Computer-based analyses of the protein constituents of transport systems catalysing export of complex carbohydrates in bacteria

Abstract

Bacteria synthesize and secrete an array of complex carbohydrates including exopolysaccharides (EPSs), capsular polysaccharides (CPSs), lipopolysaccharides (LPSs), lipo-oligosaccharides (LOSs) and teichoic acids (TCAs). We have analysed the families of homologous proteins that appear to mediate excretion of complex carbohydrates into or across the bacterial cell envelope. Two principal families of cytoplasmic-membrane transport systems appear to drive polysaccharide export: polysaccharide-specif ic transport (PST) systems and ATP-binding cassette-2 (ABC-2) systems. We present evidence that the secretion of CPSs and EPSs, but not of LPSs, LOSs or TCAs via a PST or ABC-2 system requires the presence of a cytoplasmic-membrane-periplasmic auxiliary protein (MPAI or MPA2, respectively) in both Gram-negative and Grampositive bacteria as well as an outer-membrane auxiliary (OMA) protein in Gram-negative bacteria. While all OMA proteins are included within a single family, MPAl and MPAZ family proteins are not demonstrably homologous to each other, even though they share common topological features. Moreover, MPAI family proteins (which function with PST systems), but not MPAZ family proteins (which function with ABC-2 systems), possess cytoplasmic ATPbinding domains that may either exist as separate polypeptide chains (for those from Gram-positive bacteria) or constitute the C-terminal domain of the MPAI polypeptide chain (for those from Gram-negative bacteria). The sizes, substrate specificities and regions of relative conservation and hydrophobicity are defined allowing functional and structural predictions as well as delineation of family-specific sequence motifs. Each family is characterized phylogenetically.

Translocation of Group 1 Capsular Polysaccharide in Escherichia coli Serotype K30

“…Furthermore, Wza from E. coli K30 also shares sequence similarity with a number of OM proteins associated with capsule and (secreted) extracellular polysaccharide production in other bacterial species (8,17), suggesting a conserved function. The results presented here represent the first description of the secondary and quaternary structure of a member of this class of proposed OM polysaccharide export proteins (33).…”

“…Wza is a member of the outer membrane auxiliary protein family (33). These proteins are proposed to be ␤-barrel channel proteins that transport polysaccharides across the OM (1,33).…”

“…These proteins are proposed to be ␤-barrel channel proteins that transport polysaccharides across the OM (1,33). To provide insight into the secondary structure of Wza, purified Wza and Wza His 6 were subjected to CD analysis.…”

Roles of Conserved Proline and Glycosyltransferase Motifs of EmbC in Biosynthesis of Lipoarabinomannan

“…The topology for PCP proteins (C) has been established (49), and the described proline-rich motif is located just upstream of the second TM region in these proteins. The cytoplasmic carboxyl-terminal domain differs in size and functional feature among the PCP proteins (36,47).…”

Purified, Recombinant TagF Protein from Bacillus subtilis 168 Catalyzes the Polymerization of Glycerol Phosphate onto a Membrane Acceptor in Vitro

“…Reasonable functional predictions based on sequence homology can be made for the enzymes involved in the synthesis of the prenolpyrophosphate-linked disaccharide including TagO, a probable N-acetylglucosamine-1-phosphate transferase (1), and TagA, a credible N-acetylmannosamine transferase (7). Genes tagGH appear to encode a membrane transporter of the ABC-2 family, with homology to a number of bacterial lipopolysaccharide and capsule transport systems (16), presumably with a role in exporting intracellularly synthesized teichoic acid. TagE is clearly a glycosyltransferase involved in glucosylation of poly(glycerol phosphate) (17).…”