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BackgroundCytotoxic T-lymphocytes (CTLs) recognize complexes of short peptides with major histocompatibility complex (MHC) class I molecules. MHC molecules are polymorphic, and the products of different MHC alleles bind to different subsets of peptides. This is due to differences in the shape of the peptide-binding groove on the surface of the MHC protein, especially the 'pockets' into which anchor residues at each end of the peptide fit. Non-peptidic ligands for class I molecules may be useful clinically.ResultsBy applying computer-aided design methods guided by X-ray structures, we designed and synthesized several MHC class I ligands, based on known peptide ligands, in which the tricyclic, aromatic compound phenanthridine replaced the central amino acids of the peptides. These semi-peptidic fluorescent ligands bound with high affinity and with allelic specificity to the peptide-binding groove of different MHC class I molecules, forming crystallizable complexes.ConclusionsSpecificity for binding to different MHC class I molecules can be imparted to the common phenanthridine element by judicious choice of terminal peptidic elements from either nonamer or decamer peptides. The phenanthridine-based ligands have a long bound half-life, as do antigenic peptides.

Author(s): Bing Chen [1]; Erik M. Vogan [1, 2]; Haiyun Gong [1]; John J. Skehel [3]; Don C. Wiley [1, 2, 4]; Stephen C. Harrison (corresponding author) [1, 2] The [...]

The [alpha]/[beta] T cell receptor complex transmits signals from MHC/ peptide antigens through a set of constitutively associated signaling molecules, including CD3-[epsilon]/[gamma] and CD3-[epsilon]/[delta]. We report the crystal structure at 1.9-[Angstrom] resolution of a complex between a human CD3-[epsilon]/[delta] ectodomain heterodimer and a single-chain fragment of the UCHT1 antibody. CD3-[epsilon]/[delta] and CD3-[epsilon]/[gamma] share a conserved interface between the Ig-fold ectodomains, with parallel packing of the two G strands. CD3-[delta] has a more electronegative surface and a more compact Ig fold than CD3-[gamma]; thus, the two CD3 heterodimers have distinctly different molecular surfaces. The UCHT1 antibody binds near an acidic region of CD3-[epsilon] opposite the dimer interface, occluding this region from direct interaction with the TCR. This immunodominant epitope may be a uniquely accessible surface in the TCR/CD3 complex, because there is overlap between the binding site of the UCHT1 and OKT3 antibodies. Determination of the CD3-[epsilon]/[delta] structure completes the set of TCR/CD3 globular ectodomains and contributes information about exposed CD3 surfaces. antibody-binding site | CD3 | T cell receptor

Crystal structures of alphabeta T cell receptors, bound to MHC I and II molecules along with bound antigenic peptides, demonstrate the atomic contacts, which illustrate MHC restricted T cell recognition sites.

This review describes the three recently established conformational structures of the Hong Kong influenza virus hemagglutinin. Data are discussed with reference to the activity of hemagglutinin in receptor binding, membrane fusion during viral entry, and their implications in the onset of influenza epidemics.

The structure of a stable recombinant ectodomain of influenza hemagglutinin H[A.sub.2] subunit, EH[A.sub.2] (23-185), defined by proteolysis studies of the intact bacterial-expressed ectodomain, was determined to 1.9-[Angstrom] resolution by using x-ray crystallography. The structure reveals a domain composed of N- and C-terminal residues that form an N cap terminating both the N-terminal [Alpha]-helix and the central coiled coil. The N cap is formed by a conserved sequence, and part of it is found in the neutral pH conformation of HA. The C-terminal 23 residues of the ectodomain form a 72-[Angstrom] long nonhelical structure ordered to within 7 residues of the transmembrane anchor. The structure implies that continuous [Alpha] helices are not required for membrane fusion at either the N or C termini. The difference in stability between recombinant molecules with and without the N cap sequences suggests that additional free energy for membrane fusion may become available after the formation of the central triple-stranded coiled coil and insertion of the fusion peptide into the target membrane.

The molecular arrangement of protein complexes engaged in neurotransmitter release and intracellular vesicular flow has been found to be similar to the structures of certain virus membrane fusion proteins. This arrangement is hypothesized to result in close membrane apposition since the complexes are organized for membrane fusion. Results of a study show that complexes from synapses and intracellular vesicles and the membrane fusion subunits of a set of virus glycoproteins create rod-formed alpha-helical bundles. Although they are quite dissimilar in structure, these bundles seem to possess the membrane-anchoring sequences at one of the rod ends.

Research was conducted to examine the hypothesis that a polar octapeptide fused to the N-terminal fusion peptide solubilizes the influenza virus haemagglutinin HA2 subunit ectodomain. The generated chimeric protein F185 folded spontaneously into a soluble trimer with a high alpha-helical content and a high melting temperature, which are the structural characteristics of the low-pH-induced conformation of HA2. Results suggest that the ectodomains of HA2 made in bacteria have membrane-binding properties similar to those of the same ectodomain produced by low-pH treatment of HA isolated from virus.

The ectodomain of the Ebola virus Gp2 glycoprotein was solubilized with a trimeric, isoleucine zipper derived from GCN4 (pIIGCN4) in place of the hydrophobic fusion peptide at the N terminus. This chimeric molecule forms a trimeric, highly [Alpha]-helical, and very thermostable molecule, as determined by chemical crosslinking and circular dichroism. Electron microscopy indicates that Gp2 folds into a rod-like structure like influenza HA2 and HIV-1 gp41, providing further evidence that viral fusion proteins from diverse families such as Orthomyxoviridae (Influenza), Retroviridae (HIV-1), and Filoviridae (Ebola) share common structural features, and suggesting a common membrane fusion mechanism.

The lysis of abnormal cells deficient in class I major histocompatibility complex (MHC) expression, by natural killer (NK) cells, provides a defence against pathogens and tumour cells. The MHC molecules of normal cells are recognized by NK-cell inhibitory receptors, and escape lysis. The X-ray structure of the p58 NK-cell inhibitory receptor clone 42 was determined at the 1.7-A resolution. The domain topology and their arrangement relative to each other show a link with the haematopoietic receptor family.

The structure of a complex of human T-cell receptor (TCR), viral Tax peptide and a major histocompatibility complex (MHC) molecule HLA-A2 contains diagonal binding of TCR. Variable loops in the TCR structure bury a majority of the peptide and MHC molecule. The diagonal orientation and binding mode of TCR is similar to that observed in all general class I and class II molecules. Orientation of the loops, their interaction with Tax and HLA-A2, and TCR's signalling and specificity are discussed.

A study was conducted to examine the folding of the membrane-anchoring chain of influenza virus hemagglutinin (HA2) in Escherichia coli into the low pH-induced conformation of viral HA2. Crosslinking studies showed that HA2 polypeptide residues 38-175 fold in vivo into a soluble trimer. Crystallography showed that the low pH-induced fold of HA2 is the lowest-energy-state fold of the HA2 polypeptide upon the removal of the receptor-binding chain.

The class II associated invariant chain peptide (CLIP) fragment binds to DR3 in a manner similar to that in which antigenic peptides couple to class II histocompatibility glycoproteins. The complex between HLA-DR3 and CLIP, obtained from a human cell line was analyzed using X-ray crystallography. The structure appears to be that of a substrate for the DM exchanger. The binding between DR3 and CLIP indicates that CLIP motif of Ii stabilizes class II molecules. The CLIP attached to DR3 has a half life of 4 hours.

X-ray crystallographic studies of human class I major histocompatibility complex molecule HLA-Aw68, from which alpha3 domain has been protelytically deleted, reveal no changes in structure with domain removal. Both altered and unaltered molecules exhibit similar complexation with a decameric peptide, implying that peptide binding does not require the alpha3 domain.

The three-dimensional structure of a Staphylococcus aureus superantigen, toxic shock syndrome toxin-1 (TSST-1), complexed with a human class II major histocompatibility molecule DR1), was determined by x-ray crystallography. The TSST-1 [...]