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).

This family belongs to the .



Becker, A. and A. Pühler. (1998). Specific amino acid substitutions in the proline-rich motif of the Rhizobium meliloti ExoP protein result in enhanced production of low-molecular-weight succinoglycan at the expense of high-molecular-weight succinoglycan. J. Bacteriol. 180: 395-399.

Bianco, M.I., M. Jacobs, S.R. Salinas, A.G. Salvay, M.V. Ielmini, and L. Ielpi. (2014). Biophysical characterization of the outer membrane polysaccharide export protein and the polysaccharide co-polymerase protein from Xanthomonas campestris. Protein Expr Purif 101: 42-53.

Collins, R.F., K. Beis, B.R. Clarke, R.C. Ford, M. Hulley, J.H. Naismith, and C. Whitfield. (2006). Periplasmic protein-protein contacts in the inner membrane protein Wzc form a tetrameric complex required for the assembly of Escherichia coli group 1 capsules. J. Biol. Chem. 281: 2144-2150.

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.

Hagelueken, G., H. Huang, I.L. Mainprize, C. Whitfield, and J.H. Naismith. (2009). Crystal structures of Wzb of Escherichia coli and CpsB of Streptococcus pneumoniae, representatives of two families of tyrosine phosphatases that regulate capsule assembly. J. Mol. Biol. 392: 678-688.

Huang, J. and M. Schell. (1995). Molecular characterization of the eps gene cluster of Pseudomonas solanacearum and its transcriptional regulation at a single promoter. Mol. Microbiol. 16: 977-989.

Mijakovic, I., S. Poncet, G. Boël, A. Mazé, S. Gillet, E. Jamet, P. Decottignies, C. Grangeasse, P. Doublet, P. Le Maréchal, and J. Deutscher. (2003). Transmembrane modulator-dependent bacterial tyrosine kinase activates UDP-glucose dehydrogenases. EMBO J. 22: 4709-4718.

Minic, Z., C. Marie, C. Delorme, J.M. Faurie, G. Mercier, D. Ehrlich, and P. Renault. (2007). Control of EpsE, the phosphoglycosyltransferase initiating exopolysaccharide synthesis in Streptococcus thermophilus, by EpsD tyrosine kinase. J. Bacteriol. 189:1351-1357.

Morona, J.K., J.C. Paton, D.C. Miller, and R. Morona. (2000). Tyrosine phosphorylation of CpsD negatively regulates capsular polysaccharide biosynthesis in Streptococcus pneumoniae. Mol. Microbiol. 35: 1431-1442.

Morona, J.K., R. Morona, D.C. Miller, and J.C. Paton. (2002). Streptococcus pneumonia capsule biosynthesis protein CpsB is a novel manganese-dependent phosphotyrosine-protein phosphatase. J. Bacteriol. 184: 577-583.

Morona, J.K., R. Morona, D.C. Miller, and J.C. Paton. (2003). Mutational analysis of the carboxy-terminal (YGX)4 repeat domain of CpsD, an autophosphorylating tyrosine kinase required for capsule biosynthesis in Streptococcus pneumoniae. J. Bacteriol. 185: 3009-3019.

Nadler, C., S. Koby, A. Peleg, A.C. Johnson, K.C. Suddala, K. Sathiyamoorthy, B.E. Smith, M.A. Saper, and I. Rosenshine. (2012). Cycling of Etk and Etp phosphorylation states is involved in formation of group 4 capsule by Escherichia coli. PLoS One 7: e37984.

Niemeyer, D. and A. Becker. (2001). The molecular weight distribution of succinoglycan produced by Sinorhizobium meliloti is influenced by specific tyrosine phosphorylation and ATPase activity of the cytoplasmic domain of the ExoP protein. J. Bacteriol. 183: 5163-5170.

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. Microbiology 143: 2685-2699.

Peleg, A., Y. Shifrin, O. Ilan, C. Nadler-Yona, S. Nov, S. Koby, K. Baruch, S. Altuvia, M. Elgrably-Weiss, C.M. Abe, S. Knutton, M.A. Saper, and I. Rosenshine. (2005). Identification of an Escherichia coli operon required for formation of the O-antigen capsule. J. Bacteriol. 187: 5259-5266.

Shrivastava, A., R.G. Rhodes, S. Pochiraju, D. Nakane, and M.J. McBride. (2012). Flavobacterium johnsoniae RemA is a mobile cell surface lectin involved in gliding. J. Bacteriol. 194: 3678-3688.

Temel DB., Dutta K., Alphonse S., Nourikyan J., Grangeasse C. and Ghose R. (2013). Regulatory interactions between a bacterial tyrosine kinase and its cognate phosphatase. J Biol Chem. 288(21):15212-28.

Vincent, C., B. Duclos, C. Grangeasse, E. Vaganay, M. Riberty, A.J. Cozzone, and P. Doublet. (2000). Relationship between exopolysaccharide production and protein-tyrosine phosphorylation in Gram-negative bacteria. J. Mol. Biol. 304: 311-321.

Vincent, C., P. Doublet, C. Grangeasse, E. Vaganay, A.J. Cozzone, and B. Duclos. (1999). Cells of Escherichia coli contain a protein-tyrosine kinase, Wzc, and a phosphotyrosine-protein phosphatase, Wzb. J. Bacteriol. 181: 3472-3477.

Whitfield, C. (2006). Biosynthesis and assembly of capsular polysaccharides in Escherichia coli. Annu. Rev. Biochem. 75: 39-68.

Whitfield, C. and I.S. Roberts. (1999). Structure, assembly and regulation of expression of capsules in Escherichia coli. Mol. Microbiol. 31: 1307-1319.


TC#NameOrganismal TypeExample
8.A.3.1.1MPA1-C protein component of a PST-type exopolysac-charide export system Gram-negative bacteria ExoP of Rhizobium meliloti

Capsular expopolysaccharide biosynthesis protein of 748 aas


CPS protein of Dactylococcopsis salina


Auxillary membrane polysaccharide co-polymerase protein for the export of the exo-polysaccharide, xanthan, GumC of 474 aas and 2 (N-terminal and C-terminal) α-TMSs.  Functions with GumB (TC# 1.B.18.3.7) (Bianco et al. 2014).


GumC of Xanthomonas campsetris


TC#NameOrganismal TypeExample
8.A.3.2.1MPA1 + C protein components of a PST-type capsular polysaccharide export system Gram-positive bacteria CapAB of Staphylococcus aureus
8.A.3.2.2MPA1 + C protein components of a PST-type capsular polysaccharide export system (autophosphorylating tyrosyl protein kinase)Gram-positive bacteriaCpsCD of Streptococcus pneumoniae
8.A.3.2.3MPA1 + C protein components of a PST-type exopolysaccharide export system, EpsCD (EpsD, a tyrosyl kinase, controls EpsE, the phosphoglycosyltransferase that initiates exopolysaccharide synthesis (Minic et al., 2007). EpsC and EpsD are both required for EpsE activity.Gram-positive bacteriaEpsCD of Streptococcus thermophilus
EpsC (Q93E03)
EpsD (Q93E02)

Capsular polysaccharide type 8 biosynthesis protein Cap8A


Cap8A of Staphylococcus aureus


TC#NameOrganismal TypeExample

Tyrosine protein kinase of 726 aas and 2 TMSs, Etk (MPA1-C).  Probably involved in the regulation of capsular polysaccharide export together with a tyrosine phosphatase, Etp of 148 aas (acc# P0ACZ2) (Peleg et al. 2005; Nadler et al. 2012).

Gram-negative bacteria

Etk of E. coli

8.A.3.3.2Tyrosine protein kinase, Wzc (MPA1-C) Gram-negative bacteria Wzc of E. coli
8.A.3.3.3The MPA1-C protein constituent of a PST-type exopolysaccharide exporter (EpsE; TC# 2.A.66.2.11) (Huang and Schell, 1995)BacteriaEpsB of Ralstonia solanacearum (Q45409)

The tyrosine protein kinase, Wzc.  Functions with RemC (an α-glycosyl transferase) and Wza (1.B.18.3.3) in gliding motility (Shrivastava et al. 2012).


Wzc of Flavobacterium johnsoniae


TC#NameOrganismal TypeExample

Putative EpsC homologue


Membrane protein of Streptomyces coelicolor


TC#NameOrganismal TypeExample

2 TMS auxiliary subunit of PST polysaccharide exporter; 983 aas


Auxiliary subunit of Parachlamydia acanthamoebae