SctV

SctV is a core component of bacterial type III secretion systems (T3SS), acting as part of the inner membrane export apparatus that forms the gate through which proteins are translocated from the bacterial cytoplasm into host cells. In many pathogens, SctV is encoded by the sctV gene and has homologs such as EscV in enteropathogenic and enterohemorrhagic Escherichia coli and InvA in Salmonella enterica.

Structure and organization: SctV is an integral inner membrane protein with multiple transmembrane segments that anchor the secretion apparatus in the inner membrane. A large cytoplasmic C-terminal domain participates in forming a ring-like assembly and interacts with the cytosolic ATPase SctN (also known as InvC) and with chaperone-substrate complexes. The full-length protein is challenging to crystallize, so much structural insight comes from the cytoplasmic domain and from studies of homologous systems, which indicate a nonameric ring architecture at the cytoplasmic face.

Function and mechanism: The export apparatus including SctV constitutes the secretion gate and energy coordination hub of the T3SS. SctN hydrolyzes ATP to power substrate unfolding and translocation, while SctV coordinates substrate recognition, chaperone release, and interaction with other basal body components. It participates in the hierarchical secretion of substrates, aiding the orderly secretion of early rod and needle proteins before late-effector proteins are delivered.

Clinical relevance: Type III secretion systems are virulence factors in many bacteria, and SctV is essential for secretion and pathogenicity. Because of its central role, SctV is considered a potential target for anti-virulence strategies.