title: Human CD4 Metastability Is a Function of the Allosteric Disulfide Bond in Domain 2.
abstract: CD4 is expressed on the surface of specific leukocytes where it plays a key role in the activation of immunostimulatory T-cells and acts as a primary receptor for HIV-1 entry. CD4 has four ecto-domains (D1-D4) of which D1, D2, and D4 contain disulfide bonds. Although disulfide bonds commonly serve structural or catalytic functions, a rare class of disulfide bonds possessing unusually high dihedral strain energy and a relative ease of reduction can impact protein function by shuffling their redox state. D2 of CD4 possesses one such "allosteric" disulfide. While it is becoming accepted that redox exchange of the D2 allosteric disulfide plays an essential role in regulating CD4 activity, the biophysical consequences of its reduction remain incompletely understood. By analyzing the hydrodynamic volume, secondary structure, and thermal stability of the reduced and nonreduced forms of the single D1 and D2 domains, as well as the various redox isomers of two domain CD4, we have shown that ablation of the allosteric disulfide bond in domain 2 results in both a favorable structural collapse and an increase in the stability of CD4. Conversely, ablating the structural disulfide of D1 results in destabilizing structural rearrangements in CD4. These findings expand our understanding of the mechanisms by which oxidoreduction of the D2 allosteric disulfide regulates CD4 function; they reveal the intrinsic disulfide-dependent metastability of D2 and illustrate that redox shuffling of the allosteric disulfide results in previously undescribed conformational changes in CD4 that are likely important for its interaction with its protein partners.
authors.affiliation: HIV Pathogenesis Research Unit, Department of Molecular Medicine and Haematology, Faculty of Health Sciences, University of the Witwatersrand , 7 York Road, Parktown, 2193, Johannesburg, South Africa.
authors.forename: Gavin R
authors.initials: GR
authors.lastname: Owen
authors.affiliation: Faculty of Natural Sciences, Keele University , Keele, Staffordshire ST5 5BG, United Kingdom.|Life Sciences Group, Institut Laue-Langevin , 71 Avenue des Martyrs, 38042, Grenoble, France.|European Synchrotron Radiation Facility , 71 Avenue des Martyrs, 38042, Grenoble, France.
authors.forename: Jennifer A
authors.initials: JA
authors.lastname: Channell
authors.affiliation: Faculty of Natural Sciences, Keele University , Keele, Staffordshire ST5 5BG, United Kingdom.|Life Sciences Group, Institut Laue-Langevin , 71 Avenue des Martyrs, 38042, Grenoble, France.
authors.forename: V Trevor
authors.initials: VT
authors.lastname: Forsyth
authors.affiliation: Life Sciences Group, Institut Laue-Langevin , 71 Avenue des Martyrs, 38042, Grenoble, France.
authors.forename: Michael
authors.initials: M
authors.lastname: Haertlein
authors.affiliation: Faculty of Natural Sciences, Keele University , Keele, Staffordshire ST5 5BG, United Kingdom.|European Synchrotron Radiation Facility , 71 Avenue des Martyrs, 38042, Grenoble, France.
authors.forename: Edward P
authors.initials: EP
authors.lastname: Mitchell
authors.affiliation: HIV Pathogenesis Research Unit, Department of Molecular Medicine and Haematology, Faculty of Health Sciences, University of the Witwatersrand , 7 York Road, Parktown, 2193, Johannesburg, South Africa.
authors.forename: Alexio
authors.initials: A
authors.lastname: Capovilla
authors.affiliation: HIV Pathogenesis Research Unit, Department of Molecular Medicine and Haematology, Faculty of Health Sciences, University of the Witwatersrand , 7 York Road, Parktown, 2193, Johannesburg, South Africa.
authors.forename: Maria
authors.initials: M
authors.lastname: Papathanasopoulos
authors.affiliation: HIV Pathogenesis Research Unit, Department of Molecular Medicine and Haematology, Faculty of Health Sciences, University of the Witwatersrand , 7 York Road, Parktown, 2193, Johannesburg, South Africa.
authors.forename: Nichole M
authors.initials: NM
authors.lastname: Cerutti
mesh_terms: D000495:Allosteric Site; D015704:CD4 Antigens / Q000737:chemistry* / Q000378:metabolism*; D004220:Disulfides / Q000737:chemistry*; D015699:HIV Envelope Protein gp120 / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D008285:Major Histocompatibility Complex; D008958:Models, Molecular; D010084:Oxidation-Reduction; D011485:Protein Binding; D017510:Protein Folding; D054730:Protein Interaction Domains and Motifs; D055550:Protein Stability; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013696:Temperature
chemical_list: D015704:CD4 Antigens; D004220:Disulfides; D015699:HIV Envelope Protein gp120; C514793:gp120 protein, Human immunodeficiency virus 1