Crystallization and preliminary X-ray analysis of the low-affinity complex between human leukocyte antigen-g (HLA-G) and leukocyte ig-like receptor b2 (LILRB2)

Human leukocyte antigen-G (HLA-G) is a nonclassical MHC class I (MHCI) molecule that is expressed mainly on placenta trophoblast cells. Leukocyte Ig-like receptor B2 (LILRB2) is a human inhibitory immune receptor that recognizes HLA-G with a higher affinity than any other MHCI although this interaction is only in the μM range. The interaction between HLA-G and LILRB2 seems to play a dominant role in the escape of the fetus from the maternal immune response. Here we report the crystallization and x-ray analysis of the LILRB2/HLA-G complex. The extracellular domains of HLA-G and LILRB2 were expressed in Escherichia coli, refolded and purified. The initial crystallization trials using novel PEG-based screening sets provided crystals of the LILRB2/HLA-G complex with 40-50% PEG400 as the precipitant. These crystals belong to space group P3₁21 (a=b=81.4 Å, c=186.7 Å, γ=120°). Dehydration of the crystals by soaking them in a solution containing a higher concentration of PEG400 dramatically improved the resolution and also the mosaicity.