The localization of KpsC, S and T, and KfiA, C and D proteins involved in the biosynthesis of the Escherichia coli K5 capsular polysaccharide: evidence for a membrane-bound complex.

Biosynthesis of the Escherichia coli K5 polysaccharide requires the KfiA, KfiB, KfiC and KfiD proteins. The subsequent transport of the polysaccharide onto the cell surface requires the KpsC, KpsD, KpsE, KpsM, KpsS and KpsT proteins, which are conserved between different group II capsular polysaccharides. The KfiA and KfiC, together with the KpsC, KpsS and KpsT proteins, were purified and polyclonal antisera to each protein generated. These antisera, together with one previously generated (by others) against the purified KfiD protein, were used in Western blot analysis to locate the corresponding proteins within the cell. Analysis of membrane fractions revealed that KfiA (involved in initiation of polysaccharide synthesis), KfiC (K5 glycosyl transferase) and the KfiD protein (UDP-glucose dehydrogenase) were associated with the inner membrane. The KpsC, KpsS, and KpsT proteins involved in polysaccharide transport were associated with the inner membrane and this membrane association occurred in the absence of any other capsule-related proteins. The effect of mutations in individual kps genes on the localization of each protein was determined. Mutations in the kpsC, kpsM, kpsS and kpsT genes resulted in a loss of membrane targeting for KfiA and KfiC, suggesting some form of hetero-oligomeric membrane-bound biosynthetic complex. Osmotic shock caused the release of KfiA, KfiC, KpsC and KpsS from the inner membrane into the periplasm, suggesting that the polysaccharide biosynthetic complex may be associated with sites of adhesion between the inner and outer membrane.