pseudopilus

Pseudopilus is a short, pilus-like filament found in certain Gram-negative bacteria as a component of the type II secretion system (T2SS) and related pathways. It is composed primarily of pseudopilin proteins, including a major pseudopilin and several minor pseudopilins, which are typically processed by the prepilin peptidase PilD and assembled at the cytoplasmic membrane.

The pseudopilus is believed to function as a piston-like structure that drives the secretion of folded substrates through the outer membrane secretin channel (GspD). Unlike bona fide type IV pili, the pseudopilus is generally not observed as a long, surface-exposed appendage; its exact topology may be limited to the periplasmic and inner membrane intervals, with some models proposing extension into the periplasm to push substrates outward.

Pseudopili are required for the secretion of a wide range of substrates, including proteases, lipases, and other virulence factors in pathogenic bacteria, as well as enzymes used in nutrient acquisition. The assembly is powered by the cytoplasmic ATPase GspE, which fuels polymerization of pseudopilin subunits, and disassembly or turnover allows cycles of secretion.

In contrast to type IV pili used for adhesion and motility, pseudopili specialize in secretion through the secretin complex and do not typically mediate surface motility. The study of pseudopili continues to clarify their precise architecture, dynamics, and range of substrates across species.