pilQ

PilQ is a secretin protein that forms the outer membrane component of the type IV pilus system in many Gram-negative bacteria. It oligomerizes to create a multimeric pore that spans the outer membrane, providing a conduit for pilus extension and retraction and, in some species, for the uptake of DNA during competence.

Structure and biogenesis

PilQ belongs to the secretin family and is synthesized with a signal peptide that targets it to the periplasm and outer membrane. It assembles into a ring-shaped complex, typically composed of a dozen to more than a dozen subunits, creating a central channel through which the pilus filament passes. The channel usually features an extended periplasmic vestibule and a C-terminal beta-barrel that anchors the complex in the outer membrane. Outer membrane localization and assembly are aided by a pilotin and by interactions that link the outer membrane secretin to the inner membrane assembly components.

Function and mechanism

The PilQ secretin forms the gateway for type IV pili, enabling the extrusion of pilus fibers to the cell surface and, in many bacteria, retraction as well. This outer membrane pore collaborates with inner membrane components of the pilus system to coordinate assembly and depolymerization of the pilus. In naturally competent species such as Neisseria meningitidis, PilQ is also involved in DNA uptake, linking pilus dynamics to genetic transformation.

Genetic context and significance

PilQ is encoded within pil gene clusters that include other assembly factors such as PilN, PilO, and PilP. The function of PilQ is essential for pilus-mediated adhesion, motility (twitching), biofilm formation, and, in some organisms, genetic exchange, making it a focal point in studies of bacterial pathogenesis and horizontal gene transfer.