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.