An Escherichia coli undecaprenyl-pyrophosphate phosphatase implicated in undecaprenyl phosphate recycling

Undecaprenyl phosphate (Und-P) is a universal lipid carrier of glycan biosynthetic intermediates for carbohydrate polymers that are exported to the bacterial cell envelope. Und-P arises from the dephosphorylation of undecaprenyl pyrophosphate (Und-PP) molecules produced by de novo synthesis and also from the recycling of released Und-PP after the transfer of the glycan component to other acceptor molecules. The latter reactions take place at the periplasmic side of the plasma membrane, while cytoplasmic enzymes catalyse the de novo synthesis. Four Und-PP pyrophosphatases were recently identified in Escherichia coli. One of these, UppP (formerly BacA), accounts for 75 % of the total cellular Und-PP pyrophosphatase activity and has been suggested to participate in the Und-P de novo synthesis pathway. Unlike UppP, the other three pyrophosphatases (YbjG, YeiU and PgpB) have a typical acid phosphatase motif also found in eukaryotic dolichyl-pyrophosphate-recycling pyrophosphatases. This study shows that double and triple deletion mutants in the genes uppP and ybjG, and uppP, ybjG and yeiU, respectively, are supersensitive to the Und-P de novo biosynthesis inhibitor fosmidomycin. In contrast, single or combined deletions including pgpB have no effect on fosmidomycin supersensitivity. Experimental evidence is also presented that the acid phosphatase motifs of YbjG and YeiU face the periplasmic space. Furthermore, the quadruple deletion mutant DeltauppP-DeltaybjG-DeltayeiU-DeltawaaL has a growth defect and abnormal cell morphology, suggesting that accumulation of unprocessed Und-PP-linked O antigen polysaccharides is toxic for these cells. Together, the results support the notion that YbjG, and to a lesser extent YeiU, exert their enzymic activity on the periplasmic side of the plasma membrane and are implicated in the recycling of periplasmic Und-PP molecules.

Functional characterization and membrane topology of Escherichia coli WecA, a sugar-phosphate transferase initiating the biosynthesis of enterobacterial common antigen and O-antigen lipopolysaccharide

WecA is an integral membrane protein that initiates the biosynthesis of enterobacterial common antigen and O-antigen lipopolysaccharide (LPS) by catalyzing the transfer of N-acetylglucosamine (GlcNAc)-1-phosphate onto undecaprenyl phosphate (Und-P) to form Und-P-P-GlcNAc. WecA belongs to a large family of eukaryotic and prokaryotic prenyl sugar transferases. Conserved aspartic acids in putative cytoplasmic loops 2 (Asp90 and Asp91) and 3 (Asp156 and Asp159) were targeted for replacement mutagenesis with either glutamic acid or asparagine. We examined the ability of each mutant protein to complement O-antigen LPS synthesis in a wecA-deficient strain and also determined the steady-state kinetic parameters of the mutant proteins in an in vitro transfer assay. Apparent K(m) and V(max) values for UDP-GlcNAc, Mg(2+), and Mn(2+) suggest that Asp156 is required for catalysis, while Asp91 appears to interact preferentially with Mg(2+), possibly playing a role in orienting the substrates. Topological analysis using the substituted cysteine accessibility method demonstrated the cytosolic location of Asp90, Asp91, and Asp156 and provided a more refined overall topological map of WecA. Also, we show that cells expressing a WecA derivative C terminally fused with the green fluorescent protein exhibited a punctate distribution of fluorescence on the bacterial surface, suggesting that WecA localizes to discrete regions in the bacterial plasma membrane.

Interplay of the Wzx translocase and the corresponding polymerase and chain length regulator proteins in the translocation and periplasmic assembly of lipopolysaccharide o antigen

Genetic evidence suggests that a family of bacterial and eukaryotic integral membrane proteins (referred to as Wzx and Rft1, respectively) mediates the transbilayer movement of isoprenoid lipid-linked glycans. Recent work in our laboratory has shown that Wzx proteins involved in O-antigen lipopolysaccharide (LPS) assembly have relaxed specificity for the carbohydrate structure of the O-antigen subunit. Furthermore, the proximal sugar bound to the isoprenoid lipid carrier, undecaprenyl-phosphate (Und-P), is the minimal structure required for translocation. In Escherichia coli K-12, N-acetylglucosamine (GlcNAc) is the proximal sugar of the O16 and enterobacterial common antigen (ECA) subunits. Both O16 and ECA systems have their respective translocases, WzxO16 and WzxE, and also corresponding polymerases (WzyO16 and WzyE) and O-antigen chain-length regulators (WzzO16 and WzzE), respectively. In this study, we show that the E. coli wzxE gene can fully complement a wzxO16 translocase deletion mutant only if the majority of the ECA gene cluster is deleted. In addition, we demonstrate that introduction of plasmids expressing either the WzyE polymerase or the WzzE chain-length regulator proteins drastically reduces the O16 LPS-complementing activity of WzxE. We also show that this property is not unique to WzxE, since WzxO16 and WzxO7 can cross-complement translocase defects in the O16 and O7 antigen clusters only in the absence of their corresponding Wzz and Wzy proteins. These genetic data are consistent with the notion that the translocation of O-antigen and ECA subunits across the plasma membrane and the subsequent assembly of periplasmic O-antigen and ECA Und-PP-linked polymers depend on interactions among Wzx, Wzz, and Wzy, which presumably form a multiprotein complex.

Genetic suppression analysis of an asgA missense mutation in Myxococcus xanthus

The asgA gene is required for generation of extracellular A signal, which serves as a cell-density signal for fruiting body development in Myxococcus xanthus. The AsgA protein is a histidine protein kinase and consists of a receiver domain that is conserved among response regulators of two-component signal transduction systems, followed by a histidine protein kinase domain that is conserved among sensor proteins of two-component systems. AsgA is thought to function in a signal transduction pathway that leads to expression of genes required for A-signal generation. A genetic suppression analysis of an asgA missense mutation was undertaken in order to identify genes that may provide information regarding the role of AsgA in A-signal generation and fruiting body formation. Twenty-two independent strains containing mutations that suppress asgA473 were isolated by selecting for production of heat-resistant spores under conditions that promote fruiting body development in wild-type cells. Ten of the 22 suppressor strains contained bypass suppressors. All the suppressor strains had direct spore counts at least three to four times greater than the original asgA473 mutant, and three strains had viable counts that exceeded wild-type by more than one order of magnitude. Surprisingly, none of the suppressor strains produced wild-type levels of extracellular A-signal.

Horse complement protein C9: primary structure and cytotoxic activity

Lack of hemolytic activity of horse serum is an inherent property of horse C9. To understand the molecular reasons for this deficiency we have cloned C9 cDNA from a horse liver cDNA library and have sequenced the cDNA yielding the complete coding sequence for horse C9. Purification of C9 from horse plasma and microsequencing established the N-terminus of the mature protein and verified that the correct horse C9 cDNA clone had been isolated. The deduced amino acid sequence corresponds to a mature protein of 526 amino acids that is 77% identical to human C9. It has the same domain structure as human C9 and contains 22 cysteines and four invariant tryptophans. The few differences include the N-terminus, which is an unblocked glycine in horse C9 but pyroglutamine in human C9, and three potential N-glycosylation sites compared to two in human C9. The N-terminal difference is unimportant since microsequencing of bovine C9, which is strongly hemolytic, established that it also has an unblocked glycine identical to horse C9. There are no obvious structural differences apparent that could resolve the differences in hemolytic potency between the two molecules. Aside from a few conservative replacements, both C9 sequences are identical between positions 250 and 360. This region includes the membrane interaction domain in C9 and the postulated transmembrane segment that is thought to constitute the wall of a putative transmembrane pore and, therefore, should be required for cytotoxicity. In agreement with this prediction we have observed that, in contrast to the marked decrease in hemolytic activity, horse C9 is very efficient in killing a variety of Gram-negative bacteria. These results demonstrate that horse C9 is a structurally competent molecule with efficient cytotoxic activity. Its inability to lyse erythrocytes may be related to the action of control proteins on target cell membranes.