8.A.3 The Cytoplasmic Membrane-Periplasmic Auxiliary-1 (MPA1) Protein with Cytoplasmic (C) Domain (MPA1-C or MPA1 C) Family

Proteins of the MPA1-C family, also called the polysaccharide copolymerase (PCP2a) family, have been proposed to function in conjunction with PST porters (TC #2.A.66.2) and polysaccharide polymerases for the polymerization and export of complex polysaccharides (Paulsen et al., 1997). The cytoplasmic (C) domain (the B. subtilis YwqF protein) is a tryrosyl kinase that phosphorylates and activates UDP-glucose dehydrogenase, an essential enzyme for exopolysaccharide synthesis (Mijakovic et al., 2003). This enzyme is activated by the membrane domain/protein, YwqC. Wzc of E. coli also exhibits protein-tyrosine autokinase activity (Vincent et al., 1999). It forms a tetrameric complex required for the assembly of group 1 capsules (Collins et al., 2006).

Mutation of the tyrosyl phosphorylation site of ExoP in S. meliloti changes the ratio of high MW to low MW succinoglycan (Niemeyer and Becker, 2001). In the Streptococcus pneumoniae homologue, CpsD, the tyrosines in the C-terminal (YGX)4 repeats are essential for activity (Morona et al., 2003). Wzb is its cognate phosphotyrosyl protein phosphatase; they are required for the production/assembly of high molecular weight forms of colanic acid capsular polysaccharide. Thus, phosphorylation of Wzc prevents production while Wzb catalyzed dephosphorylation restores production (Vincent et al., 2000). Similarly, autophosphorylation of CpsD of Streptococcus pneumoniae negatively regulates synthesis of capsular polysaccharide (Morona et al., 2000).

The MPA1 proteins span the cytoplasmic membrane twice as putative α-helical spanners and possess large periplasmic ''loop'' domains that might connect the cytoplasmic membrane PST porter with the outer membrane auxiliary (OMA; TC #1.B.18) protein which may exist as an oligomeric β-structure-type pore across the outer membrane, but this postulate has not been substantiated. These proteins function only in exo- or capsular polysaccharide synthesis/export (not in lipopolysaccharide export) in Gram-negative bacteria.

Capsules, protective structures on the surfaces of many bacteria, include almost 80 capsular serotypes in E. coli. Biosynthesis and translocation of capsular polysaccharides to the cell surface are probably temporally and spatially coupled by multiprotein complexes that span the cell envelope (Whitfield 2006). Crystal structures of Wzc, a tyrosyl kinase, and Wzb, a tyrosyl protein-P phosphatase, of E. coli are known (Hagelueken et al. 2009). The cyclic process of auto-phosphorylation of the C-terminal tyrosine cluster of a BY-kinase (Bacterial tyrosine kinase), and its subsequent dephosphorylation following interactions with a counteracting tyrosine phosphatase, regulates diverse physiological processes including biosynthesis and export of polysaccharides responsible for the formation of biofilms or virulence-determining capsules (Temel et al. 2013).