COATING THE SURFACE - A MODEL FOR EXPRESSION OF CAPSULAR POLYSIALIC ACID IN ESCHERICHIA-COLI K1

Capsules are well-studied components of the bacterial surface that mod ulate interactions between the cell and its environment. Generally com posed of polysaccharide, they are key virulence determinants in invasi ve infections in humans and other animals. Genetic determinants involv ed 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 o f 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 concurr ent translocation of the capsular polysialic acid through sites of inn er and outer membrane fusion, We view the ATP-binding cassette (ABC) t ransporter, KpsMT, to be central to the functioning of the complex, in teracting with the biosynthetic apparatus as well as the extracytoplas mic components of the cluster to co-ordinate synthesis and translocati on. The model provides the basis for additional experimentation and re flects emerging similarities among systems responsible for macromolecu lar export in Gram-negative bacteria.