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 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.
Bacterial contact-dependent growth inhibition (CDI) systems use a type Vb secretion mechanism to export large CdiA toxins across the outer membrane by dedicated outer membrane transporters called CdiB. Guerin et al. 2020 reported the first crystal structures of two CdiB transporters from Acinetobacter baumannii and E. coli. CdiB transporters adopt a TpsB fold, containing a 16-stranded transmembrane beta-barrel connected to two periplasmic domains. The lumen of the CdiB pore is occluded by an N-terminal alpha-helix and the conserved extracellular loop 6; these two elements adopt different conformations in the structures. A conserved DxxG motif is located on strand beta1 that connects loop 6 through different networks of interactions. Structural modifications of DxxG induce rearrangement of extracellular loops and alter interactions with the N-terminal alpha-helix, preparing the system for alpha-helix ejection. The barrel pore is probably primed for CdiA toxin secretion (Guerin et al. 2020).
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)
membrane protein component of a toxin-immunity protein module, which
functions as a cellular contact-dependent growth inhibition (CDI)
system. CdiB is of 584 aas and possibly two TMSs, one N-terminal, and one C-terminal. CDI modules allow bacteria to communicate with and inhibit the
growth of closely related neighboring bacteria in a contact-dependent
fashion. CdiB is required for secretion and assembly of the
CdiA toxin protein. Expression
of the cdiAIB locus in B. thailandensis confers protection against
other bacteria carrying the locus; growth inhibition requires cellular
contact (Nikolakakis et al. 2012). The 3-d structures of two such systems have been determined (see family description) (Guerin et al. 2020).
CdiB2 of Burkholderia pseudomallei
CdiB (FhaC) of 562 aas and one N-terminal TMS.
CdiB of Yersinia pestis
CdiB of 579 aas and one N-terminal α-TMS. The 3-D structure has been determined (see family description) (Guerin et al. 2020).
CdiB of Acinetobacter baumannii
Outer membrane transporter essential for contact-dependent growth inhibition, CdiB, of 588 aas and possibly two TMSs, one N-terminal and one near the C-terminus (Q3YL97). It exports the protein toxin, CdiA (AAZ57198) (Aoki et al., 2005). It 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). The 3-d structure of this secretion system and one other have been determined (Guerin et al. 2020).
CdiB of E. coli (AAZ57197)
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.
OptB of E. coli (Q8XAN8)
The HrpA/HrpB TPS adhesin system (HrpB = HecB) (Schmitt et al., 2007)
HecB of Neisseria meningitidis (Q9JY22)
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).
FhaC of Bordetella pertussis (P35077)
Portra domain containing ShlB-type family protein of 354 aas and 2 α-TMSs, one N-terminal and one C-terminal.
ShlB-type protein of E. coli
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
Outer membrane exporter of the ChlA exotoxin, ChlB, of 566 aas (Brumbach et al. 2007).
ChlB of Chromobacterium violaceum
Hypothetical protein of 579 aas
HP of Erythrobacter litoralis
The ShlB/FhaC/HecB family hemolysin secretion/activation protein of 555 aas.
Cytotoxin of Cupriavidus taiwanensis
Putative type Vb secretion system, beta-barrel domain proteinof 584 aas.
cytotoxin exporter of Bradyrhizobium japonicum
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.
HMW1B of Haemophilus influenzae (Q4QJR3)
EtpB, a functionally asymmetric pore with three conductance states (Meli et al., 2009).
EtpB of E. coli (Q29XT8)
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.
BpaB of Burkholderia pseudomallei (Q6Y659)
Hypothetical protein of 576 aas
HP of Chlorobium chlorochromatii
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).
PdtB of Pseudomonas aeruginosa
Uncharacterized protein of 455 aas and 1 N-terminal TMS.
UP of Oscillatoriales cyanobacterium
Pore-forming outer membrane constituent CptB of 441 aas, of an export system for cytotoxic, CptA (TC# 1.C.75.1.8) (Gentile et al. 2020).
CptB of Sneathia amnii
Uncharacterized protein of 497 aas and 1 N-terminal TMS.
UP of Phocoenobacter uteri