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EcoSal Plus

Domain 2: Cell Architecture and Growth

Structure and Assembly of Capsules

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  • Author: Chris Whitfield1
  • Editor: James M. Slauch2
  • VIEW AFFILIATIONS HIDE AFFILIATIONS
    Affiliations: 1: Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada; 2: The Schoold of Molecular and Cellular Biology, University of Illinois at Urbana-Champaign, Urbana, IL
  • Received 06 June 2008 Accepted 04 August 2008 Published 02 January 2009
  • Address correspondence to Chris Whitfield cwhitfie@uoguelph.ca
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  • Abstract:

    The capsule is a cell surface structure composed of long-chain polysaccharides that envelops many isolates of . It protects the cell against host defenses or physical environmental stresses, such as desiccation. The component capsular polysaccharides (CPSs) are major surface antigens in . They are named K antigens (after the German word ). Due to variations in CPS structures, more than 80 serologically unique K antigens exist in . Despite the hypervariability in CPS structures, only two capsule-assembly strategies exist in . These have led to the assignment of group 1 and group 2 capsules, and many of the key elements of the corresponding assembly pathways have been resolved. Structural features, as well as genetic and regulatory variations, give rise to additional groups 3 and 4. These employ the same biosynthesis processes described in groups 2 and 1, respectively. Each isolate possesses a distinctive set of cytosolic and inner-membrane enzymes, which generate a precise CPS structure, defining a given K serotype. Once synthesized, a multiprotein complex is needed to translocate the nascent CPS across the Gram-negative cell envelope to the outer surface of the outer membrane, where the capsule structure is assembled. While the translocation machineries for group 1 and group 2 CPSs are fundamentally different from one another, they possess no specificity for a given CPS structure. Each is conserved in all isolates producing capsules belonging to a particular group.

  • Citation: Whitfield C. 2009. Structure and Assembly of Capsules, EcoSal Plus 2009; doi:10.1128/ecosalplus.4.7.3

Key Concept Ranking

Type I Secretion System Proteins
0.32319313
0.32319313

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ecosalplus.4.7.3.citations
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/content/journal/ecosalplus/10.1128/ecosalplus.4.7.3
2009-01-02
2017-12-14

Abstract:

The capsule is a cell surface structure composed of long-chain polysaccharides that envelops many isolates of . It protects the cell against host defenses or physical environmental stresses, such as desiccation. The component capsular polysaccharides (CPSs) are major surface antigens in . They are named K antigens (after the German word ). Due to variations in CPS structures, more than 80 serologically unique K antigens exist in . Despite the hypervariability in CPS structures, only two capsule-assembly strategies exist in . These have led to the assignment of group 1 and group 2 capsules, and many of the key elements of the corresponding assembly pathways have been resolved. Structural features, as well as genetic and regulatory variations, give rise to additional groups 3 and 4. These employ the same biosynthesis processes described in groups 2 and 1, respectively. Each isolate possesses a distinctive set of cytosolic and inner-membrane enzymes, which generate a precise CPS structure, defining a given K serotype. Once synthesized, a multiprotein complex is needed to translocate the nascent CPS across the Gram-negative cell envelope to the outer surface of the outer membrane, where the capsule structure is assembled. While the translocation machineries for group 1 and group 2 CPSs are fundamentally different from one another, they possess no specificity for a given CPS structure. Each is conserved in all isolates producing capsules belonging to a particular group.

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Figures

Image of Figure 1
Figure 1

This group 1 capsule was preserved by a freeze-substitution process ( 3 ). Adapted from the ( 4 ) with the permission of the publisher.

Citation: Whitfield C. 2009. Structure and Assembly of Capsules, EcoSal Plus 2009; doi:10.1128/ecosalplus.4.7.3
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Image of Figure 2
Figure 2

Group 1 capsules always contain a hexose (glucose or galactose) residue in the main chain. Negative charges are provided by uronic acids, and the polymers are frequently decorated by (nonstoichiometric) -acetyl or ketal substituents. They share structural features with colanic acid. The polysaccharides forming group 4 capsules may have either O or K antigen status, depending on whether an additional LPS-linked O antigen is also present. These polysaccharides all contain a main-chain acetamido sugar (GlcNAc or GalNAc). Group 2 and 3 capsules are structurally diverse, with no conserved themes in their repeating units. Like group 1 capsules, they can be modified by -acetyl groups, and in some cases, like that of serotype K1, the O acetylation is subject to on-off form variation (denoted by *). Nonstoichiometric modification of the repeat units by amino acid residues is a feature of both groups 3 and 4. Qui4NMal, 4-(2-carboxyacetamido)-4,6-dideoxyglucose; Col, colitose (3,6-dideoxygalactose); Gro, glycerol; Rit, ribitol. Adapted from the ( 4 ) with the permission of the publisher.

Citation: Whitfield C. 2009. Structure and Assembly of Capsules, EcoSal Plus 2009; doi:10.1128/ecosalplus.4.7.3
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Image of Figure 3
Figure 3

The group 1 (K30) locus begins with four genes (, , , and ) required for the high-level polymerization of the capsular polysaccharide. is the first in a series of genes whose products are involved in repeat unit biosynthesis; these include genes encoding the characteristic PHPT (orange), Wzy (red), and Wzx (purple) proteins. The colanic acid biosynthesis genes show an overall similar organization, although is missing and the locus is transcriptionally regulated by Rcs proteins. The genes encoding the group 4 capsule are located downstream of the colanic acid locus and essentially represent an O antigen biosynthesis cluster (in a position identical to that of the O16 locus in K-12). The production of the group 4 capsule occurs when these genes are supplemented by unlinked and genes in the 22-min locus. The genes are also involved in group 4 assembly, and Rcs-regulated homologs of these genes () are located near a -linked locus, but it is not yet known whether these genes participate in the biosynthesis of colanic acid or group 1 capsules. Several housekeeping genes are shown as reference points (green), and the relative positions of these genes and the various loci suggest extensive recombination and rearrangement in this region of the chromosome. Points of regulation by RfaH and/or Rcs proteins and the stem-loop attenuator are identified by vertical black arrows. Adapted from the ( 4 ) with the permission of the publisher.

Citation: Whitfield C. 2009. Structure and Assembly of Capsules, EcoSal Plus 2009; doi:10.1128/ecosalplus.4.7.3
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Image of Figure 4
Figure 4

Panel A shows the crystal structure of the Wza octamer ( 69 ). In the ribbon diagram on the left, each of the monomers is identified in a different color and the space-filling shape representing the central cavity is highlighted in light brown. The four rings are clearly evident. When the side chains are filled in, in the diagram on the right, the amphora structure of the complex is revealed, with an open neck and a closed bottom (oxygen, red; nitrogen, blue; selenium and sulfur, gray). Lipid chains are identified as black spheres. Panel B shows the three-dimensional structure of the Wza-Wzc complex reconstructed by electron microscopy analysis of cryonegatively stained single particles. The images on the left are a surface-rendered representation. On the right, slices through the structure illustrate the central cavity ( 65 ). In Panel C, the crystal structure of Wza (purple) and the electron microscopy structure of the Wzc tetramer (yellow) are superimposed onto the overall structure of the Wza-Wzc complex ( 65 ), identified as a green wire frame. The orange regions on the Wzc structure denote the C termini containing the kinase domains; these were identified by Ni-nitrilotriacetic acid-gold labeling of an N-terminal hexahistidine tag. This image highlights the broadening of R4 (PES domain) that occurs upon interaction with Wzc. OM, outer membrane. The crystal structure of the Wza octamer is adapted from ( 69 ), and the illustration of the central cavity and the structure of the Wza-Wzc complex are adapted from ( 65 ) with the permission of the publishers.

Citation: Whitfield C. 2009. Structure and Assembly of Capsules, EcoSal Plus 2009; doi:10.1128/ecosalplus.4.7.3
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Image of Figure 5
Figure 5

Und-PP-linked repeat units are synthesized by the PHPT protein, WbaP, and additional glycosyltransferases. Wzx exports (flips) the lipid-linked repeat units across the inner membrane. Wzy catalyzes polymerization. The tyrosine autokinase (Wzc) and its cognate phosphatase (Wzb) form a tetramer and undergoes phosphorylation-transphosphorylation-dephosphorylation events. The resulting protein occupies a central position in the overall process, by controlling the high-level polymerization of the polymer and forming, together with octamers of the outer membrane translocon protein (Wza), an envelope-spanning complex for the translocation of polymers to the cell surface. Details of each biosynthetic step are described in the text. It is uncertain whether other housekeeping functions also participate in the overall process. Additional cell envelope proteins may be required, such as the YmcDCBA/YbjEFGH paralogs. The core components of the complex are required for group 4 capsule and colanic acid biosynthesis and assembly. The diagram of the loci is adapted from the ( 4 ) with the permission of the publisher.

Citation: Whitfield C. 2009. Structure and Assembly of Capsules, EcoSal Plus 2009; doi:10.1128/ecosalplus.4.7.3
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Image of Figure 6
Figure 6

The group 2 locus comprises serotype-specific region 2 flanked by two regions (1 and 3) conserved across group 2 serotypes. The two genes in region 3 encode the ABC transporter. Within region 1, four genes () are required to various degrees in polymer export and the additional two genes () encode proteins involved in CMP-Kdo biosynthesis. Region 2 genes encode enzymes for polymer biosynthesis, and the complexity of this region corresponds to the repeat unit structures ( Fig. 2 ). While most of the loci occupy the same area on the chromosome (near ), the genes for K15 are found on a pathogenicity island. Group 3 loci contain region 3 genes and some (but not all) region 1 genes, and the loci show extensive evidence of rearrangement. Genes marked by asterisks are incomplete, perhaps as a result of past recombination-rearrangement events ( 4 ). The diagram of the loci is adapted from the ( 4 ) with the permission of the publisher.

Citation: Whitfield C. 2009. Structure and Assembly of Capsules, EcoSal Plus 2009; doi:10.1128/ecosalplus.4.7.3
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Image of Figure 7
Figure 7

Polymer formation is initiated on an unknown endogenous acceptor (potentially diacylglycerophosphate or diacylglycerophosphate-Kdo) and is extended by a glycosyltransferase function(s) that adds residues to the nonreducing terminus of the chain. It is conceivable that the polymer is synthesized on a different acceptor and then transferred to diacylglycerophosphate-Kdo prior to export. The polymer is exported via the ABC transporter (KpsMT). KpsSC proteins are essential for this process, along with other proteins (e.g., KpsFU proteins also have some influence), but the details of their involvements are unknown. The orientation of the polymeric substrate during export (i.e., is the lipid terminus exported first or last?) has not been established, and biosynthesis and export may be temporally coupled. Translocation across the periplasm and outer membrane requires KpsE and KpsD, which provide putative membrane fusion (adaptor) protein and OMA protein functions.

Citation: Whitfield C. 2009. Structure and Assembly of Capsules, EcoSal Plus 2009; doi:10.1128/ecosalplus.4.7.3
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Tables

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Table 1

Classification of capsules

Citation: Whitfield C. 2009. Structure and Assembly of Capsules, EcoSal Plus 2009; doi:10.1128/ecosalplus.4.7.3

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