Residue Trp-48 of Tva is critical for viral entry but not for high-affinity binding to the SU glycoprotein of subgroup A avian leukosis and sarcoma viruses.
Previously, mutant Tva receptors were classified as either partially or completely defective in mediating subgroup A avian leukosis and sarcoma virus (ALSV-A) entry (C. Bélanger, K. Zingler, and J. A. T. Young, J. Virol. 69:1019-1024, 1995; K. Zingler, C. Bélanger, R. Peters, D. Agard, and J. A. T. Young, J. Virol. 69:4261-4266, 1995). To specifically test the abilities of these mutant Tva proteins to bind ALSV-A surface (SU) protein, binding studies were performed with a subgroup A SU-immunoadhesin. This fusion protein is composed of the subgroup A Schmidt-Ruppin SU protein fused in frame to a rabbit immunoglobulin constant region. This reagent was conjugated to fluorescein isothiocyanate and used for flow cytometric analysis with transfected human 293 cells expressing different forms of Tva. The SU-immunoadhesin bound the wild-type Tva protein with a KD of approximately 1.5 nM. Amino acid substitutions that reduced viral entry at Asp-46 and at Cys-35 and Cys-50, which are predicted to form an intrachain disulfide bond in Tva, drastically reduced the binding affinity for the SU-immunoadhesin. Thus, the effects on viral entry of some mutations could be explained solely by changes in the binding affinity for ALSV-A SU. However, this was not true for other mutations tested, especially those with amino acid substitutions that replaced Trp-48. Compared with the wild-type receptor, these latter mutations led to approximately 43- to 200-fold reductions in viral infectivity but only to approximately 2.5- to 3.4-fold reductions in the binding affinity for the SU-immunoadhesin. These results support a role for Trp-48 of Tva in mediating steps of viral entry subsequent to binding ALSV-A SU.
Department of Microbiology and Immunology, University of California School of Medicine, San Francisco 94143, USA.
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Zingler
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Young
D000818:Animals; D001354:Avian Leukosis Virus / Q000378:metabolism*; D030161:Avian Proteins; D001358:Avian Sarcoma Viruses / Q000378:metabolism*; D001665:Binding Sites; D002461:Cell Line, Transformed; D003001:Cloning, Molecular; D006023:Glycoproteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D011991:Receptors, Virus / Q000235:genetics / Q000378:metabolism*; D014357:Trypsin; D014759:Viral Envelope Proteins / Q000235:genetics / Q000378:metabolism*
D030161:Avian Proteins; D006023:Glycoproteins; D011991:Receptors, Virus; C088745:Tva receptor; D014759:Viral Envelope Proteins; D014357:Trypsin