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

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.

Type III secretion system chaperones: a helping hand for secretion

The extent to which chaperones are universally required for effector secretion is evaluated, through analysis of the conservation of chaperone sequence and structure, to discuss how these proteins interact with and support the secretion of diverse substrates.

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

It is postulate that Skp in Neisseria must utilize a different mechanism than in E. coli to stabilize substrates by expanding and contracting along its long axis to accommodate substrates of differing sizes.

Discovery of a regulatory node that coordinates cell envelope assembly in mycobacteria

Mycobacteria possess a complex double-membrane cell envelope critical for survival and pathogenesis. Proper assembly of this architecture requires the biosynthesis and transport of major components, including arabinogalactan (AG) polysaccharides and mycolic acids (MAs), but how these processes are effectively coordinated is unknown. Here, we discover an essential membrane complex that serves as a regulatory node in mycobacterial envelope biogenesis. The acyltransferase TmaT and the arabinofuranosyltransferase AftD physically interact; cryo-EM structures reveal a 1:1 stoichiometry, and present a novel fold for TmaT, featuring a central channel that binds co-factor for acetylation in the periplasm. We establish that the TmaT-AftD interaction, and the catalytic activities of both enzymes, are required for MA transport across the cell envelope, as well as AG ligation to the cell wall, the final stage of AG biosynthesis. The TmaT-AftD complex coordinates the two major envelope assembly pathways, presenting a structural vulnerability for future anti-mycobacterial drug development.

Crystal structure of pilus-specific sortase from Enterococcus faecalis reveals both open and closed conformations: insights into loop dynamics and substrate recognition.

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.