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 protein that forms SDS-stable octamers; it is synthesized as a precursor with a cleavable 20-residue amino-terminal signal sequence 6. Cys 21 is modified by a thioetherlinked 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. 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 (CPS) 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.