YscD

YscD is a core component of the Ysc-Yop type III secretion system (T3SS) found in pathogenic Yersinia species, including Yersinia pestis, Y. enterocolitica, and Y. pseudotuberculosis. It is an inner membrane protein that contributes to the assembly and stability of the T3SS basal body, the multi-protein structure that spans the bacterial inner and outer membranes to deliver effector proteins into host cells.

The yscD gene is part of the virulence plasmid-encoded operon that supplies the core components of the Yersinia T3SS. YscD is thought to function as an architectural link within the basal body, helping to connect inner membrane components to the periplasmic assembly and to the outer membrane secretin complex. In this framework, YscD is believed to interact with periplasmic components such as YscJ and with the outer membrane secretin YscC, forming a continuous basal body that supports secretion.

Functionally, YscD is essential for proper assembly of the secretion apparatus and for the translocation of Yersinia Yop effector proteins into host cells. Mutations or deletions that disrupt YscD often result in defective secretion and attenuated virulence in mammalian models.

Conservation and context: YscD belongs to a broader family of type III secretion system inner membrane proteins (often referred to as SctD in homologous systems) and has orthologs across many Gram-negative bacteria with T3SS. This conserved role in basal body assembly underpins the virulence of Yersinia species and related pathogens.