Table 1.

Structural components of the flagellar and injectisome T3SS. As the nomenclature for injectisome components is currently species-specific, we will use the unified Sct nomenclature [24] in this review. Where no Sct name exists, we will use the Ysc nomenclature (denoted by *). Refer to electronic supplementary material, table S1 for a list of names of injectisome components in specific organisms. Grey shading denotes homologous proteins, italics denotes that the protein can be exported by the T3SS. Extra, extracellular location; OM, outer membrane; PP, periplasm; IM, inner membrane; cyto, cytosolic.

cellular locationflagellar componentinjectisome componentdegree of similarityafunction
extraFliDFilament-capping protein required for proper folding of the flagellin
extraFliCFlagellin forming the filament
extraFlgLHook-filament junction proteins connecting the filament and the hook
extraFlgK
extraYopD*Pore-forming hydrophobic translocators
extraYopB*
extraLcrV*Tip protein (hydrophilic translocator)
extraFlgDHook-capping protein, scaffold protein required for hook assembly
extraFlgESctFlowHook/needle subunit, helical arrangement with 5–6 subunits per turn
OMSctCSecretin ring in the OM, requires assistance of pilotin lipoprotein for integration in OM
OMFlgHL-ring, lipoprotein, part of bushing
PPFlgIP-ring, part of bushing
PPSctIInner rod in the injectisome. Exported T3SS substrate in some systems. Involved in needle anchoring and/or length determination, not required for connection of the membrane rings
PPFlgJRod-capping protein with muramidase function
PPFlgBFlagellar rod/transmission shaft. Requires functional T3SS export for assembly
PPFlgC
PPFlgD
PPFlgF
PPFliE
IMSctDBitopic outer MS ring protein, connecting the secretin (SctC) in the OM and the inner MS ring (SctJ) in the IM
IMFliFSctJmediumMS ring protein, thought to surround the export apparatus. Large bitopic protein in the flagellum, significantly smaller and predominantly or completely periplasmic in the injectisome
IMbFliOFlagellum-specific IM export apparatus protein regulating FliP during assembly of the flagellum
IMbFliPSctRhighIM export apparatus proteins, mainly transmembrane helices and periplasmic domains, possibly forming the pore in the IM
IMbFliQSctShigh
IMbFliRSctThigh
IMbFlhBSctUhighIM export apparatus protein with C-terminal cytosolic domain, involved in substrate specificity switch upon hook/needle completion
IMbFlhASctVhighIM export apparatus protein with large C-terminal cytosolic domain. Present as a multimer, probably forming a nonameric ring
cytoFliJSctOlowStalk protein with homology to γ subunit of FoF1-ATP synthase. Also shown to bind empty chaperones (‘chaperone escort’ function). T3SS substrate in some systems
cytoFliISctNhighHexameric ATPase, probably required for chaperone release and substrate unfolding
cytoFliHSctLmediumNegative regulator of ATPase, also shown to interact with stalk and large export apparatus component, part of injectisome sorting platform
cytoFliGPart of the flagellar switch complex controlling the rotation direction (together with FliM and FliN)
cytoFliMSctQlowForming the cytosolic C-ring, part of the switch complex in the flagellum and the injectisome sorting platform
cytoFliNSctQChighPart of the cytosolic C-ring. SctQC is expressed from an internal translation initiation site in SctQ [25,26]. FliM + FliN and SctQ + SctQC form stable complexes (with a 1 : 4 or 1 : 2 ratio, respectively)
cytoSctKAccessory protein interacting with the C-ring, part of injectisome sorting platform.
  • aDegree of similarity between flagellar and injectisome components in Y. enterocolitica (see electronic supplementary material, table S2 for details): high, E < 10−5; medium, 10−5 < E < 0.01; low, E > 0.01.

  • bLocated within MS ring, most likely in a membrane-like environment.