gspD

gspD is the gene encoding the secretin protein GspD, a component of the Type II secretion system (T2SS) found in many Gram-negative bacteria. GspD forms an outer membrane channel that enables the export of folded substrates from the periplasm to the extracellular environment, functioning as a critical gateway in the T2SS.

In the T2SS, GspD interacts with inner membrane components (such as GspC, GspF, and the ATPase GspE) and with pseudopilins (GspG to GspK). The secretion process is powered by ATP hydrolysis by GspE, which drives the assembly of a pseudopilus that helps propel substrates through the GspD channel. A pilotin, typically GspS, assists in the correct localization and assembly of GspD in the outer membrane.

Structurally, GspD contains an N-terminal periplasmic region and a C-terminal secretin domain that forms the outer membrane pore. Many GspD proteins assemble into oligomeric rings, commonly as dodecamers or tetradecamers, creating a large periplasmic vestibule that guides substrates into the pore.

Genetically, gspD is part of gsp operons (alongside gspC, gspE, gspF, gspG, and others) and has orthologs across diverse bacteria, including epsD in Vibrio species and PulD in various members of the Enterobacterales. The presence and integrity of gspD are typically essential for the secretion of extracellular enzymes such as proteases, lipases, and other virulence factors; loss of GspD often results in secretion defects and attenuated virulence.

Due to its central role in secretion, GspD is sometimes considered a potential target for anti-virulence strategies aimed at disarming pathogenic bacteria without killing them.