pseudopili

Pseudopili are pilus-like fiber structures associated with certain bacterial secretion systems, most notably the type II secretion system (T2SS) and related assembly machineries. They are distinguished from true pili by their primary function and subunit composition: pseudopili are components of secretion apparatus rather than organelles for adhesion or twitching motility.

Molecular composition and structure: A pseudopilus is assembled from pseudopilin subunits. The major pseudopilin GspG (often with homologs such as PulG) forms the core polymer, while minor pseudopilins GspH, GspI, GspJ, and GspK accompany it. These proteins are embedded in the inner membrane or exported to the periplasm, where they assemble into a fiber that extends toward the periplasmic face and through the outer membrane via the secretin channel. The assembly is supported by a cytoplasmic ATPase (GspE, related to PilB of type IV pilus systems) and an inner-membrane platform formed by GspF, GspL, and GspM.

Function and mechanism: The pseudopilus acts as a piston-like structure that helps push secreted substrate proteins through the outer membrane secretin pore (GspD) as part of the secretion cycle. It is a dynamic, transient fiber that can extend and retract during secretion but is not used for surface attachment or motility.

Distribution and significance: Pseudopili are found in a broad range of Gram-negative bacteria and are essential for the proper function of the T2SS, which secretes enzymes and toxins implicated in virulence. Although related to the machinery behind type IV pili, pseudopili are specialized for secretion rather than pilus-based adhesion.