Phosphorylation of the inner core oligosaccharide of lipopolysaccharide mediates recognition and phagocytosis of Gram-negative bacteria by Brain Angiogenesis Inhibitor 1 (BAI1)

Abstract

Recognition of lipopolysaccharide (LPS) is critical for host defense against Gram-negative bacteria and the regulation of local inflammatory responses at the host-microbial interface. We have previously shown that the adhesion G-protein-coupled receptor (aGPCR) BAI1 acts as a phagocytic pattern recognition receptor (PRR) that selectively recognizes Gram-negative bacteria through a series of five Type I thrombospondin repeats (TSRs) in its extracellular domain. Unlike Toll-like receptor 4 (TLR4), which recognizes the hydrophobic lipid A region of LPS, the BAI1 TSRs bind the core oligosaccharide of canonical LPS structures. In this study, we report that BAI1 requires the phosphorylated inner core L-glycero-d-manno-heptose moiety of LPS for recognition, in the context of intact, live bacteria. Mutant strains of Salmonella enterica serovar Typhimurium or E. coli BW25113 K-12 that fail to phosphorylate the inner core oligosaccharide, specifically heptose I, are not recognized by BAI1. Phosphorylation of inner core oligosaccharides is critical for membrane stability and function and is conserved across most Gram-negative species, thus allowing BAI1 to recognize a broad range of organisms.