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I Complexes (i + complex)
Selected AbstractsRemoval of the PsaF Polypeptide Biases Electron Transfer in Favor of the PsaB Branch of Cofactors in Triton X-100 Photosystem I Complexes from Synechococcus sp.PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2008PCC 700 Continuous wave (CW) and transient electron paramagnetic resonance studies have implied that when PsaF is removed genetically, the double reduction of A1A is facile, the lifetime of A1A, is shorter and the ratio of fast to slow kinetic phases increases in PS I complexes isolated with Triton X-100 (Van der Est, A., A. I. Valieva, Y. E. Kandrashkin, G. Shen, D. A. Bryant and J. H. Golbeck [2004] Biochemistry43, 1264,1275). Changes in the lifetimes of A1A, and A1B, are characteristic of mutants involving the quinone binding sites, but changes in the relative amplitudes of A1A, and A1B, are characteristic of mutants involving the primary electron acceptors, A0A and A0B. Here, we measured the fast and slow phases of electron transfer from A1B, and A1A, to FX in psaF and psaE psaF null mutants using time-resolved CW and pump-probe optical absorption spectroscopy. The lifetime of the fast kinetic phase was found to be unaltered, but the lifetime of the slow kinetic phase was shorter in the psaF null mutant and even more so in the psaE psaF null mutant. Concomitantly, the amplitude of the fast kinetic phase increased by a factor of 1.8 and 2.0 in the psaF and psaE psaF null mutants, respectively, at the expense of the slow kinetic phase. The change in ratio of the fast to slow kinetic phases is explained as either a redirection of electron transfer through A1B at the expense of A1A, or a shortening of the lifetime of A1A, to become identical to that of A1B,. The constant lifetime and the characteristics of the near-UV spectrum of the fast kinetic phase favor the former explanation. A unified hypothesis is presented of a displacement of the A-jk(1) ,-helix and switchback loop, which would weaken the H-bond from Leu722 to A1A, accounting for the acceleration of the slow kinetic phase, as well as weaken the H-bond from Tyr696 to A0A, accounting for the bias of electron transfer in favor of the PsaB branch of cofactors. [source] Complex assembly, crystallization and preliminary X-ray crystallographic studies of duck MHC class I moleculeACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 1 2010Jianhua Zhang In order to understand the biological properties of the immune systems of waterfowl and to establish a system for structural studies of duck class I major histocompatibility complex (DuMHC I), a complex of DuMHC I with duck ,2 -microglobulin (Du,2m) and the peptide AEIEDLIF (AF8) derived from H5N1 NP residues 251,258 was assembled. The complex was crystallized; the crystals belonged to space group C2221, with unit-cell parameters a = 54.7, b = 72.4, c = 102.2,Å, and diffracted to 2.3,Å resolution. Matthews coefficient calculation and initial structure determination by molecular replacement showed that the crystals did not contain the whole DuMHC I complex, but instead contained the DuMHC I ,3 domain and a Du,2m molecule (DuMHC I ,3+,2m). Another complex of DuMHC I with the peptide IDWFDGKE derived from a chicken fusion protein also generated the same results. The stable structure of DuMHC I ,3+,2m may reflect some unique characteristics of DuMHC I and pave the way for novel MHC structure-related studies in the future. [source] Enhanced immunogenicity of CTL antigens through mutation of the CD8 binding MHC class,I invariant regionEUROPEAN JOURNAL OF IMMUNOLOGY, Issue 5 2007Linda Wooldridge Abstract CD8+ cytotoxic T,lymphocytes (CTL) are key determinants of immunity to intracellular pathogens and neoplastic cells. Recognition of specific antigens in the form of peptide-MHC class,I complexes (pMHCI) presented on the target cell surface is mediated by T cell receptor (TCR) engagement. The CD8 coreceptor binds to invariant domains of pMHCI and facilitates antigen recognition. Here, we investigate the biological effects of a Q115E substitution in the ,2,domain of human leukocyte antigen (HLA)-A*0201 that enhances CD8 binding by,,50% without altering TCR/pMHCI interactions. Soluble and cell surface-expressed forms of Q115E HLA-A*0201 exhibit enhanced recognition by CTL without loss of specificity. These CD8-enhanced antigens induce greater CD3 ,,chain phosphorylation in cognate CTL leading to substantial increases in cytokine production, proliferation and priming of naive T cells. This effect provides a fundamental new mechanism with which to enhance cellular immunity to specific T cell antigens. [source] From HLA-B27 to spondyloarthritis: a journey through the ERIMMUNOLOGICAL REVIEWS, Issue 1 2010Robert A. Colbert Summary:, Almost four decades of research into the role of human leukocyte antigen-B27 (HLA-B27) in susceptibility to spondyloarthritis has yet to yield a convincing answer. New results from an HLA-B27 transgenic rat model now demonstrate quite convincingly that CD8+ T cells are not required for the inflammatory phenotype. Discoveries that the HLA-B27 heavy chain has a tendency to misfold during the assembly of class I complexes in the endoplasmic reticulum (ER) and to form aberrant disulfide-linked dimers after transport to the cell surface have forced the generation of new ideas about its role in disease pathogenesis. In transgenic rats, HLA-B27 misfolding generates ER stress and leads to activation of the unfolded protein response, which dramatically enhances the production of interleukin-23 (IL-23) in response to pattern recognition receptor agonists. These findings have led to the discovery of striking T-helper 17 cell activation and expansion in this animal model, consistent with results emerging from humans with spondyloarthritis and the discovery of IL23R as an additional susceptibility gene for ankylosing spondylitis. Together, these results suggest a novel link between HLA-B27 and the T-helper 17 axis through the consequences of protein misfolding and open new avenues of investigation as well as identifying new targets for therapeutic intervention in this group of diseases. [source] E5 protein of human papillomavirus type 16 selectively downregulates surface HLA class IINTERNATIONAL JOURNAL OF CANCER, Issue 2 2005G. Hossein Ashrafi Abstract Papillomaviruses have evolved mechanisms that result in escape from host immune surveillance. The E5 protein is expressed early in papillomavirus infection in the deep layers of the infected epithelium. It is localized to the Golgi apparatus (GA) and endoplasmic reticulum. The E5 protein of bovine papillomavirus (BPV) impairs the synthesis and stability of major histocompatibility (MHC) class I complexes and prevents their transport to the cell surface due to retention in the GA. Here we show that human papillomavirus type 16 (HPV-16) E5 also causes the retention of MHC (HLA) class I complexes in the GA and impedes their transport to the cell surface, which is rescued by treatment with interferon. Unlike BPV E5, HPV-16 E5 does not affect the synthesis of HLA class I heavy chains or the expression of the transporter associated with antigen processing TAP. These results show that downregulation of surface MHC class I molecules is common to both BPV and HPV E5 proteins. Moreover, we determined that HPV-16 E5 downregulates surface expression of HLA-A and HLA-B, which present viral peptides to MHC class I-restricted cytotoxic T lymphocytes (CTLs), but not the natural killer (NK) cell inhibitory ligands HLA-C and HLA-E. Selective downregulation of cell surface HLA class I molecules may allow the virus to establish infection by avoiding immune clearance of virus-infected cells by both CTLs and NK cells. [source] Variations in Photosystem I Properties in the Primordial Cyanobacterium Gloeobacter violaceus PCC 7421PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 1 2010Mamoru Mimuro We compared the optical properties of the trimeric photosystem (PS) I complexes of the primordial cyanobacterium Gloeobacter violaceus PCC 7421 with those of Synechocystis sp. PCC 6803. Gloeobacter violaceus PS I showed (1) a shorter difference maximum of P700 by approximately 2 nm, (2) a smaller antenna size by approximately 10 chlorophyll (Chl) a molecules and (3) an absence of Red Chls. The energy transfer kinetics in the antennae at physiological temperatures were very similar between the two species due to the thermal equilibrium within the antenna; however, they differed at 77 K where energy transfer to Red Chls was clearly observed in Synechocystis sp. PCC 6803. Taken together with the lower P700 redox potential in G. violaceus by approximately 60 mV, we discuss differences in the optical properties of the PS I complexes with respect to the amino acid sequences of core proteins and further to evolution of cyanobacteria. [source] Ligand binding to the inhibitory and stimulatory GTP cyclohydrolase I/GTP cyclohydrolase I feedback regulatory protein complexesPROTEIN SCIENCE, Issue 4 2001Toshie Yoneyama BH4, 6R - l - erythro -5,6,7,8-tetrahydrobiopterin; GFRP, GTP cyclohydrolase I feedback regulatory protein Abstract GTP cyclohydrolase I feedback regulatory protein (GFRP) mediates feedback inhibition of GTP cyclohydrolase I activity by 6R - l - erythro -5,6,7,8-tetrahydrobiopterin (BH4), which is an essential cofactor for key enzymes producing catecholamines, serotonin, and nitric oxide as well as phenylalanine hydroxylase. GFRP also mediates feed-forward stimulation of GTP cyclohydrolase I activity by phenylalanine at subsaturating GTP levels. These ligands, BH4 and phenylalanine, induce complex formation between one molecule of GTP cyclohydrolase I and two molecules of GFRP. Here, we report the analysis of ligand binding using the gel filtration method of Hummel and Dreyer. BH4 binds to the GTP cyclohydrolase I/GFRP complex with a Kd of 4 ,M, and phenylalanine binds to the protein complex with a Kd of 94 ,M. The binding of BH4 is enhanced by dGTP. The binding stoichiometrics of BH4 and phenylalanine were estimated to be 10 molecules of each per protein complex, in other words, one molecule per subunit of protein, because GTP cyclohydrolase I is a decamer and GFRP is a pentamer. These findings were corroborated by data from equilibrium dialysis experiments. Regarding ligand binding to free proteins, BH4 binds weakly to GTP cyclohydrolase I but not to GFRP, and phenylalanine binds weakly to GFRP but not to GTP cyclohydrolase I. These results suggest that the overall structure of the protein complex contributes to binding of BH4 and phenylalanine but also that each binding site of BH4 and phenylalanine may be primarily composed of residues of GTP cyclohydrolase I and GFRP, respectively. [source] | |||||||||||||