Secretins: dynamic channels for protein transport across membranes

Abstract

Secretins form mega-Dalton bacterial membrane channels in at least four sophisticated multi-protein systems that are crucial for translocation of proteins and assembled fibers across the outer membrane of many species of bacteria. Secretin subunits contain multiple domains, which interact with numerous other proteins, including pilotins, secretion system partner proteins and exoproteins. Our understanding of the structure of secretins is rapidly progressing, and we now recognize that features common to all secretins include a cylindrical arrangement of 12–15 subunits, a large periplasmic vestibule with a wide opening on one end and a periplasmic gate at the other end. Secretins might also play a key role in the biogenesis of their cognate secretion systems.

Structure and assembly of an inner membrane platform for initiation of type IV pilus biogenesis

“…An important clue comes from work on the role of secretin-binding proteins in T4P biogenesis and the T2SS in other bacteria. Secretins form channels in the outer membrane for the passage of secreted proteins and assembled T4P (31). The T2SS secretin GspD and the T4P secretin PilQ form dodecameric assemblies enclosing chambers through which the secreted proteins pass (8, 32).…”

Unusual N-terminal ααβαββα Fold of PilQ from Thermus thermophilus Mediates Ring Formation and Is Essential for Piliation

“…Secretin subunits exhibit multiple domains specifically acting in substrate recognition, interaction with other proteins, or regulation of transport, respectively, thereby forming characteristic cylindrical structures comprising a large N-terminal periplasmic vestibule and an extracellular chamber containing the secretin-specific periplasmic gate formed by the conserved C-terminal secretin domains (35). Recent crystallographic studies of domains of the type II protein secretion system secretin GspD from enterotoxigenic E. coli and the type III protein secretion system secretins EscC and InvG from enteropathogenic E. coli and Salmonella typhimurium provided insights into the domain structure, indicating general ring- (PilQ ⌬25-34 ), -pilQ ⌬25-64 his (PilQ ⌬25-64 ), or -pilQ ⌬25-125 his (PilQ ⌬25-125 ) were stab-inoculated on minimal medium plates containing 1% BSA and incubated for 3 days at 68°C under humid conditions.…”

A comprehensive guide to pilus biogenesis in Gram-negative bacteria

“…2a). The OM secretin subcomplex includes PilQ, a multimeric OM pore protein through which the growing pilus is assembled and disassembled [65][66][67][68] , and the pilotin protein The final structural component completing the T4P system is the helical pilus filament, composed of major and minor pilin subunits and adhesin molecules 81,82 (FIG. 2d).…”