title: Determining the structure of an unliganded and fully glycosylated SIV gp120 envelope glycoprotein.
abstract: HIV/SIV envelope glycoproteins mediate the first steps in viral infection. They are trimers of a membrane-anchored polypeptide chain, cleaved into two fragments known as gp120 and gp41. The structure of HIV gp120 bound with receptor (CD4) has been known for some time. We have now determined the structure of a fully glycosylated SIV gp120 envelope glycoprotein in an unliganded conformation by X-ray crystallography at 4.0 A resolution. We describe here our experimental and computational approaches, which may be relevant to other resolution-limited crystallographic problems. Key issues were attention to details of beam geometry mandated by small, weakly diffracting crystals, and choice of strategies for phase improvement, starting with two isomorphous derivatives and including multicrystal averaging. We validated the structure by analyzing composite omit maps, averaged among three distinct crystal lattices, and by calculating model-based, SeMet anomalous difference maps. There are at least four ordered sugars on many of the thirteen oligosaccharides.
authors.affiliation: Children's Hospital Laboratory of Molecular Medicine, Harvard Medical School, Boston, Massachusetts 02115, USA.
authors.forename: Bing
authors.initials: B
authors.lastname: Chen
authors.forename: Erik M
authors.initials: EM
authors.lastname: Vogan
authors.forename: Haiyun
authors.initials: H
authors.lastname: Gong
authors.forename: John J
authors.initials: JJ
authors.lastname: Skehel
authors.forename: Don C
authors.initials: DC
authors.lastname: Wiley
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Harrison
mesh_terms: D000818:Animals; D015704:CD4 Antigens / Q000737:chemistry / Q000378:metabolism; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D006031:Glycosylation; D008024:Ligands; D008562:Membrane Glycoproteins / Q000737:chemistry* / Q000502:physiology; D008958:Models, Molecular; D017434:Protein Structure, Tertiary; D014759:Viral Envelope Proteins / Q000737:chemistry* / Q000502:physiology
chemical_list: D015704:CD4 Antigens; D008024:Ligands; D008562:Membrane Glycoproteins; D014759:Viral Envelope Proteins; C069623:gp120 protein, Simian immunodeficiency virus