James A. Letts

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  Available from: James A. Letts

  Article: Functional reconstitution of purified human Hv1 H+ channels.

  Seok-Yong Lee, James A Letts, Roderick MacKinnon
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  ABSTRACT: Voltage-dependent H(+) (Hv) channels mediate proton conduction into and out of cells under the control of membrane voltage. Hv channels are unusual compared to voltage-dependent K(+), Na(+), and Ca(2+) channels in that Hv channel genes encode a voltage sensor domain (VSD) without a pore domain. The H(+) currents observed when Hv channels are expressed heterologously suggest that the VSD itself provides the pathway for proton conduction. In order to exclude the possibility that the Hv channel VSD assembles with an as yet unknown protein in the cell membrane as a requirement for H(+) conduction, we have purified Hv channels to homogeneity and reconstituted them into synthetic lipid liposomes. The Hv channel VSD by itself supports H(+) flux.
  Journal of Molecular Biology 03/2009; 387(5):1055-60. · 4.00 Impact Factor
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  Available from: James A. Letts

  Article: Dimeric subunit stoichiometry of the human voltage-dependent proton channel Hv1.

  Seok-Yong Lee, James A Letts, Roderick Mackinnon
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  ABSTRACT: In voltage-gated Na(+), K(+), and Ca(2+) channels, four voltage-sensor domains operate on a central pore domain in response to membrane voltage. In contrast, the voltage-gated proton channel (Hv) contains only a voltage-sensor domain, lacking a separate pore domain. The subunit stoichiometry and organization of Hv has been unknown. Here, we show that human Hv1 forms a dimer in the membrane and define regions that are close to the dimer interface by using cysteine cross-linking. Two dimeric interfaces appear to exist in Hv1, one mediated by S1 and the adjacent extracellular loop, and the other mediated by a putative intracellular coiled-coil domain. It may be significant that Hv1 uses for its dimer interface a surface that corresponds to the interface between the voltage sensor and pore in Kv channels.
  Proceedings of the National Academy of Sciences 07/2008; 105(22):7692-5. · 9.68 Impact Factor
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  Article: ABO(H) blood group A and B glycosyltransferases recognize substrate via specific conformational changes.

  Javier A Alfaro, Ruixiang Blake Zheng, Mattias Persson, James A Letts, Robert Polakowski, Yu Bai, Svetlana N Borisova, Nina O L Seto, Todd L Lowary, Monica M Palcic, Stephen V Evans
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  ABSTRACT: The final step in the enzymatic synthesis of the ABO(H) blood group A and B antigens is catalyzed by two closely related glycosyltransferases, an alpha-(1-->3)-N-acetylgalactosaminyltransferase (GTA) and an alpha-(1-->3)-galactosyltransferase (GTB). Of their 354 amino acid residues, GTA and GTB differ by only four "critical" residues. High resolution structures for GTB and the GTA/GTB chimeric enzymes GTB/G176R and GTB/G176R/G235S bound to a panel of donor and acceptor analog substrates reveal "open," "semi-closed," and "closed" conformations as the enzymes go from the unliganded to the liganded states. In the open form the internal polypeptide loop (amino acid residues 177-195) adjacent to the active site in the unliganded or H antigen-bound enzymes is composed of two alpha-helices spanning Arg(180)-Met(186) and Arg(188)-Asp(194), respectively. The semi-closed and closed forms of the enzymes are generated by binding of UDP or of UDP and H antigen analogs, respectively, and show that these helices merge to form a single distorted helical structure with alternating alpha-3(10)-alpha character that partially occludes the active site. The closed form is distinguished from the semi-closed form by the ordering of the final nine C-terminal residues through the formation of hydrogen bonds to both UDP and H antigen analogs. The semi-closed forms for various mutants generally show significantly more disorder than the open forms, whereas the closed forms display little or no disorder depending strongly on the identity of residue 176. Finally, the use of synthetic analogs reveals how H antigen acceptor binding can be critical in stabilizing the closed conformation. These structures demonstrate a delicately balanced substrate recognition mechanism and give insight on critical aspects of donor and acceptor specificity, on the order of substrate binding, and on the requirements for catalysis.
  Journal of Biological Chemistry 04/2008; 283(15):10097-108. · 4.77 Impact Factor
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  Article: The effect of heavy atoms on the conformation of the active-site polypeptide loop in human ABO(H) blood-group glycosyltransferase B.

  James A Letts, Mattias Persson, Brock Schuman, Svetlana N Borisova, Monica M Palcic, Stephen V Evans
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  ABSTRACT: The human ABO(H) blood-group antigens are oligosaccharide structures that are expressed on erythrocyte and other cell surfaces. The terminal carbohydrate residue differs between the blood types and determines the immune reactivity of this antigen. Individuals with blood type A have a terminal N-acetylgalactosamine residue and those with blood type B have a terminal galactose residue. The attachment of these terminal carbohydrates are catalyzed by two different glycosyltransferases: an alpha(1-->3)N-acetylgalactosaminyltransferase (GTA) and an alpha(1-->3)galactosyltransferase (GTB) for blood types A and B, respectively. GTA and GTB are homologous enzymes that differ in only four of 354 amino-acid residues (Arg/Gly176, Gly/Ser235, Leu/Met266 and Gly/Ala268 in GTA and GTB, respectively). Diffraction-quality crystals of GTA and GTB have previously been grown from as little as 10 mg ml(-1) stock solutions in the presence of Hg, while diffraction-quality crystals of the native enzymes require much higher concentrations of protein. The structure of a single mutant C209A has been determined in the presence and absence of heavy atoms and reveals that when mercury is complexed with Cys209 it forces a significant level of disorder in a polypeptide loop (amino acids 179-195) that is known to cover the active site of the enzyme. The observation that the more highly disordered structure is more amenable to crystallization is surprising and the derivative provides insight into the mobility of this polypeptide loop compared with homologous enzymes.
  Acta Crystallographica Section D Biological Crystallography 09/2007; 63(Pt 8):860-5. · 12.62 Impact Factor
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  Article: Structural basis for red cell phenotypic changes in newly identified, naturally occurring subgroup mutants of the human blood group B glycosyltransferase.

  Bahram Hosseini-Maaf, James A Letts, Mattias Persson, Elizabeth Smart, Pierre-Yves LePennec, Hein Hustinx, Zhihon Zhao, Monica M Palcic, Stephen V Evans, M Alan Chester, Martin L Olsson
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  ABSTRACT: Four amino-acid-changing polymorphisms differentiate the blood group A and B alleles. Multiple missense mutations are associated with weak expression of A and B antigens but the structural changes causing subgroups have not been studied. Individuals or families having serologically weak B antigen on their red cells were studied. Alleles were characterized by sequencing of exons 1 through 7 in the ABO gene. Single crystal X-ray diffraction, three-dimensional-structure molecular modeling, and enzyme kinetics showed the effects of the B allele mutations on the glycosyltransferases. Seven unrelated individuals with weak B phenotypes possessed seven different B alleles, five of which are new and result in substitution of highly conserved amino acids: M189V, I192T, F216I, D262N, and A268T. One of these (F216I) was due to a hybrid allele resulting from recombination between B and O(1v) alleles. The two other alleles were recently described in other ethnic groups and result in V175M and L232P. The first crystal-structure determination (A268T) of a subgroup glycosyltransferase and molecular modeling (F216I, D262N, L232P) indicated conformational changes in the enzyme that could explain the diminished enzyme activity. The effect of three mutations could not be visualized since they occur in a disordered loop. The genetic background for B(w) phenotypes is very heterogeneous but usually arises through seemingly random missense mutations throughout the last ABO exon. The targeted amino acid residues, however, are well conserved during evolution. Based on analysis of the resulting structural changes in the glycosyltransferase, the mutations are likely to disrupt molecular bonds of importance for enzymatic function.
  Transfusion 06/2007; 47(5):864-75. · 3.22 Impact Factor