1.B.20 The Two-partner Secretion (TPS) Family

The first member of the TPS family to be characterized was the ShlB (HlyB) protein of Serratia marcescens which exports the ShlA hemolysin from the periplasm of the Gram-negative bacterial envelope into the external medium (Poole et al., 1988). ShlA reaches the periplasm by export from the cytoplasm via the general secretory pathway (GSP or IISP; TC #3.A.5). ShlB and some, but not all TPS homologues, include domains with both an outer membrane export channel and a 'hemolysin activator protein' which activates ShlA by derivatization with phosphatidyl ethanolamine (Hertle et al., 1997). Several ShlB homologues have been functionally characterized (Hirono et al, 1997; Jacob-Debuisson et al., 1997, Jacob-Debuisson et al., 2000; Palmer and Munson, 1995). The channel activities of some of these homologues have been demonstrated (Jacob-Debuisson et al., 1999; Kànninger et al., 1999), and topological features of these β-barrel porins have been studied (Guédin et al., 2000; Kànninger et al., 1999). Specificity for particiular protein substrates has been demonstrated (Jacob-Debuisson et al., 1997). One such protein, FhaC of B. pertussis, exhibits a surface exposed N-terminus and an odd number of β-strands with large surface loops and small periplasmic loops (Guédin et al., 2000; Kànninger et al., 1999; Méli et al., 2006).

Substrates of TPS family secretins include Ca+-independent cytolysins, an ion acquisition protein and several adhesins. The hallmarks of TPS systems are the presence of (1) an N-proximal module where specific secretion signals in the substrate protein are found, and (2) a β-barrel channel (TpsB) homologue (Jacob-Debuisson et al., 2000). Usually, the genes encoding these two proteins occur within an operon. While transport via the GSP occurs in the unfolded state, the substrate protein probably folds in the periplasm and on the periplasmic surface of the outer membrane before it is exported via the TPS porin (Jacob-Debuisson et al., 2000). Evidence for secretion-dependent folding of mature exoproteins has also been obtained (Mazar and Cotter, 2007).

FhaC of B. pertussis, the TpsB protein that transports the TpsA partner, FHA, exhibits a surface-exposed N-terminus and 16 β-strands with large surface loops and small periplasmic turns (Méli et al., 2006; Clantin et al., 2007). These features may be characteristic of the family. Surface exposed residues and pore formation in artificial membranes have been characterized with FhaC and the ShlB outer membrane porins. Méli et al. (2006) reported that FhaC (TpsB) exhibits ion channel properties, and mutants altered for FHA (TpsA) transport affected ion channel activity. The N-terminal 200 residues probably form a functionally distinct domain that modulates the pore properties and may participate in FHA recognition. The C-terminal two-thirds of TpsB forms the transmembrane channel-forming β-barrel domain. A C-proximal motif (the family signature sequence) appeared to be essential for pore formation (Méli et al., 2006). FHA moves through the periplasm in an extended conformation maintained by the DegP chaparone before it is exported and folds into a β-helical structure (Baud et al., 2009).

In Gram-negative bacteria and eukaryotic organelles, ß-barrel proteins of the outer membrane protein YaeT (TC#1.B.33) / two-partner secretion B (TC#1.B.20) (Omp85-TpsB) superfamily are essential components of protein transport machineries. The TpsB transporter, FhaC (TC# 1.B.20.2.1) mediates the secretion of Bordetella pertussis filamentous hemagglutinin (FHA). The 3.15 Å crystal structure of FhaC has been reported (Clantin et al., 2007). The transporter comprises a 16-stranded ß barrel that is occluded by an N-terminal {alpha} helix and an extracellular loop and a periplasmic module composed of two aligned polypeptide-transport–associated (POTRA) domains. Functional data reveal that FHA binds to the POTRA 1 domain via its N-terminal domain and likely translocates the adhesin-repeated motifs in an extended hairpin conformation, with folding occurring at the cell surface. General features of the mechanism are likely to apply throughout the superfamily.

Proteins showing large regions of sequence similarity to established members of the TPS family have been identified in Gram-positive bacteria, yeast, plants and animals. They clearly share homologous domains. In Gram-negative bacteria, the two-partner secretion (TPS) pathway is dedicated to the secretion of large, mostly virulence-related proteins. The secreted TpsA proteins carry a characteristic 250-residue-long N-terminal 'TPS domain' essential for secretion, while their TpsB transporters are pore-forming proteins that specifically recognize their respective TpsA partners and mediate their translocation across the outer membrane. Bordetella pertussis secretes its major adhesin filamentous haemagglutinin (FHA) via the TpsB transporter, FhaC (TC# 1.B.20.2.1). Specific interactions between an N-terminal fragment of FHA containing the TPS domain and FhaC occur (Hodak et al., 2006). FhaC recognizes only non-native conformations of the TPS domain, and in vivo, periplasmic FHA is not folded. Interaction determinants forming the secretion signal have been identified (Hodak et al., 2006). They are found far into the TPS domain and include both conserved and variable residues, which most likely explains the specificity of the TpsA-TpsB interaction. The N-terminal domain of FhaC is involved in the FHA-FhaC interaction, in agreement with its proposed function and periplasmic localization.

In Gram-negative bacteria, most surface-associated proteins are present as integral outer-membrane proteins. Exceptions include the Haemophilus influenzae HMW1 and HMW2 adhesins and a subset of other proteins secreted by the two-partner secretion system. HMW1 forms hair-like fibres on the bacterial surface and is usually present as pairs that appear to be joined together at one end. HMW1 is anchored to the multimeric HMW1B outer membrane translocator, resulting in a direct correlation between the level of surface-associated HMW1 and the quantity of HMW1B in the outer membrane. Anchoring of HMW1 requires the C-terminal 20 amino acids of the protein and is dependent upon disulphide bond formation between two conserved cysteine residues in this region (Buscher et al., 2006). The immediate C-terminus of HMW1 is inaccessible to surface labelling, suggesting that it remains buried in HMW1B. These observations may have broad relevance to many proteins secreted by the two-partner secretion system, especially given the conservation of C-terminal cysteine residues among surface-associated proteins in this family. 

Omp85 transporters and Two Partner Secretion (TPS) systems have a single conserved architecture, with POTRA domains that interact with substrate proteins, a 16-stranded transmembrane beta barrel, and an extracellular loop, L6, folded back in the barrel pore. Guérin et al. 2015 showed that the L6 loop of  FhaC changes conformation and modulates channel opening. Those conformational changes involve breaking the conserved interaction between the tip of L6 and the inner beta-barrel wall. The membrane-proximal POTRA domain also exchanges between several conformations, and the binding of FHA displaces this equilibrium. There is dynamic, physical communication between the POTRA domains and L6 within the beta barrel (Guérin et al. 2015).

The transport reaction catalyzed by bacterial members of the TPS family is:

Partially folded protein (periplasm) → Folded protein (external milieu)



This family belongs to the Outer Membrane Pore-forming Protein (OMPP) Superfamily I.

 

References:

Aoki, S.K., J.C. Malinverni, K. Jacoby, B. Thomas, R. Pamma, B.N. Trinh, S. Remers, J. Webb, B.A. Braaten, T.J. Silhavy, and D.A. Low. (2008). Contact-dependent growth inhibition requires the essential outer membrane protein BamA (YaeT) as the receptor and the inner membrane transport protein AcrB. Mol. Microbiol. 70: 323-340.

Aoki, S.K., R. Pamma, A.D. Hernday, J.E. Bickham, B.A. Braaten, and D.A. Low. (2005). Contact-dependent inhibition of growth in Escherichia coli. Science 309: 1245-1248.

Basso, P., M. Ragno, S. Elsen, E. Reboud, G. Golovkine, S. Bouillot, P. Huber, S. Lory, E. Faudry, and I. Attrée. (2017). Pseudomonas aeruginosa Pore-Forming Exolysin and Type IV Pili Cooperate To Induce Host Cell Lysis. MBio 8:.

Baud, C., H. Hodak, E. Willery, H. Drobecq, C. Locht, M. Jamin, and F. Jacob-Dubuisson. (2009). Role of DegP for two-partner secretion in Bordetella. Mol. Microbiol. 74: 315-329.

Baud, C., J. Guérin, E. Petit, E. Lesne, E. Dupré, C. Locht, and F. Jacob-Dubuisson. (2014). Translocation path of a substrate protein through its Omp85 transporter. Nat Commun 5: 5271.

Brumbach, K.C., B.D. Eason, and L.K. Anderson. (2007). The Serratia-type hemolysin of Chromobacterium violaceum. FEMS Microbiol. Lett. 267: 243-250.

Buscher, A.Z., S. Grass, J. Heuser, R. Roth, and J.W. St Geme JW III. (2006). Surface anchoring of a bacterial adhesin secreted by the two-partner secretion pathway. Mol. Microbiol. 61: 470-83.

Choi, P.S., A.J. Dawson, and H.D. Bernstein. (2007). Characterization of a novel two-partner secretion system in Escherichia coli O157:H7. J. Bacteriol. 189: 3452-3461.

Clantin, B., A.S. Delattre, P. Rucktooa, N. Saint, A.C. Méli, C. Locht, F. Jacob-Dubuisson, and V. Villeret. (2007). Structure of the Membrane Protein FhaC: A Member of the Omp85-TpsB Transporter Superfamily. Science. 317(5840):957-961.

Delattre, A.S., B. Clantin, N. Saint, C. Locht, V. Villeret, and F. Jacob-Dubuisson. (2010). Functional importance of a conserved sequence motif in FhaC, a prototypic member of the TpsB/Omp85 superfamily. FEBS J. 277: 4755-4765.

Duret, G., M. Szymanski, K.J. Choi, H.J. Yeo, and A.H. Delcour. (2008). The TpsB Translocator HMW1B of Haemophilus influenzae Forms a Large Conductance Channel. J. Biol. Chem. 283: 15771-15778.

Elsen, S., P. Huber, S. Bouillot, Y. Couté, P. Fournier, Y. Dubois, J.F. Timsit, M. Maurin, and I. Attrée. (2014). A type III secretion negative clinical strain of Pseudomonas aeruginosa employs a two-partner secreted exolysin to induce hemorrhagic pneumonia. Cell Host Microbe 15: 164-176.

Faure, L.M., S. Garvis, S. de Bentzmann, and S. Bigot. (2014). Characterization of a novel two-partner secretion system implicated in the virulence of Pseudomonas aeruginosa. Microbiology 160: 1940-1952.

Guédin, S., E. Willery, J. Tommassen, E. Fort, H. Drobecq, C. Locht and F. Jacob-Dubuisson (2000). Novel topological features of FhaC, the outer membrane transporter involved in the secretion of the Bordetella pertussis filamentous hemagglutinin. J. Biol. Chem. 275: 30202-30210.

Guerin J., Baud C., Touati N., Saint N., Willery E., Locht C., Vezin H. and Jacob-Dubuisson F. (2014). Conformational dynamics of protein transporter FhaC: large-scale motions of plug helix. Mol Microbiol. 92(6):1164-76.

Guerin J., Saint N., Baud C., Meli AC., Etienne E., Locht C., Vezin H. and Jacob-Dubuisson F. (2015). Dynamic interplay of membrane-proximal POTRA domain and conserved loop L6 in Omp85 transporter FhaC. Mol Microbiol. 98(3):490-501.

Hertle, R. (2005). The family of Serratia type pore forming toxins. Curr. Prot. Pept. Sci. 6: 313-325.

Hertle, R., S. Brutsche, W. Groeger, S. Hobbie, W. Kock, U. Könninger, and V. Braun. (1997). Specific phosphatidylethanolamine dependence of Serratia marcescens cytotoxin activity. Mol. Microbiol. 26: 853-865.

Hirono, I., N. Tange, and T. Aoki. (1997). Iron-regulated haemolysin gene from Edwardsiella tarda. Mol. Microbiol. 24: 851-856.

Hodak, H., B. Clantin, E. Willery, V. Villeret, C. Locht, and F. Jacob-Dubuisson. (2006). Secretion signal of the filamentous haemagglutinin, a model two-partner secretion substrate. Mol. Microbiol. 61: 368-382.

Jacob-Dubuisson, F., B. Kehoe, E. Willery, N. Reveneau , C. Locht, and D.A. Relman. (2000). Molecular characterization of Bordetella bronchiseptica filamentous haemagglutinin and its secretion machinery. Microbiology 146: 1211-1221.

Jacob-Dubuisson, F., C. Buisine, E. Willery, G. Renauld-Mongénie, and C. Locht. (1997). Lack of fundamental complementation between Bordetella pertussis filamentous hemagglutinin and Proteus mirabilis HpmA hemolysin secretion machineries. J. Bacteriol. 179: 775-783.

Jacob-Dubuisson, F., C. El-Hamel, N. Saint, S. Guèdin, E. Willery, G. Molle, and C. Locht. (1999). Channel formation by FhaC, the outer membrane protein involved in the secretion of the Bordetella pertussis filamentous hemagglutinin. J. Biol. Chem. 274: 37731-37735.

Jacob-Dubuisson, F., V. Villeret, B. Clantin, A.S. Delattre, and N. Saint. (2009). First structural insights into the TpsB/Omp85 superfamily. Biol Chem 390: 675-684.

Könninger, U.W., S. Hobbie, R. Benz, and V. Braun. (1999). The haemolysin-secreting ShlB protein of the outer membrane of Serratia marcescens: determination of surface-exposed residues and formation of ion-permeable pores by ShlB mutants in artificial lipid bilayer membranes. Mol. Microbiol. 32: 1212-1225.

Li, H., S. Grass, T. Wang, T. Liu, and J.W. St Geme 3rd. (2007). Structure of the Haemophilus influenzae HMW1B translocator protein: evidence for a twin pore. J. Bacteriol. 189: 7497-7502.

Mazar, J., and P.A. Cotter. (2007). New insight into the molecular mechanisms of two-partner secretion. Trends Microbiol. 15: 508-515.

Méli, A.C., H. Hodak, B. Clantin, C. Locht, G. Molle, F. Jacob-Dubuisson, and N. Saint. (2006). Channel properties of TpsB transporter FhaC point to two functional domains with a C-terminal protein-conducting pore. J. Biol. Chem. 281: 158-166.

Meli, A.C., M. Kondratova, V. Molle, L. Coquet, A.V. Kajava, and N. Saint. (2009). EtpB is a pore-forming outer membrane protein showing TpsB protein features involved in the two-partner secretion system. J. Membr. Biol. 230: 143-154.

Palmer, K.L. and R.S. Munson, Jr. (1995). Cloning and characterization of the genes encoding the hemolysin of Haemophilus ducreyi. Mol. Microbiol. 18: 821-830.

Poole, K., E. Schiebel, and V. Braun. (1988). Molecular characterization of the hemolysin determinant of Serratia marcescens. J. Bacteriol. 170: 3177-3188.

Schmitt, C., D. Turner, M. Boesl, M. Abele, M. Frosch, and O. Kurzai. (2007). A functional two-partner secretion system contributes to adhesion of Neisseria meningitidis to epithelial cells. J. Bacteriol. 189: 7968-7976.

Surana, N.K., A.Z. Buscher, G.G. Hardy, S. Grass, T. Kehl-Fie, and J.W. St Geme, 3rd. (2006). Translocator proteins in the two-partner secretion family have multiple domains. J. Biol. Chem. 281: 18051-18058.

Examples:

TC#NameOrganismal TypeExample
1.B.20.1.1Outer membrane toxin channel protein, ShlB Gram-negative bacteria ShlB of Serratia marcescens
 
1.B.20.1.2Outer membrane hemolysin secretion protein, HpmA Gram-negative bacteria HpmA of Proteus mirabilis
 
1.B.20.1.3

Outer membrane transporter essential for contact-dependent growth inhibition, CdiB; exports CdiA (AAZ57198) (Aoki et al., 2005).  Mediates contact-dependent growth inhibition (CDI), a phenomenon by which bacterial cell growth is regulated by direct cell-to-cell contact. The CdiA/CdiB two-partner secretion system appears to play a direct role (Aoki et al. 2008).

Gram-negative bacteria

CdiB of E. coli (AAZ57197)

 
1.B.20.1.4

The outer membrane haemolysin-like OptA exporter, OptB (OptA, AAG55657, resembles Alveicin B, 1.C.75.1.1) (Choi et al., 2007). Choi and Bernstein (2010) have demonstrated that BpaA is secreted in a two step process, and the C-terminus of OtpA enters the OtpB pore before the N-terminus.

Gram-negative bacteria

OptB of E. coli (Q8XAN8)

 
1.B.20.1.5

The HrpA/HrpB TPS adhesin system (HrpB = HecB) (Schmitt et al., 2007)

Gram-negative bacteria

HecB of Neisseria meningitidis (Q9JY22)

 
1.B.20.1.6

Outer membrane hemagglutinin secretion protein, FhaC. Functionally important conserved motifs have been identified (Delattre et al., 2010).  The x-ray structure reveals a beta-barrel pore obstructed by two structural elements conserved in all two partner secretion systems, an N-terminal α-helix and an extracellular loop.  FhaC goes from the closed to the open state in the presence of the filamentous haemagglutinin adhesin, FHA.  The N-terminal α-helix is displaced into the periplasm during FHA secretion (Guérin et al. 2014).  With two POTRA domains in the periplasm, a transmembrane beta barrel and a large loop harboring a functionally important motif, FhaC epitomizes the conserved features of the superfamily (Jacob-Dubuisson et al. 2009). The conserved secretion domain of FHA interacts with the POTRA domains, specific extracellular loops and strands of FhaC and the inner beta-barrel surface. The interaction map indicates a funnel-like pathway, with conformationally flexible FHA entering the channel in a non-exclusive manner and exiting along a four-stranded beta-sheet at the surface of the FhaC barrel. This sheet of FhaC guides the secretion domain of FHA along discrete steps of translocation and folding (Baud et al. 2014).  The membrane-proximal POTRA domain exists in several conformations, and the binding of FHA displaces this equilibrium (Guérin et al. 2015).

Proteobacteria

FhaC of Bordetella pertussis (P35077)

 
1.B.20.1.7

Portra domain containing ShlB-type family protein of 354 aas

Proteobacteria

ShlB-type protein of E. coli

 
1.B.20.1.8

Hemolysin activator protein, ExlB of 570 aas. Exports the exotoxin, ExlA (TC# 1.C.73.1.1) (Elsen et al. 2014; Basso et al. 2017).

ExlB of Pseudomonas aeruginosa

 
1.B.20.1.9

Outer membrane exporter of the ChlA exotoxin, ChlB, of 566 aas (Brumbach et al. 2007).

ChlB of Chromobacterium violaceum

 
Examples:

TC#NameOrganismal TypeExample
1.B.20.2.1

Hypothetical protein of 579 aas

Proteobacteria

HP of Erythrobacter litoralis

 
Examples:

TC#NameOrganismal TypeExample
1.B.20.3.1Heme-hemopexin utilization protein B precursor Gram-negative bacteria Hxb2 of Haemophilus influenzae
 
1.B.20.3.2

HMW1B outer membrane exporter, required for secretion of HMW1A and HMW2A adhesins (exhibit a twin pore dimeric structure) (Li et al., 2007) and forms a large-conductance channel (Duret et al., 2008).  The protein has a modular three domain structure: an N-terminal  membrane domain, a central periplasmic domain and a C-terminal membrane anchor domain that oligomerizes and forms a pore (Surana et al. 2006).  The periplasmic domain is required for secretion.

Gram-negative bacteria

HMW1B of Haemophilus influenzae (Q4QJR3)

 
1.B.20.3.3

EtpB, a functionally asymmetric pore with three conductance states (Meli et al., 2009).

Gram-negative Bacteria

EtpB of E. coli (Q29XT8)

 
1.B.20.3.4

The BpaB outer membrane channel protein. Exports BpaA (Brown et al., 2004). BpaA is very large (~530kDa) and contains 3 repeats, each ~700aas in length.

Bacteria

BpaB of Burkholderia pseudomallei (Q6Y659)

 
1.B.20.3.5

Hypothetical protein of 576 aas

Chlorobi

HP of Chlorobium chlorochromatii

 
1.B.20.3.6

Two component virulence-related protein exporter, PdtB of 544 aas.  Exports the PdtA adhesin (4180 aas; Q9I5N6) to the cell surface for processing (Faure et al. 2014).

Proteobacteria

PdtB of Pseudomonas aeruginosa