Recombinant hepatitis B surface antigen and anionic phospholipids share a binding region in the fifth domain of β2-glycoprotein I (apolipoprotein H)

Abstract

Human β 2-glycoprotein I (β 2GPI) binds to recombinant hepatitis B surface antigen (rHBsAg), but the location of the binding domain on β 2GPI is unknown. It has been suggested that the lipid rather than the protein moiety of rHBsAg binds to β 2GPI. Since β 2GPI binds to anionic phospholipids (PL) through its lipid-binding region in the fifth domain of β 2GPI, we predicted that this lipid-binding region may also be involved in binding rHBsAg. In this study, we examined rHBsAg binding to two naturally occurring mutants of β 2GPI, Cys306Gly and Trp316Ser, or evolutionarily conserved hydrophobic amino acid sequence, Leu313-Ala314-Phe315 in the fifth domain of β 2GPI. The two naturally occurring mutations and two mutagenized amino acids, Leu313Gly or Phe315Ser, disrupted the binding of recombinant β 2GPI (rβ 2GPI) to both rHBsAg and cardiolipin (CL), an anionic PL. These results suggest that rHBsAg and CL share the same region in the fifth domain of β 2GPI. Credence to this conclusion was further provided by competitive ELISA, where CL-bound rβ 2GPI was incubated with increasing amounts of rHBsAg. As expected, pre-incubation of rβ 2GPI with CL precluded binding to rHBsAg, indicating that CL and rHBsAg bind to the same region on β 2GPI. Our data provide evidence that the lipid (PL) rather than the protein moiety of rHBsAg binds to β 2GPI and that this binding region is located in the fifth domain of β 2GPI, which also binds to anionic PL.