CHAPTER 9:Lipopolysaccharide Induces Raft Domain Expansion in a Cholesterol-Containing Membrane

Lipopolysaccharide (LPS) is a key molecule that triggers the immediate response of the innate immune system in higher animals against invading Gram-negative microbes. Solid-state NMR spectroscopy revealed how ReLPS, a deep rough mutant LPS of Escherichia coli, effects the morphology, phase state and molecular motion of the membrane. ReLPS exerted a substantial influence on the phase state of raft-forming membranes comprising equimolar 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine (DEPE), sphingomyelin (SM) and cholesterol (Chol), as observed using 13C NMR. Both liquid-ordered (Lo) and liquid-disordered (Ld) phases coexist in the DEPE/SM/Chol membrane, whereas the Ld phase disappeared on addition of LPS. This clearly indicates that ReLPS has a capability to expand the “raft” area in raft-forming membranes. NMR relaxation times indicate that the effects of ReLPS incorporation on the motions of the DEPE and DEPE/SM/Chol membranes are opposite. In the DEPE membrane, lipid dynamics were slower than that in the absence of ReLPS. By contrast, in the raft-forming membrane, ReLPS reduced the packing effect of Chol and led to faster lipid motions. Thus, in addition to its role as ligand during the innate immune recognition, LPS may act autocatalytically in animal cell membranes by inducing the expansion of raft regions and rapid motions in membranes to facilitate molecular interactions.