Contributions of polysaccharide and lipid regions of lipopolysaccharide to the recognition by spike G protein of bacteriophage phi X174

A histidine-tagged G protein of bacteriophage phi X174 (HisG) bound strongly with lipopolysaccharide (LPS) of Escherichia coli C, one of a phi X174-sensitive Ra strain. The dissociation constant, Kd, was measured to be 0.16 +/- 0.04 microM by fluorometric titration. HisG showed slightly less affinity to LPSs of the insensitive Rc and Rd 2 strains having shorter R-core polysaccharide sequences than that of the sensitive Ra strains. The difference between the two types of LPS was demonstrated by CD spectra; LPSs of the sensitive strains increased the signal intensity for beta-sheet, while the insensitive strains decreased it. The chemically degraded LPS derivatives lacking a hydrophobic lipid region showed much less affinity to HisG, indicating the importance of the lipid region of LPS for strong binding with HisG. On the other hand, since the degraded derivatives increased the intensity of CD spectra, the polysaccharide region is thought to contribute to the conformation change of the protein.