Molecular structure of bacterial endotoxin (Escherichia coli Re lipopolysaccharide): implications for formation of a novel heterogeneous lattice structure

Analyses of crystals of Escherichia coli Re lipopolysaccharide (LPS) formed after storage in 1% triethylamine indicate that the LPS molecules are assembled to form a monolayered structure consisting of a novel heterogeneous lattice structure, the greater part of which is occupied by one kind of lattice (lattice I), corresponding to the acyl chain portion of lipid A, and the remainder is occupied by the other kind of lattice (lattice II), corresponding to the 3-deoxy-Dmanno-octulosonic acid (dOclA) dimer and the N-acetylglucosamine disaccharide of lipid A. X-ray diffraction reveals that the type of cell is monoclinic (a = 5.53 A, b = 27.2 A, c = 6.47 A, alpha = 90 degrees, beta = 125.8 degrees, gamma = 90 degrees ). Atomic force microscopy shows that crystals consist of multiple layers; the thickness of a layer corresponds to the b-axis value, and two types of surface topographies are visualized. One, regarded as the view onto the acyl chain ends, is two-dimensional arrays of oval bodies that constitute the lattice, with the lattice constants corresponding to the a- and c-axes and the angle of beta (lattice I). The other, regarded as the view onto the dOclA dimers, is two-dimensional arrays of dromedary-back-like bodies that constitute the lattice with axes of 9.0 and 10.7 A and the angle of 65 degrees formed by both axes (lattice II). Based on these results, we present the molecular model of E. coli Re LPS.