MHC-II dynamics are maintained in HLA-DR allotypes to ensure catalyzed peptide exchange.

Presentation of antigenic peptides by major histocompatibility complex class II (MHC-II) proteins determines T helper cell reactivity. The MHC-II genetic locus displays a large degree of allelic polymorphism influencing the peptide repertoire presented by the resulting MHC-II protein allotypes. During antigen processing, the human leukocyte antigen (HLA) molecule HLA-DM (DM) encounters these distinct allotypes and catalyzes exchange of the placeholder peptide CLIP by exploiting dynamic features of MHC-II. Here, we investigate 12 highly abundant CLIP-bound HLA-DRB1 allotypes and correlate dynamics to catalysis by DM. Despite large differences in thermodynamic stability, peptide exchange rates fall into a target range that maintains DM responsiveness. A DM-susceptible conformation is conserved in MHC-II molecules, and allosteric coupling between polymorphic sites affects dynamic states that influence DM catalysis. As exemplified for rheumatoid arthritis, we postulate that intrinsic dynamic features of peptide-MHC-II complexes contribute to the association of individual MHC-II allotypes with autoimmune disease.

Target Recognition in Tandem WW Domains: Complex Structures for Parallel and Antiparallel Ligand Orientation in h-FBP21 Tandem WW.

Protein-protein interactions often rely on specialized recognition domains, such as WW domains, which bind to specific proline-rich sequences. The specificity of these protein-protein interactions can be increased by tandem repeats, i.e., two WW domains connected by a linker. With a flexible linker, the WW domains can move freely with respect to each other. Additionally, the tandem WW domains can bind in two different orientations to their target sequences. This makes the elucidation of complex structures of tandem WW domains extremely challenging. Here, we identify and characterize two complex structures of the tandem WW domain of human formin-binding protein 21 and a peptide sequence from its natural binding partner, the core-splicing protein SmB/B'. The two structures differ in the ligand orientation and, consequently, also in the relative orientation of the two WW domains. We analyze and probe the interactions in the complexes by molecular simulations and NMR experiments. The workflow to identify the complex structures uses molecular simulations, density-based clustering, and peptide docking. It is designed to systematically generate possible complex structures for repeats of recognition domains. These structures will help us to understand the synergistic and multivalency effects that generate the astonishing versatility and specificity of protein-protein interactions.

FBP21's C-Terminal Domain Remains Dynamic When Wrapped around the c-Sec63 Unit of Brr2 Helicase.

Based on our recent finding that FBP21 regulates human Brr2 helicase activity involved in the activation of the spliceosomal B-complex, we investigated the structural and dynamic contribution of FBP21 to the interaction. By using NMR spectroscopy, we could show that the 50 C-terminal residues of FBP21 (FBP21326-376), which are sufficient to fully form the interaction with the C-terminal Sec63 unit of Brr2 (Brr2C-Sec63), adopt a random-coil conformation in their unbound state. Upon interaction with Brr2C-Sec63, 42 residues of FBP21326-376 cover the large binding site on Brr2C-Sec63 in an extended conformation. Short charged motifs are steering complex formation, still allowing the bound state to retain dynamics. Based on fragment docking in combination with experimental restraints, we present models of the complex structure. The FBP21326-376/Brr2C-Sec63 interaction thus presents an example of an intrinsically disordered protein/ordered-protein interaction in which a large binding site provides high specificity and, in combination with conformational disorder, displays a relatively high affinity.

A new role for FBP21 as regulator of Brr2 helicase activity.

Splicing of eukaryotic pre-mRNA is carried out by the spliceosome, which assembles stepwise on each splicing substrate. This requires the concerted action of snRNPs and non-snRNP accessory proteins, the functions of which are often not well understood. Of special interest are B complex factors that enter the spliceosome prior to catalytic activation and may alter splicing kinetics and splice site selection. One of these proteins is FBP21, for which we identified several spliceosomal binding partners in a yeast-two-hybrid screen, among them the RNA helicase Brr2. Biochemical and biophysical analyses revealed that an intrinsically disordered region of FBP21 binds to an extended surface of the C-terminal Sec63 unit of Brr2. Additional contacts in the C-terminal helicase cassette are required for allosteric inhibition of Brr2 helicase activity. Furthermore, the direct interaction between FBP21 and the U4/U6 di-snRNA was found to reduce the pool of unwound U4/U6 di-snRNA. Our results suggest FBP21 as a novel key player in the regulation of Brr2.

Intrinsically Disordered Protein Ntr2 Modulates the Spliceosomal RNA Helicase Brr2.

Precursor messenger RNA splicing is mediated by the spliceosome, a large and dynamic molecular machine composed of five small nuclear RNAs and numerous proteins. Many spliceosomal proteins are predicted to be intrinsically disordered or contain large disordered regions, but experimental validation of these predictions is scarce, and the precise functions of these proteins are often unclear. Here, we show via circular dichroism spectroscopy, dynamic light scattering, and NMR spectroscopy that the yeast spliceosomal disassembly factor Ntr2 is largely intrinsically disordered. Peptide SPOT analyses, analytical size-exclusion chromatography, and surface plasmon resonance measurements revealed that Ntr2 uses an N-terminal region to bind the C-terminal helicase unit of the Brr2 RNA helicase, an enzyme involved in spliceosome activation and implicated in splicing catalysis and spliceosome disassembly. NMR analyses suggested that Ntr2 does not adopt a tertiary structure and likely remains disordered upon complex formation. RNA binding and unwinding studies showed that Ntr2 downregulates Brr2 helicase activity in vitro by modulating the fraction of helicase molecules productively bound to the RNA substrate. Our data clarify the nature of a physical link between Brr2 and Ntr2, and point to the possibility of a functional Ntr2-Brr2 interplay during splicing.

HLA-DMA polymorphisms differentially affect MHC class II peptide loading.

During the adaptive immune response, MHCII proteins display antigenic peptides on the cell surface of APCs for CD4(+) T cell surveillance. HLA-DM, a nonclassical MHCII protein, acts as a peptide exchange catalyst for MHCII, editing the peptide repertoire. Although they map to the same gene locus, MHCII proteins exhibit a high degree of polymorphism, whereas only low variability has been observed for HLA-DM. As HLA-DM activity directly favors immunodominant peptide presentation, polymorphisms in HLA-DM (DMA or DMB chain) might well be a contributing risk factor for autoimmunity and immune disorders. Our systematic comparison of DMA*0103/DMB*0101 (DMA-G155A and DMA-R184H) with DMA*0101/DMB*0101 in terms of catalyzed peptide exchange and dissociation, as well as direct interaction with several HLA-DR/peptide complexes, reveals an attenuated catalytic activity of DMA*0103/DMB*0101. The G155A substitution dominates the catalytic behavior of DMA*0103/DMB*0101 by decreasing peptide release velocity. Preloaded peptide-MHCII complexes exhibit ∼2-fold increase in half-life in the presence of DMA*0103/DMB*0101 when compared with DMA*0101/DMB*0101. We show that this effect leads to a greater persistence of autoimmunity-related Ags in the presence of high-affinity competitor peptide. Our study therefore reveals that HLA-DM polymorphic residues have a considerable impact on HLA-DM catalytic activity.

Analysis of Phosphorylation-dependent Protein Interactions of Adhesion and Degranulation Promoting Adaptor Protein (ADAP) Reveals Novel Interaction Partners Required for Chemokine-directed T cell Migration.

Stimulation of T cells leads to distinct changes of their adhesive and migratory properties. Signal propagation from activated receptors to integrins depends on scaffolding proteins such as the adhesion and degranulation promoting adaptor protein (ADAP)(1). Here we have comprehensively investigated the phosphotyrosine interactome of ADAP in T cells and define known and novel interaction partners of functional relevance. While most phosphosites reside in unstructured regions of the protein, thereby defining classical SH2 domain interaction sites for master regulators of T cell signaling such as SLP76, Fyn-kinase, and NCK, other binding events depend on structural context. Interaction proteomics using different ADAP constructs comprising most of the known phosphotyrosine motifs as well as the structured domains confirm that a distinct set of proteins is attracted by pY571 of ADAP, including the ζ-chain-associated protein kinase of 70 kDa (ZAP70). The interaction of ADAP and ZAP70 is inducible upon stimulation either of the T cell receptor (TCR) or by chemokine. NMR spectroscopy reveals that the N-terminal SH2 domains within a ZAP70-tandem-SH2 construct is the major site of interaction with phosphorylated ADAP-hSH3(N) and microscale thermophoresis (MST) indicates an intermediate binding affinity (Kd = 2.3 µm). Interestingly, although T cell receptor dependent events such as T cell/antigen presenting cell (APC) conjugate formation and adhesion are not affected by mutation of Y571, migration of T cells along a chemokine gradient is compromised. Thus, although most phospho-sites in ADAP are linked to T cell receptor related functions we have identified a unique phosphotyrosine that is solely required for chemokine induced T cell behavior.

Susceptibility to HLA-DM protein is determined by a dynamic conformation of major histocompatibility complex class II molecule bound with peptide.

HLA-DM mediates the exchange of peptides loaded onto MHCII molecules during antigen presentation by a mechanism that remains unclear and controversial. Here, we investigated the sequence and structural determinants of HLA-DM interaction. Peptides interacting nonoptimally in the P1 pocket exhibited low MHCII binding affinity and kinetic instability and were highly susceptible to HLA-DM-mediated peptide exchange. These changes were accompanied by conformational alterations detected by surface plasmon resonance, SDS resistance assay, antibody binding assay, gel filtration, dynamic light scattering, small angle x-ray scattering, and NMR spectroscopy. Surprisingly, all of those changes could be reversed by substitution of the P9 pocket anchor residue. Moreover, MHCII mutations outside the P1 pocket and the HLA-DM interaction site increased HLA-DM susceptibility. These results indicate that a dynamic MHCII conformational determinant rather than P1 pocket occupancy is the key factor determining susceptibility to HLA-DM-mediated peptide exchange and provide a molecular mechanism for HLA-DM to efficiently target unstable MHCII-peptide complexes for editing and exchange those for more stable ones.

Bidirectional binding of invariant chain peptides to an MHC class II molecule.

T-cell recognition of peptides bound to MHC class II (MHCII) molecules is a central event in cell-mediated adaptive immunity. The current paradigm holds that prebound class II-associated invariant chain peptides (CLIP) and all subsequent antigens maintain a canonical orientation in the MHCII binding groove. Here we provide evidence for MHCII-bound CLIP inversion. NMR spectroscopy demonstrates that the interconversion from the canonical to the inverse alignment is a dynamic process, and X-ray crystallography shows that conserved MHC residues form a hydrogen bond network with the peptide backbone in both orientations. The natural catalyst HLA-DM accelerates peptide reorientation and the exchange of either canonically or inversely bound CLIP against antigenic peptide. Thus, noncanonical MHC-CLIP displays the hallmarks of a structurally and functionally intact antigen-presenting complex.

Kinetics, Thermodynamics, and Structural Effects of Quinoline-2-Carboxylates, Zinc-Binding Inhibitors of New Delhi Metallo-ß-lactamase-1 Re-sensitizing Multidrug-Resistant Bacteria for Carbapenems.

Carbapenem resistance mediated by metallo-ß-lactamases (MBL) such as New Delhi metallo-ß-lactamase-1 (NDM-1) has become a major factor threatening the efficacy of essential ß-lactam antibiotics. Starting from hit fragment dipicolinic acid (DPA), 8-hydroxy- and 8-sulfonamido-quinoline-2-carboxylic acids were developed as inhibitors of NDM-1 with highly improved inhibitory activity and binding affinity. The most active compounds formed reversibly inactive ternary protein-inhibitor complexes with two zinc ions as proven by native protein mass spectrometry and bio-layer interferometry. Modification of the NDM-1 structure with remarkable entropic gain was shown by isothermal titration calorimetry and NMR spectroscopy of isotopically labeled protein. The best compounds were potent inhibitors of NDM-1 and other representative MBL with no or little inhibition of human zinc-binding enzymes. These inhibitors significantly reduced the minimum inhibitory concentrations (MIC) of meropenem for multidrug-resistant bacteria recombinantly expressing blaNDM-1 as well as for several multidrug-resistant clinical strains at concentrations non-toxic to human cells.

Proline-rich sequence recognition: II. Proteomics analysis of Tsg101 ubiquitin-E2-like variant (UEV) interactions.

The tumor maintenance protein Tsg101 has recently gained much attention because of its involvement in endosomal sorting, virus release, cytokinesis, and cancerogenesis. The ubiquitin-E2-like variant (UEV) domain of the protein interacts with proline-rich sequences of target proteins that contain P(S/T)AP amino acid motifs and weakly binds to the ubiquitin moiety of proteins committed to sorting or degradation. Here we performed peptide spot analysis and phage display to refine the peptide binding specificity of the Tsg101 UEV domain. A mass spectrometric proteomics approach that combines domain-based pulldown experiments, binding site inactivation, and stable isotope labeling by amino acids in cell culture (SILAC) was then used to delineate the relative importance of the peptide and ubiquitin binding sites. Clearly "PTAP" interactions dominate target recognition, and we identified several novel binders as for example the poly(A)-binding protein 1 (PABP1), Sec24b, NFkappaB2, and eIF4b. For PABP1 and eIF4b the interactions were confirmed in the context of the corresponding full-length proteins in cellular lysates. Therefore, our results strongly suggest additional roles of Tsg101 in cellular regulation of mRNA translation. Regulation of Tsg101 itself by the ubiquitin ligase TAL (Tsg101-associated ligase) is most likely conferred by a single PSAP binding motif that enables the interaction with Tsg101 UEV. Together with the results from the accompanying article (Kofler, M., Schuemann, M., Merz, C., Kosslick, D., Schlundt, A., Tannert, A., Schaefer, M., Lührmann, R., Krause, E., and Freund, C. (2009) Proline-rich sequence recognition: I. Marking GYF and WW domain assembly sites in early spliceosomal complexes. Mol. Cell. Proteomics 8, 2461-2473) on GYF and WW domain pathways our work defines major proline-rich sequence-mediated interaction networks that contribute to the modular assembly of physiologically relevant protein complexes.

Major histocompatibility complex (MHC) class I and class II proteins: impact of polymorphism on antigen presentation.

The major histocompatibility complex (MHC) loci are amongst the most polymorphic regions in the genomes of vertebrates. In the human population, thousands of MHC gene variants (alleles) exist that translate into distinct allotypes equipped with overlapping but unique peptide binding profiles. Understanding the differential structural and dynamic properties of MHC alleles and their interaction with critical regulators of peptide exchange bears the potential for more personalized strategies of immune modulation in the context of HLA-associated diseases.

Type 1 Diabetes and the HLA Region: Genetic Association Besides Classical HLA Class II Genes.

Type 1 diabetes is an autoimmune disease with rising incidence in high-income countries. Genetic and environmental predisposing factors contribute to the etiology of the disease, although their interaction is not sufficiently understood to allow for preventive action. Strongest known associations with genetic variation map to classical HLA class II genes. Because of its genetic complexity, the HLA region has been under-represented in genome-wide association studies, having potentially hindered the identification of relevant associations underlying the etiology of the disease. Here, we performed a comprehensive HLA-wide genetic association analysis of type 1 diabetes including multi-allelic and rare variants. We used high-density whole-exome sequencing data of the HLA region in the large UK Biobank dataset to apply gene-based association tests with a carefully defined type 1 diabetes phenotype (97 cases and 48,700 controls). Exon-based and single-variant association tests were used to complement the analysis. We replicated the known association of type 1 diabetes with the classical HLA-DQ gene. Tailoring the analysis toward rare variants, we additionally identified the lysine methyl transferase EHMT2 as associated. Deeper insight into genetic variation associated with disease as presented and discussed in detail here can help unraveling mechanistic details of the etiology of type 1 diabetes. More specifically, we hypothesize that genetic variation in EHMT2 could impact autoimmunity in type 1 diabetes development.

Exchange catalysis by tapasin exploits conserved and allele-specific features of MHC-I molecules.

The repertoire of peptides presented by major histocompatibility complex class I (MHC-I) molecules on the cell surface is tailored by the ER-resident peptide loading complex (PLC), which contains the exchange catalyst tapasin. Tapasin stabilizes MHC-I molecules and promotes the formation of stable peptide-MHC-I (pMHC-I) complexes that serve as T cell antigens. Exchange of suboptimal by high-affinity ligands is catalyzed by tapasin, but the underlying mechanism is still elusive. Here we analyze the tapasin-induced changes in MHC-I dynamics, and find the catalyst to exploit two essential features of MHC-I. First, tapasin recognizes a conserved allosteric site underneath the α2-1-helix of MHC-I, 'loosening' the MHC-I F-pocket region that accomodates the C-terminus of the peptide. Second, the scoop loop11-20 of tapasin relies on residue L18 to target the MHC-I F-pocket, enabling peptide exchange. Meanwhile, tapasin residue K16 plays an accessory role in catalysis of MHC-I allotypes bearing an acidic F-pocket. Thus, our results provide an explanation for the observed allele-specificity of catalyzed peptide exchange.

The HIV-1 capsid protein C-terminal domain in complex with a virus assembly inhibitor.

Immature HIV particles bud from infected cells after assembly at the cytoplasmic side of cellular membranes. This assembly is driven by interactions between Gag polyproteins. Mature particles, each containing a characteristic conical core, are later generated by proteolytic maturation of Gag in the virion. The C-terminal domain of the HIV-1 capsid protein (C-CA) has been shown to contain oligomerization determinants essential for particle assembly. Here we report the 1.7-A-resolution crystal structure of C-CA in complex with a peptide capable of inhibiting immature- and mature-like particle assembly in vitro. The peptide inserts as an amphipathic alpha-helix into a conserved hydrophobic groove of C-CA, resulting in formation of a compact five-helix bundle with altered dimeric interactions. This structure thus reveals the details of an allosteric site in the HIV capsid protein that can be targeted for antiviral therapy.

A peptide inhibitor of HIV-1 assembly in vitro.

Formation of infectious HIV-1 involves assembly of Gag polyproteins into immature particles and subsequent assembly of mature capsids after proteolytic disassembly of the Gag shell. We report a 12-mer peptide, capsid assembly inhibitor (CAI), that binds the capsid (CA) domain of Gag and inhibits assembly of immature- and mature-like capsid particles in vitro. CAI was identified by phage display screening among a group of peptides with similar sequences that bind to a single reactive site in CA. Its binding site was mapped to CA residues 169-191, with an additional contribution from the last helix of CA. This result was confirmed by a separate X-ray structure analysis showing that CAI inserts into a conserved hydrophobic groove and alters the CA dimer interface. The CAI binding site is a new target for antiviral development, and CAI is the first known inhibitor directed against assembly of immature HIV-1.

The Na,K-ATPase acts upstream of phosphoinositide PI(4,5)P2 facilitating unconventional secretion of Fibroblast Growth Factor 2.

FGF2 is a tumor cell survival factor that is exported from cells by an ER/Golgi-independent secretory pathway. This unconventional mechanism of protein secretion is based on direct translocation of FGF2 across the plasma membrane. The Na,K-ATPase has previously been shown to play a role in this process, however, the underlying mechanism has remained elusive. Here, we define structural elements that are critical for a direct physical interaction between FGF2 and the α1 subunit of the Na,K-ATPase. In intact cells, corresponding FGF2 mutant forms were impaired regarding both recruitment at the inner plasma membrane leaflet and secretion. Ouabain, a drug that inhibits both the Na,K-ATPase and FGF2 secretion, was found to impair the interaction of FGF2 with the Na,K-ATPase in cells. Our findings reveal the Na,K-ATPase as the initial recruitment factor for FGF2 at the inner plasma membrane leaflet being required for efficient membrane translocation of FGF2 to cell surfaces.

A xenon-129 biosensor for monitoring MHC-peptide interactions.

Caged in: The formation of a complex between a peptide ligand and a major histocompatibility complex (MHC) class II protein is detected by a (129)Xe biosensor. Cryptophane molecules that trap Xe atoms are modified with a hemagglutinin (HA) peptide, which binds to the MHC protein. The interaction can be monitored by an NMR chemical shift change of cage-HA bound (129)Xe.

Exon Inclusion Modulates Conformational Plasticity and Autoinhibition of the Intersectin 1 SH3A Domain.

The scaffolding protein intersectin 1 plays important roles in clathrin-mediated endocytosis and in the replenishment of release-ready synaptic vesicles (SV). Two splice variants of intersectin's SH3A domain are expressed in the brain, and association of the neuron-specific variant with synapsin I has been shown to enable sustained neurotransmission and to be regulated by an adjacent C-terminal motif. Here, we demonstrate that the ubiquitously expressed short SH3A variant of intersectin 1 interacts with an N-terminal intramolecular sequence that operates synergistically with the C-terminal motif. NMR spectroscopic investigations show that the five-amino acid insertion into the ß strand 2 of the neuronal SH3A variant introduces conformational plasticity incompatible with binding of the N-terminal sequence. The difference in the autoregulatory mechanism of the domain's variants differentially affects its synaptic binding partners, thereby establishing alternative splicing in conjunction with autoinhibitory motif variation as a mechanism to regulate protein interaction networks.

Major Histocompatibility Complex (MHC) Class I and MHC Class II Proteins: Conformational Plasticity in Antigen Presentation.

Antigen presentation by major histocompatibility complex (MHC) proteins is essential for adaptive immunity. Prior to presentation, peptides need to be generated from proteins that are either produced by the cell's own translational machinery or that are funneled into the endo-lysosomal vesicular system. The prolonged interaction between a T cell receptor and specific pMHC complexes, after an extensive search process in secondary lymphatic organs, eventually triggers T cells to proliferate and to mount a specific cellular immune response. Once processed, the peptide repertoire presented by MHC proteins largely depends on structural features of the binding groove of each particular MHC allelic variant. Additionally, two peptide editors-tapasin for class I and HLA-DM for class II-contribute to the shaping of the presented peptidome by favoring the binding of high-affinity antigens. Although there is a vast amount of biochemical and structural information, the mechanism of the catalyzed peptide exchange for MHC class I and class II proteins still remains controversial, and it is not well understood why certain MHC allelic variants are more susceptible to peptide editing than others. Recent studies predict a high impact of protein intermediate states on MHC allele-specific peptide presentation, which implies a profound influence of MHC dynamics on the phenomenon of immunodominance and the development of autoimmune diseases. Here, we review the recent literature that describe MHC class I and II dynamics from a theoretical and experimental point of view and we highlight the similarities between MHC class I and class II dynamics despite the distinct functions they fulfill in adaptive immunity.