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.
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.
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)
CtrB of Neisseria meningitidis