T3SS

The type III secretion system (T3SS) is a specialized protein complex used by many Gram-negative bacteria to deliver effector proteins directly into the cytoplasm of eukaryotic host cells. It is sometimes called an injectisome when referring to pathogenic systems that inject effectors to manipulate host processes, and it shares evolutionary roots with the bacterial flagellum.

Structurally, the T3SS forms a multi-subunit apparatus that spans the bacterial inner and outer membranes and extends a needle-like structure toward the host cell. The basal body provides a conduit through the cell envelope, while the needle and tip contact the host membrane and recruit translocators to form a pore in the host plasma membrane. A cytosolic secretion and sorting platform coordinates substrate recognition and delivery. Energy for secretion is provided by ATP hydrolysis and the proton motive force across the inner membrane, and secretion substrates are often escorted by dedicated chaperones to the apparatus.

Substrates are organized into secretion classes, including early assembling components, translocators that form the host membrane pore, and effector proteins that function inside the host cell to alter signaling, cytoskeletal dynamics, vesicular trafficking, and immune responses. Secretion is typically triggered by contact with a host cell or environmental cues and is tightly regulated to ensure timely delivery.

T3SSs are central to the virulence of numerous pathogens, such as Salmonella, Shigella, Yersinia, and Pseudomonas, as well as plant pathogens like Xanthomonas. They are distinct from other secretion systems and have been explored as antimicrobial targets and as tools for delivering proteins into cells in research contexts. Some bacterial systems use a flagellar type III secretion mechanism derived from the same ancestral complex.