Pathway Summary from MetaCyc

The enterobacterial common antigen (ECA) is an outer membrane glycolipid shared by all members of the Enterobacteriaceae and is restricted to this family. A notable exception is Dickeya chrysanthemi, which does not produce ECA [Kuhn88].

ECA was discovered in 1962 in antisera against Escherichia coli O14, and originally named "Kunin antigen" [Kunin62]. It is located in the outer leaflet of the outer membrane, and is a glycophospholipid built up by an aminosugar heteropolymer linked to an L-glycerophosphatidyl residue.

The carbohydrate portion consists of trisaccharide repeat units of N-acetyl-α-D-glucosamine, N-acetyl-β-D-mannosaminuronate and N-acetylthomosamine [Mannel78, Lugowski83]. These repeating units are concatenated into longer linear heteropolysaccharide chains [MeierDieter90], and each polysaccharide chain is covalently linked to diacylglycerolphosphate via a glycosidic linkage between the potential reducing terminal GlcNAc residue and the phosphate residue of the aglycone [Rick98]. The phosphoglyceride aglycone anchors the ECA chains to the outer membrane [Rinno80, Acker81]. The number of repeats units in each chain varies within the organism and among organisms. For example, in Escherichia coli K-12 the chains range from 1 to 14 repeat units [Barr99].

The ECA trisaccharide repeat unit is assembled on the cytosolic face of the cytoplasmic membrane as an undecaprenyldiphosphate-linked intermediate (lipid III). However, the Wzy-catalyzed polymerization of repeat units to form linear polysaccharide chains occurs on the periplasmic face of the membrane. This requires the translocation of lipid III to the periplasmic face of the membrane, which is mediated by a flippase encoded by the wzxE gene [Macpherson95].

Polymerization is followed by the transfer of polysaccharide chains from the lipid carrier to either an as yet unidentified acceptor to yield phosphoglyceride-linked chains or to the core region of lipopolysaccharides. ECA linked to phosphoglycerides is known as ECA_PG and ECA linked to the core region of a lipopolysaccharide is known as ECA_LPS [Kiss78, Kuhn88]. Both of those forms are translocated, by different mechanisms, to the outer membrane. A third form is ECA_CYC, a cyclic water-soluble polymer that contains only ECA trisaccharide repeat units and is found in the periplasm [Dell84, Vinogradov94, Staaf01, Erbel03].