Fiyaz Mohammed

High-resolution structure of myo-inositol monophosphatase, the putative target of lithium therapy

Inositol monophosphatase is a key enzyme of the phosphatidylinositol signalling pathway and the putative target of the mood-stabilizing drug lithium. The crystal structure of bovine inositol monophosphatase has been determined at 1.4 A resolution in complex with the physiological magnesium ion ligands. Three magnesium ions are octahedrally coordinated at the active site of each of the two subunits of the inositol monophosphatase dimer and a detailed three-metal mechanism is proposed. Ligands to the three metals include the side chains of Glu70, Asp90, Asp93 and Asp220, the backbone carbonyl group of Ile92 and several solvent molecules, including the proposed nucleophilic water molecule (W1) ligated by both Mg-1 and Mg-3. Modelling of the phosphate moiety of inositol monophosphate to superpose the axial phosphate O atoms onto three active-site water molecules orientates the phosphoester bond for in-line attack by the nucleophilic water which is activated by Thr95. Modelling of the pentacoordinate transition state suggests that the 6-OH group of the inositol moiety stabilizes the developing negative charge by hydrogen bonding to a phosphate O atom. Modelling of the post-reaction complex suggests a role for a second water molecule (W2) ligated by Mg-2 and Asp220 in protonating the departing inositolate. This second water molecule is absent in related structures in which lithium is bound at site 2, providing a rationale for enzyme inhibition by this simple monovalent cation. The higher resolution structural information on the active site of inositol monophosphatase will facilitate the design of substrate-based inhibitors and aid in the development of better therapeutic agents for bipolar disorder (manic depression).

The Atomic Resolution Structure of Methanol Dehydrogenase from Methylobacterium Extorquens

The crystal structure of methanol dehydrogenase (MDH) from Methylobacterium extorquens has been refined without stereochemical restraints at a resolution of 1.2 A. The high-resolution data have defined the conformation of the tricyclic pyrroloquinoline quinone (PQQ) cofactor ring as entirely planar. The detailed definition of the active-site geometry has shown many features that are similar to the quinohaemo-protein alcohol dehydrogenases from Comamonas testosteroni and Pseudomonas putida, both of which possess MDH-like and cytochrome c-like domains. Conserved features between the two types of PQQ-containing enzyme suggest a common pathway for electron transfer between MDH and its physiological electron acceptor cytochrome cL. A pathway for proton transfer from the active site to the bulk solvent is also suggested.

Structure of human porphobilinogen deaminase at 2.8 Å: the molecular basis of acute intermittent porphyria

Mutations in the human PBGD (porphobilinogen deaminase) gene cause the inherited defect AIP (acute intermittent porphyria). In the present study we report the structure of the human uPBGD (ubiquitous PBGD) mutant, R167Q, that has been determined by X-ray crystallography and refined to 2.8 A (1 A=0.1 nm) resolution (Rfactor=0.26, Rfree=0.29). The protein crystallized in space group P2(1)2(1)2 with two molecules in the asymmetric unit (a=81.0 A, b=104.4 A and c=109.7 A). Phases were obtained by molecular replacement using the Escherichia coli PBGD structure as a search model. The human enzyme is composed of three domains each of approx. 110 amino acids and possesses a dipyrromethane cofactor at the active site, which is located between domains 1 and 2. An ordered sulfate ion is hydrogen-bonded to Arg26 and Ser28 at the proposed substrate-binding site in domain 1. An insert of 29 amino acid residues, present only in mammalian PBGD enzymes, has been modelled into domain 3 where it extends helix alpha2(3) and forms a beta-hairpin structure that contributes to a continuous hydrogen-bonding network spanning domains 1 and 3. The structural and functional implications of the R167Q mutation and other mutations that result in AIP are discussed.

Secondary anchor polymorphism in the HA-1 minor histocompatibility antigen critically affects MHC stability and TCR recognition.

T cell recognition of minor histocompatibility antigens (mHags) underlies allogeneic immune responses that mediate graft-versus-host disease and the graft-versus-leukemia effect following stem cell transplantation. Many mHags derive from single amino acid polymorphisms in MHC-restricted epitopes, but our understanding of the molecular mechanisms governing mHag immunogenicity and recognition is incomplete. Here we examined antigenic presentation and T-cell recognition of HA-1, a prototypic autosomal mHag derived from single nucleotide dimorphism (HA-1H versus HA-1R) in the HMHA1 gene. The HA-1H peptide is restricted by HLA-A2 and is immunogenic in HA-1R/R into HA-1H transplants, while HA-1R has been suggested to be a “null allele” in terms of T cell reactivity. We found that proteasomal cleavage and TAP transport of the 2 peptides is similar and that both variants can bind to MHC. However, the His>Arg change substantially decreases the stability and affinity of HLA-A2 association, consistent with the reduced immunogenicity of the HA-1R variant. To understand these findings, we determined the structure of an HLA-A2-HA-1H complex to 1.3Å resolution. Whereas His-3 is accommodated comfortably in the D pocket, incorporation of the lengthy Arg-3 is predicted to require local conformational changes. Moreover, a soluble TCR generated from HA-1H-specific T-cells bound HA-1H peptide with moderate affinity but failed to bind HA-1R, indicating complete discrimination of HA-1 variants at the level of TCR/MHC interaction. Our results define the molecular mechanisms governing immunogenicity of HA-1, and highlight how single amino acid polymorphisms in mHags can critically affect both MHC association and TCR recognition.

Clonal selection in the human Vδ1 T cell repertoire indicates γδ TCR-dependent adaptive immune surveillance

γδ T cells are considered to be innate-like lymphocytes that respond rapidly to stress without clonal selection and differentiation. Here we use next-generation sequencing to probe how this paradigm relates to human Vδ2neg T cells, implicated in responses to viral infection and cancer. The prevalent Vδ1 T cell receptor (TCR) repertoire is private and initially unfocused in cord blood, typically becoming strongly focused on a few high-frequency clonotypes by adulthood. Clonal expansions have differentiated from a naive to effector phenotype associated with CD27 downregulation, retaining proliferative capacity and TCR sensitivity, displaying increased cytotoxic markers and altered homing capabilities, and remaining relatively stable over time. Contrastingly, Vδ2+ T cells express semi-invariant TCRs, which are present at birth and shared between individuals. Human Vδ1+ T cells have therefore evolved a distinct biology from the Vδ2+ subset, involving a central, personalized role for the γδ TCR in directing a highly adaptive yet unconventional form of immune surveillance.

Crystal structure of leukocyte Ig-like receptor LILRB4 (ILT3/LIR-5/CD85k): a myeloid inhibitory receptor involved in immune tolerance

The myeloid inhibitory receptor LILRB4 (also called ILT3, LIR-5, CD85k), a member of the leukocyte immunoglobulin-like receptors (LILRs/LIRs), is an important mediator of immune tolerance. Up-regulated on tolerogenic dendritic cells, it has been shown to modulate immune responses via induction of T cell anergy and differentiation of CD8+ T suppressor cells and may play a role in establishing immune tolerance in cancer. Consequently, characterizing the molecular mechanisms involved in LILRB4 function and in particular its structure and ligands is a key aim but has remained elusive to date. Here we describe the production, crystallization, and structure of the LILRB4 ectodomain to 1.7 Å using an expression strategy involving engineering of an additional disulfide bond in the D2 domain to enhance protein stability. LILRB4 comprises two immunoglobulin domains similar in structure to other LILRs; however, the D2 domain, which is most closely related to the D4 domains of other family members, contains 310 helices not previously observed. At the D1-D2 interface, reduced interdomain contacts resulted in an obtuse interdomain angle of ∼107°. Comparison with MHC class I binding Group 1 LILRs suggests LILRB4 is both conformationally and electrostatically unsuited to MHC ligation, consistent with LILRB4 status as a Group 2 LILR likely to bind novel non-MHC class I ligands. Finally, examination of the LILRB4 surface highlighted distinctive surface patches on the D1 domain and D1D2 hinge region, which may be involved in ligand binding. These findings will facilitate our attempts to precisely define the role of LILRB4 in the regulation of immune tolerance.

Crystallization and preliminary X-ray analysis of the controller protein C.AhdI from Aeromonas hydrophilia

Single crystals of purified homodimeric controller protein from Aeromonas hydrophilia (C.AhdI) have been grown under several different conditions using vapour diffusion. X-ray diffraction data have been collected using synchrotron radiation from crystals of both the native and a selenomethionine (SeMet) derivative of the protein. The native crystal form belongs to space group P2(1) and data were collected to a resolution of 2.2 A. Two crystal forms of the SeMet protein have been obtained and were found to belong to space groups P1 and P2(1); data have been recorded to 2.0 and 1.7 A resolution, respectively, for the two crystal forms. Three-wavelength MAD data were collected to 1.7 A for the SeMet derivative crystal, which is isomorphous with the native P2(1) crystal.

BTN3A1 discriminates γδ T cell phosphoantigens from non-antigenic small molecules via a conformational sensor in its B30.2 domain

Human Vγ9/Vδ2 T-cells detect tumor cells and microbial infections by recognizing small phosphorylated prenyl metabolites termed phosphoantigens (P-Ag). The type-1 transmembrane protein Butyrophilin 3A1 (BTN3A1) is critical to the P-Ag-mediated activation of Vγ9/Vδ2 T-cells; however, the molecular mechanisms involved in BTN3A1-mediated metabolite sensing are unclear, including how P-Ags are discriminated from nonantigenic small molecules. Here, we utilized NMR and X-ray crystallography to probe P-Ag sensing by BTN3A1. Whereas the BTN3A1 immunoglobulin variable domain failed to bind P-Ag, the intracellular B30.2 domain bound a range of negatively charged small molecules, including P-Ag, in a positively charged surface pocket. However, NMR chemical shift perturbations indicated BTN3A1 discriminated P-Ag from nonantigenic small molecules by their ability to induce a specific conformational change in the B30.2 domain that propagated from the P-Ag binding site to distal parts of the domain. These results suggest BTN3A1 selectively detects P-Ag intracellularly via a conformational antigenic sensor in its B30.2 domain and have implications for rational design of antigens for Vγ9/Vδ2-based T-cell immunotherapies.

Structural and energetic evidence for highly peptide-specific tumor antigen targeting via allo-MHC restriction

Immunotherapies targeting peptides presented by allogeneic MHC molecules offer the prospect of circumventing tolerance to key tumor-associated self-antigens. However, the degree of antigen specificity mediated by alloreactive T cells, and their ability to discriminate normal tissues from transformed cells presenting elevated antigen levels, is poorly understood. We examined allorecognition of an HLA-A2–restricted Hodgkins lymphoma-associated antigen and were able to isolate functionally antigen-specific allo-HLA-A2–restricted T cells from multiple donors. Binding and structural studies, focused on a prototypic allo-HLA-A2–restricted T-cell receptor (TCR) termed NB20 derived from an HLA-A3 homozygote, suggested highly peptide-specific allorecognition that was energetically focused on antigen, involving direct recognition of a distinct allopeptide presented within a conserved MHC recognition surface. Although NB20/HLA-A2 affinity was unremarkable, TCR/MHC complexes were very short-lived, consistent with suboptimal TCR triggering and tolerance to low antigen levels. These data provide strong molecular evidence that within the functionally heterogeneous alloreactive repertoire, there is the potential for highly antigen-specific “allo-MHC–restricted” recognition and suggest a kinetic mechanism whereby allo-MHC–restricted T cells may discriminate normal from transformed tissue, thereby outlining a suitable basis for broad-based therapeutic targeting of tolerizing tumor antigens.

Mechanism of intermediate filament recognition by plakin repeat domains revealed by envoplakin targeting of vimentin

Plakin proteins form critical connections between cell junctions and the cytoskeleton; their disruption within epithelial and cardiac muscle cells cause skin-blistering diseases and cardiomyopathies. Envoplakin has a single plakin repeat domain (PRD) which recognizes intermediate filaments through an unresolved mechanism. Herein we report the crystal structure of envoplakins complete PRD fold, revealing binding determinants within its electropositive binding groove. Four of its five internal repeats recognize negatively charged patches within vimentin via five basic determinants that are identified by nuclear magnetic resonance spectroscopy. Mutations of the Lys1901 or Arg1914 binding determinants delocalize heterodimeric envoplakin from intracellular vimentin and keratin filaments in cultured cells. Recognition of vimentin is abolished when its residues Asp112 or Asp119 are mutated. The latter slot intermediate filament rods into basic PRD domain grooves through electrosteric complementarity in a widely applicable mechanism. Together this reveals how plakin family members form dynamic linkages with cytoskeletal frameworks.