SctQ

SctQ is a cytosolic protein component of bacterial Type III secretion systems (T3SS). These systems enable Gram-negative bacteria to inject effector proteins into host cells and are essential for virulence in many pathogens. SctQ is part of the conserved cytoplasmic sorting platform that sits at the base of the T3SS and is present in many bacterial species, with homologs such as YscQ in Yersinia and SpaQ in Shigella.

Functions of SctQ center on substrate recognition and delivery to the secretion channel. It forms a complex with other cytosolic components, notably SctK and SctL, and it interacts with the ATPase SctN that powers secretion. Through these interactions, SctQ helps coordinate the recruitment and secretion of effector and translocator proteins, contributing to the timing and substrate specificity of the secretion process. Mutations in SctQ often reduce or abolish secretion and can attenuate virulence in animal models, underscoring its essential role in T3SS function.

Structure and dynamics of SctQ are not yet fully resolved. It is believed to assemble into dynamic, multimeric cytosolic complexes that associate with the inner membrane–embedded basal body. The protein likely shuttles between the cytosol and the sorting platform as secretion proceeds, allowing the system to adapt to different stages of assembly and secretion.

Significance beyond basic biology includes the role of SctQ in pathogenicity and as a potential target for anti-virulence strategies. By disrupting the sorting platform, it may be possible to impair toxin delivery without killing the bacteria, offering an approach to mitigate bacterial infections that rely on T3SSs.