism for substrate-dependent hexamerization and priming of VPS4 for its ESCRT-III remodeling activity.

Detection of Ring Structure and Its Flexibility in Periplasmic Region of FliL, Bacterial Flagellar Stator-Associate Protein, in Marine Vibrio.

Analysis of oligomeric organization and structural plasticity of the periplasmic domain of polar-flagellum FliL from Vibrio alginolyticus and compared with detergent-solubilized full-length pofFliL suggests that such plasticity enables FliL to act as an adaptable scaffold for stator engagement and mechanosensitive remodeling of the motor.

Intrinsic flexibility of Type VII secretion central pore is required for substrate translocation

Structural and Functional Interrogation of Active Streptococcus pneumoniae Sortase A

Sortase A (SrtA) enzymes covalently anchor surface proteins to Gram-positive bacterial cell walls, promoting colonization and virulence. In Streptococcus pneumoniae, previous studies identified both a domain-swapped dimer and an active refolded monomer, but the active enzyme has not been characterized at the structural and residue-specific level. Here, we performed quantitative proteomic comparisons of wild-type and SrtA knockout strains that confirmed the loss of multiple LPxTG-containing virulence factors, including ZmpB, NanA, and IgA1 protease, consistent with an essential role for SrtA in surface protein anchoring. To enable mechanistic studies, we established a biochemical framework to produce monomeric Streptococcus pneumoniae SrtA by refolding and developed a gel-based assay using recombinant substrates to monitor catalytic activity. The refolded monomer, but not the swapped dimer, catalyzed cleavage and transpeptidation of a canonical LPxTG substrate in a metal-independent manner under the conditions examined. We further report high-resolution NMR backbone assignments for the active monomer and identify substrate-induced chemical shift perturbations that localize to the active site. Together, these findings provide an integrated proteomic, biochemical, and NMR characterization of monomeric, catalytically active Streptococcus pneumoniae SrtA and reveal residue-specific interactions with a canonical LPNTG recognition peptide.

Structural determinants of endopilus assembly, stability, and functional specificity in bacterial type II secretion.

Key determinants of secretion specificity and endopilus stability are identified, revealing how minor sequence variations in conserved nanomachines drive functional adaptation to diverse environments.