sctD

SctD is a protein component of bacterial type III secretion systems (T3SS), a complex nanomachine used by many Gram-negative pathogens to inject effector proteins into host cells. In the architecture of the T3SS, SctD sits in the inner membrane and contributes to the basal body's inner ring, linking the cytoplasmic secretion machinery to the periplasm and the outer membrane ring formed by SctC.

Function and structure: SctD is an inner membrane protein that spans the inner membrane and interacts with other basal-body components to form a continuous secretion channel. It participates in assembly and stabilization of the basal body, enabling the substrate secretory path from cytoplasm to the extracellular milieu. While precise topology differs among species, SctD generally provides a structural scaffold and interfaces with cytoplasmic ATPases such as SctN and with outer-ring components to coordinate secretion.

Genetics and nomenclature: The gene is part of the T3SS gene clusters and is highly conserved. Across bacteria it is known by different names depending on the organism: escD in enteropathogenic and enterohemorrhagic Escherichia coli; yscD in Yersinia species; invD or sctD in other systems. SctD homologs are typically co-located with sctC (outer-ring), sctJ/SctK (inner components), and sctN (ATPase).

Importance: SctD is essential for proper assembly of the T3SS basal body and for efficient secretion of translocated effectors. Mutations or deletions impair basal-body formation, secretion, and virulence, making SctD a focus of studies on T3SS mechanisms and potential therapeutic targeting.