title: Receptor-induced conformational changes in the SU subunit of the avian sarcoma/leukosis virus A envelope protein: implications for fusion activation.
abstract: The avian sarcoma/leukosis virus (ASLV) is activated for fusion by a two-step mechanism. For ASLV subgroup A (ASLV-A), association with its receptor (Tva) at neutral pH converts virions to a form that can bind target membranes and, in some assays, induce the lipid-mixing stage of fusion. Low pH is necessary to complete the fusion reaction. ASLV-A env (EnvA) exists on the viral surface as a trimer of heterodimers consisting of receptor binding (SU-A) and fusion-mediating (TM-A) subunits. As the receptor binding and fusion-mediating functions reside in separate subunits, we hypothesize that SU-A and TM-A are conformationally coupled. To begin to understand the effect of the binding of a soluble 47-residue domain of the receptor (sTva) on this coupling and the subsequent function of low pH, we prepared recombinant proteins representing full-length SU-A and a nested set of deletion mutant proteins. Full-length SU-A binds sTva with high affinity, but even small deletions at either the N or the C terminus severely impair sTva binding. We have purified the full-length SU-A subunit and characterized its interactions with sTva and the subsequent effect of low pH on the complex. sTva binds SU-A with an apparent KD of 3 pM. Complex formation occludes hydrophobic surfaces and tryptophan residues and leads to a partial loss of alpha-helical structure in SU-A. Low pH does not alter the off rate for the complex, further alter the secondary structure of SU-A, or induce measurable changes in tryptophan environment. The implications of these findings for fusion are discussed.
authors.affiliation: Department of Cell Biology, UVA Health System, School of Medicine, P.O. Box 800732, Charlottesville, VA 22908-0732, USA. sed7a@virginia.edu
authors.forename: Sue E
authors.initials: SE
authors.lastname: Delos
authors.forename: Jesse A
authors.initials: JA
authors.lastname: Godby
authors.forename: Judith M
authors.initials: JM
authors.lastname: White
mesh_terms: D001354:Avian Leukosis Virus / Q000737:chemistry* / Q000502:physiology; D030161:Avian Proteins; D001358:Avian Sarcoma Viruses / Q000737:chemistry* / Q000502:physiology; D002942:Circular Dichroism; D006863:Hydrogen-Ion Concentration; D008561:Membrane Fusion; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D021122:Protein Subunits / Q000737:chemistry; D011991:Receptors, Virus / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017384:Sequence Deletion; D012995:Solubility; D013050:Spectrometry, Fluorescence; D020349:Surface Plasmon Resonance; D014759:Viral Envelope Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
chemical_list: D030161:Avian Proteins; D021122:Protein Subunits; D011991:Receptors, Virus; D011994:Recombinant Proteins; C088745:Tva receptor; D014759:Viral Envelope Proteins; C107403:glycoprotein 85, avian leukosis virus
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