Shaodong Dai

The Structure of a Bcl-xL/Bim Fragment Complex: Implications for Bim Function

After antigen-driven expansion, the majority of T cells involved in an immune response die rapidly by apoptosis dependent on the Bcl-2 related proteins, Bim and Bax or Bak. The details of how these proteins are activated and interact are still unclear. The crystal structure of mouse Bcl-x(L) bound to a long helical fragment of Bim indicates that the structure of Bim is very different from proteins with a Bcl-2-like fold and may leave the BH3 region of Bim constitutively exposed. Based on the structural homology between Bcl-x(L) and Bax, we predicted that binding of Bim to Bax would require displacement of the Bax penultimate alpha helix. Consistent with this prediction, truncation of this short helix was required for Bim/Bax interaction and led to spontaneous activation of Bax. Our results suggest a way in which both Bim and Bax/Bak might be required for activated T cell apoptosis.

Structural insights into histone demethylation by JMJD2 family members

Posttranslational modifications of histones regulate chromatin structure and gene expression. Histone demethylases, members of a newly emerging transcription-factor family, remove methyl groups from the lysine residues of the histone tails and thereby regulate the transcriptional activity of target genes. JmjC-domain-containing proteins have been predicted to be demethylases. For example, the JmjC-containing protein JMJD2A has been characterized as a H3-K9me3- and H3-K36me3-specific demethylase. Here, structures of the catalytic-core domain of JMJD2A with and without alpha-ketoglutarate in the presence of Fe2+ have been determined by X-ray crystallography. The structure of the core domain, consisting of the JmjN domain, the JmjC domain, the C-terminal domain, and a zinc-finger motif, revealed the unique elements that form a potential substrate binding pocket. Sited-directed mutagenesis in conjunction with demethylase activity assays allowed us to propose a molecular model for substrate selection by the JMJD2 histone demethylase family.

Evolutionarily Conserved Amino Acids That Control TCR-MHC Interaction

The rules for the conserved reaction of alphabeta T cell receptors (TCRs) with major histocompatibility complex (MHC) proteins plus peptides are poorly understood, probably because thymocytes bearing TCRs with the strongest MHC reactivity are lost by negative selection. Thus, only TCRs with an attenuated ability to react with MHC appear on mature T cells. Also, the interaction sites between TCRs and MHC may be inherently flexible and hence difficult to spot. We reevaluated contacts between TCRs and MHC in the solved structures of their complexes with these points in mind. Relatively conserved amino acids in the TCR complementarity-determining regions (CDR) 1 and CDR2 are often used to bind exposed areas of the MHC alpha-helices. These areas are exposed because of small amino acids that allow somewhat flexible binding of the TCRs. The TCR amino acids involved are specific to families of variable (V) regions and to some extent different rules may govern the recognition of MHCI versus MHCII.

Crossreactive T cells spotlight the germline rules for αβ T cell receptor interactions with MHC molecules

To test whether highly crossreactive alphabeta T cell receptors (TCRs) produced during limited negative selection best illustrate evolutionarily conserved interactions between TCR and major histocompatibility complex (MHC) molecules, we solved the structures of three TCRs bound to the same MHC II peptide (IAb-3K). The TCRs had similar affinities for IAb-3K but varied from noncrossreactive to extremely crossreactive with other peptides and MHCs. Crossreactivity correlated with a shrinking, increasingly hydrophobic TCR-ligand interface, involving fewer TCR amino acids. A few CDR1 and CDR2 amino acids dominated the most crossreactive TCR interface with MHC, including Vbeta8 48Y and 54E and Valpha4 29Y, arranged to impose the familiar diagonal orientation of TCR on MHC. These interactions contribute to MHC binding by other TCRs using related V regions, but not usually so dominantly. These data show that crossreactive TCRs can spotlight the evolutionarily conserved features of TCR-MHC interactions and that these interactions impose the diagonal docking of TCRs on MHC.

Ligand–receptor binding revealed by the TNF family member TALL-1

The tumour necrosis factor (TNF) ligand TALL-1 and its cognate receptors, BCMA, TACI and BAFF-R, were recently identified as members of the TNF superfamily, which are essential factors contributing to B-cell maturation. The functional, soluble fragment of TALL-1 (sTALL-1) forms a virus-like assembly for its proper function. Here we determine the crystal structures of sTALL-1 complexed with the extracellular domains of BCMA and BAFF-R at 2.6 and 2.5 Å, respectively. The single cysteine-rich domain of BCMA and BAFF-R both have saddle-like architectures, which sit on the horseback-like surface formed by four coil regions on each individual sTALL-1 monomer. Three novel structural modules, D2, X2 and N, were revealed from the current structures. Sequence alignments, structural modelling and mutagenesis revealed that one disulphide bridge in BAFF-R is critical for determining the binding specificity of the extracellular domain eBAFF-R to TALL-1 instead of APRIL, a closely related ligand of TALL-1, which was confirmed by binding experiments in vitro.

Structural basis of the recognition of a methylated histone tail by JMJD2A

The Jumonji C domain is a catalytic motif that mediates histone lysine demethylation. The Jumonji C-containing oxygenase JMJD2A specifically demethylates tri- and dimethylated lysine-9 and lysine-36 of histone 3 (H3K9/36me3/2). Here we present structures of the JMJD2A catalytic core complexed with methylated H3K36 peptide substrates in the presence of Fe(II) and N-oxalylglycine. We found that the interaction between JMJD2A and peptides largely involves the main chains of the enzyme and the peptide. The peptide-binding specificity is primarily determined by the primary structure of the peptide, which explains the specificity of JMJD2A for methylated H3K9 and H3K36 instead of other methylated residues such as H3K27. The specificity for a particular methyl group, however, is affected by multiple factors, such as space and the electrostatic environment in the catalytic center of the enzyme. These results provide insights into the mechanisms and specificity of histone demethylation.

Structural snapshots along the reaction pathway of ferredoxin-thioredoxin reductase.

Oxygen-evolving photosynthetic organisms regulate carbon metabolism through a light-dependent redox signalling pathway. Electrons are shuttled from photosystem I by means of ferredoxin (Fdx) to ferredoxin–thioredoxin reductase (FTR), which catalyses the two-electron-reduction of chloroplast thioredoxins (Trxs). These modify target enzyme activities by reduction, regulating carbon flow. FTR is unique in its use of a [4Fe–4S] cluster and a proximal disulphide bridge in the conversion of a light signal into a thiol signal. We determined the structures of FTR in both its one- and its two-electron-reduced intermediate states and of four complexes in the pathway, including the ternary Fdx–FTR–Trx complex. Here we show that, in the first complex (Fdx–FTR) of the pathway, the Fdx [2Fe–2S] cluster is positioned suitably for electron transfer to the FTR [4Fe–4S] centre. After the transfer of one electron, an intermediate is formed in which one sulphur atom of the FTR active site is free to attack a disulphide bridge in Trx and the other sulphur atom forms a fifth ligand for an iron atom in the FTR [4Fe–4S] centre—a unique structure in biology. Fdx then delivers a second electron that cleaves the FTR–Trx heterodisulphide bond, which occurs in the Fdx–FTR–Trx complex. In this structure, the redox centres of the three proteins are aligned to maximize the efficiency of electron transfer from the Fdx [2Fe–2S] cluster to the active-site disulphide of Trxs. These results provide a structural framework for understanding the mechanism of disulphide reduction by an iron–sulphur enzyme and describe previously unknown interaction networks for both Fdx and Trx (refs 4–6).

Identification and analysis of a bottleneck in PCB biodegradation.

The microbial degradation of polychlorinated biphenyls (PCBs) provides the potential to destroy these widespread, toxic and persistent environmental pollutants. For example, the four-step upper bph pathway transforms some of the more than 100 different PCBs found in commercial mixtures and is being engineered for more effective PCB degradation. In the critical third step of this pathway, 2,3-dihydroxybiphenyl (DHB) 1,2-dioxygenase (DHBD; EC 1.13.11.39) catalyzes aromatic ring cleavage. Here we demonstrate that ortho-chlorinated PCB metabolites strongly inhibit DHBD, promote its suicide inactivation and interfere with the degradation of other compounds. For example, kcatapp for 2′,6′-diCl DHB was reduced by a factor of ∼7,000 relative to DHB, and it bound with sufficient affinity to competitively inhibit DHB cleavage at nanomolar concentrations. Crystal structures of two complexes of DHBD with ortho-chlorinated metabolites at 1.7 Å resolution reveal an explanation for these phenomena, which have important implications for bioremediation strategies.

Crystal structure of HLA-DP2 and implications for chronic beryllium disease

Chronic beryllium disease (CBD) is a fibrotic lung disorder caused by beryllium (Be) exposure and is characterized by granulomatous inflammation and the accumulation of Be-responsive CD4+ T cells in the lung. Genetic susceptibility to CBD has been associated with certain alleles of the MHCII molecule HLA-DP, especially HLA-DPB1*0201 and other alleles that contain a glutamic acid residue at position 69 of the β-chain (βGlu69). The HLA-DP alleles that can present Be to T cells match those implicated in the genetic susceptibility, suggesting that the HLA contribution to disease is based on the ability of those molecules to bind and present Be to T cells. The structure of HLA-DP2 and its interaction with Be are unknown. Here, we present the HLA-DP2 structure with its antigen-binding groove occupied by a self-peptide derived from the HLA-DR α-chain. The most striking feature of the structure is an unusual solvent exposed acidic pocket formed between the peptide backbone and the HLA-DP2 β-chain α-helix and containing three glutamic acids from the β-chain, including βGlu69. In the crystal packing, this pocket has been filled with the guanidinium group of an arginine from a neighboring molecule. This positively charged moiety forms an extensive H-bond/salt bridge network with the three glutamic acids, offering a plausible model for how Be-containing complexes might occupy this site. This idea is strengthened by the demonstration that mutation of any of the three glutamic acids in this pocket results in loss of the ability of DP2 to present Be to T cells.

Alternate interactions define the binding of peptides to the MHC molecule IAb

We have solved the crystal structure of the MHCII molecule, IAb, containing an antigenic variant of the major IAb-binding peptide derived from the MHCII IEα chain. The four MHC pockets at p1, p4, p6, and p9 that usually bind peptide side chains are largely empty because of alanines in the peptide at these positions. The complex is nevertheless very stable, apparently because of unique alternate interactions between the IAb and peptide. In particular, there are multiple additional hydrogen bonds between the N-terminal end of the peptide and the IAb α chain and an extensive hydrogen bond network involving an asparagine at p7 position of the peptide and the IAb β chain. By using knowledge of the shape and size of the traditional side chain binding pockets and the additional possible interactions, an IAb peptide-binding motif can be deduced that agrees well with the sequences of known IAb-binding peptides.