Table 1.

Properties of Dedicated Protein Translocation Systems in Bacteria.

System	Distribution	Translocated substrates			Translocation requirements		Machine properties
		Types	Destination(s)	Function	Secretion signal(s)	Additional requirements	OM channel	IM channel	Requirements for biogenesis	Energetics
Class I: Two-step translocation systems—assemble only at the OM and translocate substrates from the periplasm across the OM
Type 5 (Auto transporters)	Gram-negative; mainly studied in Proteobacteia	Monomric/trimeric passenger domains; two-partner systems	Cell surface; extracellular milieu	Adhesin; toxins; proteases; receptor-binding proteins	N-termina Sec; Bam complex targeting signals	Autoproteolytic release from cell surface for some substrates	Hybrid anchor domain/BamA β-barrel channel	GSP	SurA/Skp/DegP chaperones, Bam complex	Charge interaction; vectorial folding TamA/B complex for subset of T5SSs
C-U pilus assembly	Gram-negative; Enterobacteriaceae	Type I/Pap pili CFA pili; CS pili; Afa/Dr fibers; F1 capsular antigen	Cell surface	Adhesion; Aggregation; Biofilms; IBCs; Activation of host cell pathways	N-terminal Sec; signal; Chaperone-pilin binding to usher	Donor strand complementation(DSC) for chaperone-pilin-contacts; Donor strand exchange(DSE) for pilin-pilin interactions	Usher: 24-strandei β-barrel with substrate binding/& pilus assembly domains	GSP	Bam-dependent Insertion of usher into OM; Dedicated chaperone binds pilins	Chaperone-pilin/usher binding affinities; Pilus subunit-subunit interaction affinities; entropy-based diffusion; evidence for a role of the TamA/B complex
Type 8(Curii)	Gram-negative; Proteobacteria, Bacteriodetes	Curli amyloid fibers	Cell surface; extracellular milieu	Adhesion biofilms; Host colonization; Innate response activation	N-terminal Sec signal; Chaperone-pilin binding to CsgG translocon	Charge-based chaperone-pilin and pilin-pilin interactions	CsgG: 36-stranded β-barrel; CsgE gate CsgB/F curli nucleator	GSP	LOL sorting/Bam-independent insertion of OM translocon; dedicated chaperone binds pilins	Peptide diffusion; entropy free-energy gradient
Class II: Two-step translocation systems—assemble across entire cell envelope but translocate substrates from the periplasm across the OM
Type 2	Gram-negative; Proteobacteria	Monomeric, multimeric proteins	Cell surface via lipid or other attachment; Extracellular milieu	Toxins; lipases, other lytic enzymes; biofilm matrix components	N-terminal Sec or Tat; signal for binding to pseudopilus	Substrate folding in the periplasm; Recruitment to pseudopilus tip for extrusion through OM secretin channel	Secretin: 15 copies of GspD::GspS complexes	GSP or Tat	LOL-sorting & Bam-independent OM insertion; Secretin assembles first and stabilizes IM platform & GspE ATPase; Pseudopilus assembles for substrate extrusion	GspE ATPase drives pseuodopilus assembly
T4P	Gram-negative & -positive; Archaea	Dynamic type 4 pili; some T4Ps export exoproteins	Cell surface	Attachment; Bio-films; Twitching motility; Archaeal T4Ps can function as flagella	N-termina Sec for insertion of pilin into IM; Signals for pilin extrusion from and reinsertion into IM	T4P assembly on IM/CM platform; Pilus extension through G-negative secretin channel or G -positive cell wall	T2SS GspD secretin	GSP for insertion of pilins into IM	IM platform and GspE/PilT ATPases for pilus assembly/retraction	GspE/PilaT ATPases & PMF for Pilus extension/retraction; PilT-independent retraction in some systems
Type 9	Gram-negative; Bacteriodetes	Monomeric proteins, including very large (~670 kDa SprB) adhesins	Cell surface; Extracellular milieu	Adhesion; Polymer degradation; Biofilms; Gliding motility: SprB adhesin moves rapidly between cell poles along closed helical loop	N-terminal Sec;Conserved CTDs for recruitment to T9SS in periplasm	Cell surface anchoring to acidic LPS by 'sortase-like' mechanism	SprA/sov: 36-strand β-barrel; PorV complex cleaves CTDs and attaches substrates to LPS	GSP	SprA/sov channel regulated by PorN,P,K,V; OM channel physically linked to IM-spanning PorM/L complex	IM PMF; secretion coupled to gliding motility
Class III: One-step translocation systems—span Gram-negative cell envelope and translocate substrates without a periplasmic intermediate
Type 1	Gram-negative	Monomeric unfolded proteins	Cell surface; Extracellular milieu	Adhesin; Proteases; Lipases; Heme-binding	C-terminal signal forbinding to MFP/ATPase complex	Lap/Iba substrates are released by environmentally-induced, post-translocation cleavage	TolC-like channel/tunnel	ATPase/MFP complex	Substrate and ATP-binding induced channel activation and recruitment of TolC	ABC ATPase; Ca2+-mediated protein folding to prevent backsliding
Type 3: Injectisomes and flagella	Gram-negative; Gram-positive flagella	Monomeric unfolded proteins	Eukaryotic cell Flagellum assembly	Effector translocation disrupts various eukaryotic cell pathways and physiological processes; Flagellar-based motility	N-terminal peptide signal or 5′ RNA signal	Ordered contacts with sorting platform; ATPase-mediated unfolding; Target-cell-contact mediated machine activation	Secretion channel through which needle complex extrudes; L-/P-rings for Flagella assembly	ATPase/sorting platform/Export apparatus	IM complex and OM secretin complex form and join together as the needle complex (NC). NC is a scaffold for substrate sorting platform and injection needle	InvC-Like ATPase for substrate unfolding; PMF for translocation
Type 4	Gram-negative and -positive; Archaea	Monomeric unfolded proteins; Multimeric A/B5 toxin, Single-stranded DNA-relaxase intermediates of MGEs	Bacterial or eukaryotic cells; Extracellular milieu; Surface-displayed conjugative pili	Conjugative DNA transfer; Effector translocation disrupts eukaryotic cell pathways and physiological processes; interbacterial toxin transmission; pilus-mediated adherence and biofilm development	C-terminal charged or hydrophobic, or internal motifs; Two-step translocation for PT export by B. pertussis Ptl system	Substrate docking & target-cell-contact mediated machine activation; Some systems elaborate conjugative pili	α-Helical OM pore connected to barrel/disc-shaped OMC	IMC:IM platform; VirB4, VirD4 ± VirB11 ATPases	LOL-sorting/Bam-independent insertion of OMC into OM; OMC assembles & stabilizes IMC and VirB4; VirB11 and VirD4 dock stably or transiently with IMC	ATPase- & substrate-induced conformational changes & PMF
Type 6	Gram-negative	Monomeric proteins; may be covalently bound to machine components	Bacterial or eukaryotic cells	Effectors modulate eukaryotic cell processes; anti-bacterial and anti-eukaryotic cell toxins	Signals for effector docking with sheath, tube, or spike proteins; Covalent binding of effectors with these machine subunits	Target-cell-contact mediated machine activation	TssL/J/M membrane complex	TssL/J/M membrane complex/Baseplate	Sheath/tube assembly & contraction	PMF for sheath/tube contraction; ClpV ATPase-mediated recycling of sheath components
Class IV: One-step translocation systems-span Gram-positive cytoplasmic membrane and translocate substrates to cell surface or beyond
Type 7	Gram-positive Mycobacteria, Actinobacteria, Firmicutes	Monomeric proteins; Homo- and hetero-dimers; WxG1OO & PPE/PE proteins	Cell surface; Extracellular milieu	Membrane permeabilization; Virulence; DNA conjugation; sliding motility; other?	C-terminal signals mediate binding to EccC ATPase	Homo- or hetero-dimer formation for some substrates	Two-step mechanism postulated for translocation across mycobacterial mycolic membrane	EccB/C/D/E membrane complex; VirD4- like EccC has 3 ATPase domains	Machine assembly & translocation mechanism unknown	EccC ATPase-mediated translocation