Pseudopilin residue E5 is essential for recruitment by the type 2 secretion system assembly platform.

Type II secretion systems (T2SSs) promote secretion of folded proteins playing important roles in nutrient acquisition, adaptation and virulence of Gram-negative bacteria. Protein secretion is associated with the assembly of type 4 pilus (T4P)-like fibres called pseudopili. Initially membrane embedded, pseudopilin and T4 pilin subunits share conserved transmembrane segments containing an invariant Glu residue at the fifth position, E5. Mutations of E5 in major T4 pilins and in PulG, the major pseudopilin of the Klebsiella T2SS abolish fibre assembly and function. Among the four minor pseudopilins, only PulH required E5 for secretion of pullulanase, the substrate of the Pul T2SS. Mass-spectrometry analysis of pili resulting from the co-assembly of PulG(E5A) variant and PulG(WT) ruled out an E5 role in pilin processing and N-methylation. A bacterial two-hybrid analysis revealed interactions of the full-length pseudopilins PulG and PulH with the PulJ-PulI-PulK priming complex and with the assembly factors PulM and PulF. Remarkably, PulG(E5A) and PulH(E5A) variants were defective in interaction with PulM but not with PulF, and co-purification experiments confirmed the E5-dependent interaction between native PulM and PulG. These results reveal the role of E5 in a recruitment step critical for assembly of the functional T2SS, likely relevant to T4P assembly systems.

Type II secretion system: a magic beanstalk or a protein escalator.

Type II protein secretion systems (T2SS) are molecular machines that promote specific transport of folded periplasmic proteins in Gram-negative bacteria, across a dedicated channel in the outer membrane. Secreted substrates, released to the milieu or displayed on the cell surface, contribute to bacterial adaptation to a range of habitats, from deep-sea waters to animal and plant tissues. The past decade has seen remarkable progress in structural, biochemical and functional analysis of T2SS and related systems, bringing new mechanistic insights into these dynamic complexes. This review focuses on recent advances in the field, and discusses open questions regarding the secretion mechanism. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Structure of the calcium-dependent type 2 secretion pseudopilus.

Many Gram-negative bacteria use type 2 secretion systems (T2SSs) to secrete proteins involved in virulence and adaptation. Transport of folded proteins via T2SS nanomachines requires the assembly of inner membrane-anchored fibres called pseudopili. Although efficient pseudopilus assembly is essential for protein secretion, structure-based functional analyses are required to unravel the mechanistic link between these processes. Here, we report an atomic model for a T2SS pseudopilus from Klebsiella oxytoca, obtained by fitting the NMR structure of its calcium-bound subunit PulG into the ~5-Å-resolution cryo-electron microscopy reconstruction of assembled fibres. This structure reveals the comprehensive network of inter-subunit contacts and unexpected features, including a disordered central region of the PulG helical stem, and highly flexible C-terminal residues on the fibre surface. NMR, mutagenesis and functional analyses highlight the key role of calcium in PulG folding and stability. Fibre disassembly in the absence of calcium provides a basis for pseudopilus length control, essential for protein secretion, and supports the Archimedes screw model for the type 2 secretion mechanism.

Distinct docking and stabilization steps of the Pseudopilus conformational transition path suggest rotational assembly of type IV pilus-like fibers.

The closely related bacterial type II secretion (T2S) and type IV pilus (T4P) systems are sophisticated machines that assemble dynamic fibers promoting protein transport, motility, or adhesion. Despite their essential role in virulence, the molecular mechanisms underlying helical fiber assembly remain unknown. Here, we use electron microscopy and flexible modeling to study conformational changes of PulG pili assembled by the Klebsiella oxytoca T2SS. Neural network analysis of 3,900 pilus models suggested a transition path toward low-energy conformations driven by progressive increase in fiber helical twist. Detailed predictions of interprotomer contacts along this path were tested by site-directed mutagenesis, pilus assembly, and protein secretion analyses. We demonstrate that electrostatic interactions between adjacent protomers (P-P+1) in the membrane drive pseudopilin docking, while P-P+3 and P-P+4 contacts determine downstream fiber stabilization steps. These results support a model of a spool-like assembly mechanism for fibers of the T2SS-T4P superfamily.

The type II secretion system - a dynamic fiber assembly nanomachine.

Type II secretion systems (T2SSs) share common origins and structure with archaeal flagella (archaella) and pili, bacterial competence systems and type IV pili. All of these systems use a conserved ATP-powered machinery to assemble helical fibers that are anchored in the plasma membrane. The T2SSs assemble pseudopili, periplasmic filaments that promote extracellular secretion of folded periplasmic proteins. Comparative analysis of T2SSs and related fiber assembly nanomachines might provide important clues on their functional specificities and dynamics. This review focuses on recent developments in the study of pseudopilus structure and biogenesis, and discusses mechanistic models of pseudopilus function in protein secretion.