Coating the surface: a model for expression of capsular polysialic acid in Escherichia coli K1

Abstract

Capsules are well-studied components of the bacterial surface that modulate interactions between the cell and its environment. Generally composed of polysaccharide, they are key virulence determinants in invasive infections in humans and other animals. Genetic determinants involved in capsule expression have been isolated from a number of organisms, but perhaps the best characterized is the kps cluster of Escherichia coli K1. In this review, the current understanding of the functions of the kps gene products is summarized. Further, a proposed mechanistic model for capsule expression is presented and discussed. The model is based on the premise that the numerous components of the kps cluster form a hetero-oligomeric complex responsible for synthesis and concurrent translocation of the capsular polysialic acid through sites of inner and outer membrane fusion. We view the ATP-binding cassette (ABC) transporter, KpsMT, to be central to the functioning of the complex, interacting with the biosynthetic apparatus as well as the extracytoplasmic components of the cluster to co-ordinate synthesis and translocation. The model provides the basis for additional experimentation and reflects emerging similarities among systems responsible for macromolecular export in Gram-negative bacteria.

Reduced polysialic acid capsule expression in Escherichia coli K1 mutants with chromosomal defects in kpsF

“…The region 3 kpsMT genes together with kpsE in region 1 code for an ATP-binding cassette type 2 (ABC-2) transporter (Reizer et al, 1992). This ABC-2 transporter may be part of the transmembrane channel used for exporting polysialic acid or other capsular polysaccharide chains to the cell surface (for a review, see Bliss and Silver, 1996). Region 2 neu genes encode specific enzymes needed for sialic acid synthesis (neuB ), activation (neuA ) or polymerization (neuS ), whereas the functions of neuD, C or E are not known but seem to be required for sialic (neuC ) or polysialic (neuD ) acid synthesis or transmembrane (neuE ) translocation (reviewed in Vimr and Steenbergen, 1993;Vimr et al, 1995).…”

Translocation of Group 1 Capsular Polysaccharide in Escherichia coli Serotype K30

“…Translocation of group 1 and 2 CPS occurs at specific sites where the IM and OM are in close apposition (so called "Bayer junctions" or zones of adhesion) (2-4), providing indirect evidence for transmembrane complexes in capsule assembly. Translocation of group 2 capsules is proposed to involve coordinated synthesis and export (1,(5)(6)(7). In the current model, group 2 CPS is polymerized in the cytoplasm by a multi-enzyme membrane-bound complex and then modified with a phospholipid anchor.…”

Evidence that KpsT, the ATP-binding component of an ATP-binding cassette transporter, is exposed to the periplasm and associates with polymer during translocation of the polysialic acid capsule of Escherichia coli K1

“…Capsule expression is mediated by the 14 gene products of the 17-kb kps cluster (4,19,23). This cluster is divided into three functional regions with the central region 2 responsible for synthesis, activation, and polymerization of the capsular polymer and the flanking regions 1 and 3 involved with translocation of the polymer from its cytoplasmic site of synthesis to the cell surface (4,19,23). We believe translocation to be mediated by a multicomponent complex of proteins, effecting translocation in a single step at sites of membrane fusion (for a review, see reference 4).…”

“…To determine if any other kps components were important for the coprecipitation of KpsT with polySia, pSR340 was transformed into a variety of chromosomal kps mutant strains including chromosomal deletions in kpsM, kpsD, and kpsE, as well as an insertion mutation in kpsC (4,19,23). Extracts from each of these strains were immunoprecipitated and analyzed as described above.…”