Component of the type III secretion system, which is required for effector protein delivery, parasitism, and pathogenicity. Probably participates in the formation of a C-ring-like assembly along with HrcQa (By similarity). Homotetramer. The four monomers assemble into two tightly bound homodimers. Interacts with HrcQa (By similarity). Or loosely associated with a membrane component, probably HrcQa. Negatively regulated by HrpV. The HrcQb-C domain interacts with the HrcQa C-terminal domain. Belongs to the FliN/MopA/SpaO family.