secretins

Secretins are a family of outer membrane proteins found in many Gram-negative bacteria. They form multimeric, channel-forming pores that span the outer membrane and function as essential components of several secretion systems. The most prominent role is in the type II secretion system (T2SS), where secretins create the conduit through which folded proteins are exported from the periplasm to the extracellular environment. Secretins also participate in other systems that assemble extracellular appendages, such as certain type IV pilus systems, and in some type III secretion systems where they form the outer membrane channel for secreted substrates.

Structurally, secretins are large beta-barrel proteins with a conserved C-terminal secretin domain that assembles into a gated channel in the outer membrane. A periplasmic N-terminal region forms a vestibule that interacts with other components of the secretion apparatus. Secretins typically assemble into oligomeric rings, often comprising 12 to 15 subunits, to create a central pore. Localization and stability in the outer membrane are usually aided by pilotins, small lipoproteins encoded adjacent to secretin genes, which help insert and position the secretin correctly.

Diversity within the secretin family includes GspD/PulD/OutD components of T2SS, PilQ in type IV pilus systems, and InvG/SctC in some type III secretion systems. While the exact architecture can vary among systems, the common theme is a secretin channel that collaborates with inner membrane and periplasmic machinery to enable translocation of substrates.

Note: The term secretin in this context is distinct from the gastrointestinal hormone secretin, a peptide that regulates pancreatic and bile secretion.