HLA-G is a nonclassical MHC class I (MHCITo date, the structural basis for HLA-G͞LILR recognition remains to be examined. Here, we report the 2.5-Å resolution crystal structure of the LILRB2͞HLA-G complex. LILRB2 exhibits an overlapping but distinct MHCI recognition mode compared with LILRB1 and dominantly recognizes the hydrophobic site of the HLA-G ␣3 domain. NMR binding studies also confirmed these LILR recognition differences on both conformed (heavy chain͞peptide͞␤ 2m) and free forms of ␤2m. Binding studies using ␤2m-free MHCIs revealed differential ␤2m-dependent LILR-binding specificities. These results suggest that subtle structural differences between LILRB family members cause the distinct binding specificities to various forms of HLA-G and other MHCIs, which may in turn regulate immune suppression.crystal structure ͉ immune suppression ͉ MHC class I ͉ ␤2m-free MHC ͉ maternal-fetal interface

HLA-G is a nonclassical major histocompatibility complex class I (MHCI) molecule, which is expressed in trophoblasts and confers immunological tolerance in the maternal-fetal interface by binding to leukocyte Ig-like receptors (LILRs, also called as LIR/ILT/CD85) and CD8. HLA-G is expressed in disulfide-linked dimer form both in solution and at the cell surface. Interestingly, MHCI dimer formations have been involved in pathogenesis and T cell activation. The structure and receptor binding characteristics of MHCI dimers have never been evaluated. Here we performed binding studies showing that the HLA-G dimer exhibited higher overall affinity to LILRB1/2 than the monomer by significant avidity effects. Furthermore, the cell reporter assay demonstrated that the dimer formation remarkably enhanced the LILRB1-mediated signaling at the cellular level. We further determined the crystal structure of the wild-type dimer of HLA-G with the intermolecular Cys 42 -Cys 42 disulfide bond. This dimer structure showed the oblique configuration to expose two LILR/CD8-binding sites upward from the membrane easily accessible for receptors, providing plausible 1:2 (HLA-G dimer:receptors) complex models. These results indicated that the HLA-G dimer conferred increased avidity in a proper structural orientation to induce efficient LILR signaling, resulting in the dominant immunosuppressive effects. Moreover, structural and functional implications for other MHCI dimers observed in activated T cells and the pathogenic allele, HLA-B27, are discussed.During pregnancy, the fetus can be the allogenic object for the maternal immune system, and thus a special system of immune tolerance is necessary for escaping from maternal immune surveillance to achieve a successful pregnancy. However, knowledge of the molecular mechanism of the maternal-fetal immune tolerance is still limited. In the maternal-fetal interface, the fetal extravillous cytotrophoblasts do not express major histocompatibility complex class I molecules (MHCIs), 5 HLA-A or -B, on the cell surface but do express minor classical MHCI, HLA-C, and nonclassical MHCIs, HLA-E and -G (1). Toward T cells, the classical MHCIs present 8 -10 amino acid peptides processed inside cells (e.g. proteasome) to induce peptide-specific T cell immune responses. Thus, the loss of HLA-A and -B expression significantly suppresses maternal T cell responses. Although HLA-C and -E are expressed in normal cells, the expression of HLA-G is restricted to a few tissues as follows: extravillous trophoblasts, thymus epithelial cells, and some tumors (1). In contrast with polymorphic classical MHCIs, HLA-G shows the limited polymorphism, suggesting that HLA-G may potentially have a role as a common ligand for generic immunosuppressive receptors in the protection of fetus cells from the maternal immune cells. Recently, several immunologically relevant cell-surface receptors were found to mediate the negative regulation of immune cells through binding to classical and nonclassical MHCIs. The receptors of HLA-G rep...

In bacteria, incorporation of selenocysteine, the 21(st) amino acid, into proteins requires elongation factor SelB, which has the unusual property of binding to both transfer RNA (tRNA) and mRNA. SelB binds to an mRNA hairpin formed by the selenocysteine insertion sequence (SECIS) with extremely high specificity, the molecular basis of which has been unknown. We have determined the crystal structure of the mRNA-binding domain of SelB in complex with SECIS RNA at a resolution of 2.3 A. This is the first example of a complex between an RNA and a winged-helix (WH) domain, a motif found in many DNA-binding proteins and recently discovered in RNA-binding proteins. Notably, RNA binding does not induce a major conformational change in the WH motif. The structure reveals a new mode of RNA recognition with a geometry that allows the complex to wrap around the small ribosomal subunit.

Leukocyte immunoglobulin-like receptor subfamily B member 1 (LILRB1/LIR1/ILT2) is an inhibitory receptor broadly expressed on leukocytes and recognizes HLA-class I and human cytomegalovirus UL18. LILRB1 is encoded within the leukocyte receptor complex on 19q13.4, previously implicated to be a susceptibility region to systemic lupus erythematosus (SLE). In this study, we screened for polymorphisms of LILRB1 and examined their association with SLE and rheumatoid arthritis (RA). In the 5' portion of LILRB1, three haplotypes containing four non-synonymous substitutions within the ligand-binding domains and two single nucleotide polymorphisms within the promoter region were identified and designated as PE01-03. In the 3' portion, two haplotypes (CY01, 02) containing a non-synonymous substitution of the cytoplasmic region were identified. CY01 and 02 did not co-segregate with PE01-03. Significant association with susceptibility to SLE or RA was not observed; however, among the subjects not carrying RA-associated HLA-DRB1 shared epitope (SE), LILRB1.PE01/01 diplotype was significantly associated with RA (odds ratio 2.05, P = 0.019 and Pc = 0.038). Gross difference was not observed in the crystal structures, thermostabilities and binding affinities to HLA-class I ligands among LILRB1.PE01-03 haplotype products; however, surface expression of LILRB1 was significantly decreased in lymphocytes and monocytes from the carriers of PE01 haplotype. These findings demonstrated that LILRB1 is highly polymorphic and is associated with susceptibility to RA in HLA-DRB1 SE negative subjects, possibly by insufficient inhibitory signaling in leukocytes. In addition, these observations suggested that the polymorphisms of LILR family members may be substantially involved in the diversity of human immune responses.

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