Injectisomes are membrane-embedded, multiprotein assemblies used by bacterial pathogens to inject virulent effector proteins into eukaryotic host cells. Protein secretion is regulated by cytosolic proteins that dynamically bind and unbind at injectisomes.
The membrane-embedded injectisome, the structural component of the virulence-associated type III secretion system (T3SS), is used by gram-negative bacterial pathogens to inject species-specific effector proteins into eukaryotic host cells. The cytosolic injectisome proteins are required for export of effectors and display both stationary, injectisome-bound populations as well as freely-diffusing cytosolic populations. How the cytosolic injectisome proteins interact with each other in the cytosol and associate with membrane-embedded injectisomes remains unclear. Here, we utilize 3D single-molecule tracking to resolve distinct cytosolic complexes of injectisome proteins in living Yersinia enterocolitica cells. Tracking of the eYFP-labeled ATPase, YeSctN, and its regulator, YeSctL, reveals that these proteins form a cytosolic complex with each other and then further with YeSctQ. YeSctNL and YeSctNLQ complexes can be observed in both in wild type cells and in ΔsctD mutants, which cannot assemble injectisomes. In ΔsctQ mutants, the relative abundance of the YeSctNL complex is considerably increased. These data indicate that distinct cytosolic complexes of injectisome proteins can form prior to injectisome binding - a feature that could contribute to injectisome functional regulation.
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