8.A.4 The Cytoplasmic Membrane-Periplasmic Auxiliary-2 (MPA2) Family Proteins of the MPA2 family function in conjunction with an ABC porter (TC #3.A.1) specific for capsular polysaccharides to allow transport across both membranes and the periplasm of the Gram-negative bacterial cell envelope in a single energy coupled step. The MPA2 proteins are not demonstrably homologous to the MPA1 proteins (TC #8.A.3), but they exhibit a similar topology with two transmembrane spanners and a large periplasmic loop. They lack the cytoplasmic 'C' domain that is associated with MPA1 proteins. The periplasmic loops of MPA2 proteins presumably interact with outer membrane auxiliary (OMA) proteins (TC #1.B.18) which provide oligomeric pores of β-structure in the outer membrane through which the polysaccharide can passively diffuse. It is also called the Polysaccharide co-polymerase (PCP) Family (Larue et al., 2011). One MPA2 family member, KpsE, has been studied topologically. Its C-terminus was shown to interact with the periplasmic face of the cytoplasmic membrane, possibly via an amphipathic α-helix. It is probably a dimer (Arrecubieta et al, 2001). It is thought that KpsE, together with KpsD, allows transport of capsular polysaccharide across the periplasm.
References:
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.
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.
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.
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. Microbiol. 143: 2685-2699.
Whitfield, C. and I. S. Roberts (1999). Structure, assembly and regulation of expression of capsules in Escherichia coli. Mol. Microbiol. 31: 1307-1319.
Examples:
TC#	Name	Organismal Type	Example
8.A.4.1.1	MPA2 protein component of an ABC-type capsular polysaccharide export system	Gram-negative bacteria	KpsE of E. coli

Examples:
TC#	Name	Organismal Type	Example
8.A.4.2.1	*MPA2 protein component of an ABC-type capsular polysaccharide export system. (functions with 3.A.1.101.3 (ABC) and 1.B.18.2.3 (OMA)) (Larue et al., 2011)*	Gram-negative bacteria	CtrB of Neisseria meningitidis

8.A.4.2.2	*MPA2 component of the ABC-type Vi polysaccharide exporter (Hashimoto et al., 1993)*	Gram-negative bacteria	VexD of Salmonella typhi (P43111)