Evolutionary conservation of the lipopolysaccharide binding site of β2-glycoprotein I

Abstract

β₂-Glycoprotein I (β₂GPI) is a highly abundant plasma protein and the major antigen for autoantibodies in the antiphospholipid syndrome. Recently, we have described a novel function of β₂GPI as scavenger of lipopolysaccharide (LPS). With this in mind we investigated the conservation of β₂GPI in vertebrates and set out to identify the binding site of LPS within β₂GPI. The genome sequences of 42 species were surveyed. Surface plasmon resonance (SPR) was performed with peptides to characterise the binding site of β₂GPI for LPS. β₂GPI could be identified in most tested vertebrates with a high overall amino acid homology of 80% or more in mammals. SPR revealed that a synthesised peptide (LAFWKTDA) from domain V of β₂GPI was able to compete for binding of β₂GPI to LPS. The AFWKTDA sequence was completely conserved in all mammals. The peptide containing the LPS binding site attenuated the inhibition by β₂GPI in a cellular model of LPS-induced tissue factor expression. Other important sites, such as the binding site for anionic phospholipids and the antiphospholipid antibody binding epitope, were also preserved. β₂GPI is highly conserved across the animal kingdom, which suggests that the function of β₂GPI may be more important than anticipated.