Lipopolysaccharide (LPS) is a major component of the

No abstract

Neisseria meningitidis serogroup B and Escherichia coli K1 bacteria produce a capsular polysaccharide (CPS) that is composed of ␣2,8-linked polysialic acid (PSA). Biosynthesis of PSA in these bacteria occurs via an ABC (ATP-binding cassette) transporter-dependent pathway. In N. meningitidis, export of PSA to the surface of the bacterium requires two proteins that form an ABC transporter (CtrC and CtrD) and two additional proteins, CtrA and CtrB, that are proposed to form a cell envelope-spanning export complex. CtrA is a member of the outer membrane polysaccharide export (OPX) family of proteins, which are proposed to form a pore to mediate export of CPSs across the outer membrane. CtrB is an inner membrane protein belonging to the polysaccharide co-polymerase (PCP) family. PCP proteins involved in other bacterial polysaccharide assembly systems form structures that extend into the periplasm from the inner membrane. There is currently no structural information available for PCP or OPX proteins involved in an ABC transporter-dependent CPS biosynthesis pathway to support their proposed roles in polysaccharide export. Here, we report cryo-EM images of purified CtrB reconstituted into lipid bilayers. These images contained molecular top and side views of CtrB and showed that it formed a conical oligomer that extended ϳ125 Å from the membrane. This structure is consistent with CtrB functioning as a component of an envelope-spanning complex. Cross-complementation of CtrA and CtrB in E. coli mutants with defects in genes encoding the corresponding PCP and OPX proteins show that PCP-OPX pairs require interactions with their cognate partners to export polysaccharide. These experiments add further support for the model of an ABC transporter-PCP-OPX multiprotein complex that functions to export CPS across the cell envelope. Capsular polysaccharides (CPSs)2 are produced by many bacteria and form a layer (the capsule) that coats the surface of the cell. The capsule helps prevent bacteria from desiccation and is important in infection because it provides protection from the host immune response by interfering with both complement-mediated killing (1-3) and the adhesion of monocytes and neutrophils to the bacteria (4, 5). Consequently, capsules are an important virulence factor for numerous pathogenic bacteria including those that cause disease in livestock, like Pasteurella multocida, and bacteria such as Escherichia coli and Neisseria meningitidis, which cause bacterial meningitis and sepsis in humans (6).CPSs vary significantly in carbohydrate composition and glycosidic linkages. Despite the diversity of structures, the majority of CPSs produced by Gram-negative bacteria are assembled via either an ATP-binding cassette (ABC) transporter-dependent biosynthesis pathway or a Wzy-dependent biosynthesis pathway (for review, see Refs. 7,8). These pathways are fundamentally different in the topology of the polysaccharide polymerization reaction at the inner membrane. In Wzy-dependent pathways, polymer repeat units are assembled at...

After infection of Escherichia coli by various double-stranded deoxyribonucleic acid (DNA) phages, the newly synthesized phage-directed DNA may sediment differently from the DNA of mature phage particles. There are several reasons why this may occur, in the cases of T4 (7), T5 (13), X (12), and P22 (2), the vegetative phage DNA may be longer than the mature phage DNA, and individual genomes of the phages X (1, 15) and P22 (10) may also be present, intracellularly, as fast-sedimenting circular molecules. Furthermore, it has been shown that some amber mutants of T4 (6) and of X (11), and some temperature-sensitive mutants of P22 (3), determine the synthesis, in nonpermissive conditions, of phage-directed DNA whose sedimentation patterns differ from that of the DNA synthesized by the wild-type phages. The study of these patterns is of considerable interest because of the clues it may provide to the steps involved in the synthesis of mature virus genomes.