FIG. 4.
Proposed model for the assembly of ECACYC in Escherichia coli K-12. It is proposed that WzxE mediates the translocation of lipid III to the periplasmic face of the cytoplasmic membrane. The transmembrane translocation of lipid III is followed by the WzyE catalyzed “block polymerization” of ECA trisaccharide repeat units and cyclization to yield ECACYC molecules containing four repeat units. The assembly of ECACYC from lipid III also appears to be dependent on a functional polysaccharide chain length regulator (WzzE); however, the role of WzzE in the assembly process has not been established. The cyclization reaction presumably involves synthesis of an intramolecular glycosidic linkage between the potential reducing terminal GlcNAc residue of Fuc4NAc-ManNAcA-GlcNAc-(Fuc4NAc-ManNAcA-GlcNAc)2-Fuc4NAc-ManNAcA-GlcNAc-P-P-Und and the hydroxyl group in position 3 of the nonreducing terminal Fuc4NAc residue, thus resulting in the release of undecaprenylpyrophosphate. Neither the putative enzyme that catalyzes this reaction nor its structural gene has been identified. It is possible that the cyclization reaction is dependent on the formation of a complex between the putative “cyclase,” WzyE and WzzE. Indeed, this complex may also include WzxE. Alternatively, it is possible that WzyE or WzzE is bifunctional and is able to catalyze the cyclization reaction as well as polymerization or chain length regulation, respectively. In either case, the assembly of ECACYC from lipid III may be dependent on the formation of a membrane-bound complex that includes some or all of these proteins.