1.B.18 The Outer Membrane Auxiliary (OMA) Protein Family

These proteins are believed to function together with an MPA2 (TC #8.A.4) and ABC- (TC #3.A.1) or an MPA1 (TC #8.A.3) and PST- (TC #2.A.66.2) type exo- or capsular polysaccharide export complex. The OMA proteins are usually of 350-400 amino acyl residues in size although a few are reported to be substantially smaller or larger. They are found exclusively in Gram-negative bacteria, are predicted to consist largely of β-structure and presumably form β-barrel-type porins in the outer membranes of the Gram-negative bacterial envelope. These proteins are in the BexD/CtrA/VexA family of Swiss Prot. In some systems, polysaccharide copolymerases (PCP) and outer membrane polysaccharide exporter (OMA or OPX) proteins interact to form a trans-envelope scaffold for polymer export (Cuthbertson et al., 2009).

One member of the OMA family, WzaK30, has been shown to be required for export of the group 1 K30 capsular polysaccharide, colanic acid, in E. coli strain E69. Mutations in the encoding gene do not interfere with the synthesis or polymerization of the K30 repeat unit but prevent appearance of the polysaccharide on the cell surface. WzaK30 is a surface-exposed outer membrane lipoprotein which forms octomeric ring-like structures resembling the secretins (TC #1.B.22). Mature Wza, the best studied OMA member, is a 359-residue lipoprotein that forms SDS-stable octamers; it is synthesized as a precursor with a cleavable 20-residue amino-terminal signal sequence. Cys 21 is modified by a thioether-linked diacylglyceryl group, and its amino group is acylated. Such structures form the channels through which polysaccharides pass to reach the cell surface from the periplasm. The ring-like homo-octamers (a tetramer of dimers) have an outer diameter of ~9 nm and a central cavity of about 2 nm (Beis et al., 2004; Nesper et al., 2003). The native acylated N-terminus is critical for proper assembly. The Wza octamer forms a complex with Wzc, an inner membrane tyrosine autokinase (Nesper et al., 2003; Reid and Whitfield, 2005).

The 2.26 Å resolution structure of the 340 kDa octamer of Wza was reported by Dong et al. (2006). It reveals a transmembrane region that is a novel α-helical barrel. The bulk of the Wza structure is located in the periplasm and comprises three novel domains forming a large central cavity. Wza is open to the extracellular environment but closed to the periplasm. The route and mechanism for translocation of the capsular polysaccharide were proposed (Dong et al., 2006).

The structural/functional features of Wza have been reviewed and evaluated by Collins and Derrick (2007). This octamer with an 8-fold axis of rotational symmetry is unusual in forming an α-helical barrel structure as noted above. The helices within the barrel are amphipathic: hydrophobic on the outside, to permit interaction with outer membrane lipid, and hydrophilic on the inside. The implication for the function of this α-helical region is clear: the helical cone forms a 'nozzle' that crosses the outer membrane, through which the secreted polysaccharide chains pass to the outside of the cell. The internal diameter of the nozzle is just sufficient to accommodate the capsular polysaccharide in an extended conformation. Interestingly, a 3D electron microscopy structure of a unique complex between Wza and the Wzc inner membrane protein has shown that Wza does indeed change conformation, opening out substantially to form a larger aperture at its base when bound to Wzc (Collins et al., 2007).

Another member of the OMA family, KpsD, has been reported to distribute between the periplasm and the two membranes (Arrecubieta et al., 2001). Together with KpsE (MPA2; TC# 8.A.4), it was suggested to facilitate transport across the periplasm as well as the outer membrane. However, more recently, McNulty et al. (2006) reported that KdpD is an outer membrane protein involved in export of group 2 capsular polysaccharides (CPSs) across this membrane. Interestingly, KdpD, KdpE and the biosynthetic complex comprise a metabolon and are located at the cell poles (McNulty et al., 2006). RhsA is a component of this complex and is required for normal CPS export.

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



Arrecubieta, C., T.C. Hammarton, B. Barrett, S. Chareonsudjai, N. Hodson, D. Rainey, and I.S. Roberts. (2001). The transport of group 2 capsular polysaccharides across the periplasmic space in Escherichia coli. J. Biol. Chem. 276: 4245-4250.

Beis, K., R.F. Collins, R.C. Ford, A.B. Kamis, C. Whitfield, and J.H. Naismith. (2004). Three-dimensional structure of Wza, the protein required for translocation of Group 1 capcular polysaccharide across the outer membrane of Escherichia coli. J. Biol. Chem. 279: 28227-28232.

Bianco, M.I., M. Jacobs, S.R. Salinas, A.G. Salvay, M.V. Ielmini, and L. Ielpi. (2014). Biophysical characterization of the outer membrane polysaccharide export protein and the polysaccharide co-polymerase protein from Xanthomonas campestris. Protein Expr Purif 101: 42-53.

Collins, R.F. and J. P. Derrick. (2007). Wza: a new structural paradigm for outer membrane secretory proteins? Trends Microbiol. 15: 96-100.

Collins, R.F., K. Beis, C. Dong, C.H. Botting, C. McDonnell, R.C. Ford, B.R. Clarke, C. Whitfield, and J.H. Naismith. (2007). The 3D structure of a periplasm-spanning platform required for assembly of group 1 capsular polysaccharides in Escherichia coli. Proc. Natl. Acad. Sci. USA 104: 2390-2395.

Cuthbertson, L., I.L. Mainprize, J.H. Naismith, and C. Whitfield. (2009). Pivotal roles of the outer membrane polysaccharide export and polysaccharide copolymerase protein families in export of extracellular polysaccharides in gram-negative bacteria. Microbiol. Mol. Biol. Rev. 73: 155-177.

Dong, C., K. Beis, J. Nesper, A.L. Brunkan-Lamontagne, B.R. Clarke, C. Whitfield, and J.H. Naismith. (2006). Wza the translocon for E. coli capsular polysaccharides defines a new class of membrane protein. Nature 444: 226-229.

Drummelsmith, J. and C. Whitfield. (2000). Translocation of group 1 capsular polysaccharide to the surface of Escherichia coli requires a multimeric complex in the outer membrane. EMBO J. 19: 57-66.

Hashimoto, Y., N. Li, H. Yokoyama, and T. Ezaki. (1993). Complete nucleotide sequence and molecular characterization of ViaB region encoding Vi antigen in Salmonella typhi. J. Bacteriol. 175: 4456-4465.

Huang, J., and M. Schell. (1995). Molecular characterization of the eps gene cluster of Pseudomonas solanacearum and its transcriptional regulation at a single promoter. Mol. Microbiol. 16: 977-989.

Kroll, J.S., B. Loynds, L.N. Brophy, and E.R. Moxon. (1990). The bex locus in encapsulated Haemophilus influenzae: a chromosomal region involved in capsule polysaccharide export. Mol. Microbiol. 4: 1853-1862.

Larue, K., R.C. Ford, L.M. Willis, and C. Whitfield. (2011). Functional and structural characterization of polysaccharide co-polymerase proteins required for polymer export in ATP-binding cassette transporter-dependent capsule biosynthesis pathways. J. Biol. Chem. 286: 16658-16668.

Mahendran, K.R., A. Niitsu, L. Kong, A.R. Thomson, R.B. Sessions, D.N. Woolfson, and H. Bayley. (2017). A monodisperse transmembrane α-helical peptide barrel. Nat Chem 9: 411-419.

Marczak, M., A. Mazur, J.E. Krol, W.I. Gruszecki, and A. Skorupska. (2006). Lipoprotein PssN of Rhizobium leguminosarum bv. trifolii: Subcellular localization and possible involvement in exopolysaccharide export. J. of Bacteriol. 188(19): 6943-6952.

McNulty, C., J. Thompson, B. Barrett, L. Lord, C. Andersen, and I.S. Roberts. (2006). The cell surface expression of group 2 capsular polysaccharides in Escherichia coli: the role of KpsD, RhsA and a multi-protein complex at the pole of the cell. Mol. Microbiol. 59: 907-922.

Nadler, C., S. Koby, A. Peleg, A.C. Johnson, K.C. Suddala, K. Sathiyamoorthy, B.E. Smith, M.A. Saper, and I. Rosenshine. (2012). Cycling of Etk and Etp phosphorylation states is involved in formation of group 4 capsule by Escherichia coli. PLoS One 7: e37984.

Nesper, J., C.M.D. Hill, A. Paiment, G. Harauz, K. Beis, J.H. Naismith, and C. Whitfield. (2003). Translocation of group 1 capsular polysaccharide in Escherichia coli serotype K30. Structure and functional analysis of the outer membrane lipoprotein Wza. J. Biol. Chem. 278: 49763-49772.

Nickerson, N.N., I.L. Mainprize, L. Hampton, M.L. Jones, J.H. Naismith, and C. Whitfield. (2014). Trapped translocation intermediates establish the route for export of capsular polysaccharides across Escherichia coli outer membranes. Proc. Natl. Acad. Sci. USA 111: 8203-8208.

Paulsen, I.T., A.M. Beness, and M.H. Saier, Jr. (1997). Computer-based analyses of the protein constituents of transport systems catalyzing export of complex carbohydrates in bacteria. Microbiology 143: 2685-2699.

Peleg, A., Y. Shifrin, O. Ilan, C. Nadler-Yona, S. Nov, S. Koby, K. Baruch, S. Altuvia, M. Elgrably-Weiss, C.M. Abe, S. Knutton, M.A. Saper, and I. Rosenshine. (2005). Identification of an Escherichia coli operon required for formation of the O-antigen capsule. J. Bacteriol. 187: 5259-5266.

Pérez-Burgos, M., I. García-Romero, J. Jung, E. Schander, M.A. Valvano, and L. Søgaard-Andersen. (2020). Characterization of the Exopolysaccharide Biosynthesis Pathway in Myxococcus xanthus. J. Bacteriol. 202:.

Reid, A.N. and C. Whitfield. (2005). Functional analysis of conserved gene products involved in assembly of Escherichia coli capsules and exopolysaccharides: evidence for molecular recognition between Wza and Wzc for colanic acid biosynthesis. J. Bacteriology 187: 5470-5481.

Shrivastava, A., R.G. Rhodes, S. Pochiraju, D. Nakane, and M.J. McBride. (2012). Flavobacterium johnsoniae RemA is a mobile cell surface lectin involved in gliding. J. Bacteriol. 194: 3678-3688.


TC#NameOrganismal TypeExample
1.B.18.1.1OMA protein component of a PST-type exopolysaccharide export system (outer membrane porin constituent)Gram-negative bacteria ExoF of Rhizobium meliloti
1.B.18.1.2OMA protein component of an ABC-type group 2 capsular polysaccharide (polysialic acid) export system (outer membrane porin constituent)Gram-negative bacteria KpsD of E. coli

TC#NameOrganismal TypeExample
1.B.18.2.1OMA protein component of an ABC-type Vi polysaccharide antigen export system, VexA (functions with VexBCD, 3.A.1.101.2) (Hashimoto et al., 1993)Gram-negative bacteria VexA of Salmonella typhi (Q04976)
1.B.18.2.2OMA component of exopolysaccharide exporter, PssN (outer membrane lipoprotein, oriented toward the periplasm; predominantly of β-structure, but with some α-structure) (Marczak et al., 2006).Gram-negative bacteriaPssN of Rhizobium leguminosarum (Q27SU9)

Capsular polysialate exporter component, CtrA. OMA of 3.A.1.101.3 (functions with 3.A.1.101.3 (ABC) and 8.B.4.2.1 (MPA2)) (Larue et al., 2011)


CtrA of Neisseria meningitidis (Q547A8)


The OMA capsular polysaccharide exporter of 394 aas, BexD (Kroll et al. 1990).


BexD of Haemophilus influenzae


TC#NameOrganismal TypeExample

OMA component of the group 1 K30 capsular polysaccharide, colanic acid, export system, Wza (Reid and Whitfield, 2005) (outer membrane porin constituent). The 3-d structure of the Wza/Wzc complex has been solved by Collins et al. (2007). It spans the periplasm, comprising a central constituent for complex formation. The x-ray structure shows an integral outer membrane transmembrane α-helical barrel with a large central cavity, similar to the secretin protein, PilQ (1.B.22.2.1) (Collins and Derrick, 2007).  Wza is an octameric α-helical outer membrane channel that directly exports nascent capsular polysaccharide chains through the Wza portal (Nickerson et al. 2014). Peptides based on the C-terminal D4 domain of Wza form transmembrane, ion conducting α-helical barrels. The helix barrel contains eight D4 peptides arranged in parallel (Mahendran et al. 2017).


Wza of E. coli (P0A930)

1.B.18.3.2The OMA protein component of a PST-type exopolysaccharide exporter (EpsE; TC# 2.A.66.2.11) (Huang and Schell, 1995)


EpsA of Ralstonia solanacearum (Q45407)

The Wza protein, an OMA homologue.  May functions with an α-glycosyl transferase, RemC (A5FNG2) which shows limited sequence similarity to the cytoplasmic domain of 2.A.38.4.5, and the Wzc tyrosine protein kinase (8.A.3.3.4) (Shrivastava et al. 2012).


Wza of Flavobactrerium johnsoniae


Outer membrane polysaccharide-specific porin, Wza of 348 aas.


Wza of Parachlamydia acanthamoebae


Polysaccharide transmembrane transporter of 741 aas.


PS exporter of Oscillatoria acuminata


Polysaccharide exporter of 680 aas


PS exporter of Rhodopirellula sallentina


The outer membrane exo-polysaccharide (xanthan) exporter, GumB of 286 aas (Bianco et al. 2014).


GumB of Xanthomonas campestris


Outer membrane auxillary lipoprotein of 698 aas and 1 N-terminal TMS, GlfD or YmcA.  Probably involved in capsular polysaccharide export (Peleg et al. 2005; Cuthbertson et al. 2009; Nadler et al. 2012).

GlfD of E. coli


Outer membrane polysaccharide biosynthesis/export protein, Opx, EpsY or MXAN_7417 of 219 aas and 1 N-terminal TMS. The gene encoding this protein is adjacent to EpsZ (MXAN_7415; TC# 9.B.18.1.6), an exopolysaccharide biosynthetic protein, and Wzx (MXAN_7416; TC# 2.A.66.12.12), a polysaccharide cytoplasmic membrane flippase (Pérez-Burgos et al. 2020).

Opx of Myxococcus xanthus