Pseudopilins

Pseudopilins are a group of small, pilin-like proteins that assemble into a pseudopilus, a dynamic filament used by the type II secretion system (T2SS) in many Gram-negative bacteria. The pseudopilus is built from major and minor pseudopilins, most notably GspG (the major pseudopilin) and several minor subunits such as GspH, GspI, GspJ, and GspK. These proteins are encoded with other T2SS components, including the secretin GspD and the cytoplasmic ATPase GspE. Pseudopilins are typically synthesized as prepilins with an N-terminal leader peptide that is cleaved by the prepilin peptidase PilD; the mature subunits are then assembled at the inner membrane periplasmic face to form the pseudopilus filament.

Function: The pseudopilus functions as a piston-like filament that, powered by ATP hydrolysis through GspE, extends through the periplasm and into the secretin channel of the outer membrane. By polymerizing and disassembling dynamically, the pseudopilus is thought to push substrate proteins through the secretin pore into the extracellular space or outer membrane channel. The secretion typically carries hydrolytic enzymes, toxins, or other proteins exported by the T2SS.

Relation to type IV pili: Pseudopilins are homologous to type IV pilins and share a common structural motif, including an N-terminal alpha-helix that forms the filament core. However, pseudopili are generally shorter and assembled in the periplasm–outer membrane secretion system rather than as extracellular, retractable filaments.

Clinical and ecological significance: T2SS and its pseudopili are essential for the secretion of virulence factors in several pathogens and contribute to nutrient acquisition and interaction with hosts. Studying pseudopilins informs understanding of bacterial secretion and pilus-related processes.