Mutations in the envelope glycoprotein of SIV/BK28 produce a virus with a cytopathic phenotype in SUPT1 cells

The acquired immunodeficiency syndrome (AIDS) pandemic continues to drain the world\u27s human and financial resources. One path toward developing strategies for therapy and prophylaxis against AIDS is to discover the mechanisms of pathogenesis of the etiologic agent, HIV. The envelope glycoproteins of HIV have been shown to contribute significantly to viral cytopathogenesis. In an effort to dissect the viral and cellular factors involved in envelope function, we have derived and characterized two viruses with markedly different biological phenotypes (NC-MAC and CP-MAC) from a single molecular clone of SIV (BK28). NC-MAC was shown to infect Sup-T1 cells with slow kinetics, induced little CD4 downmodulation, and caused no cytopathic effects. In contrast, CP-MAC infected this cell type rapidly, completely downmodulated CD4, and caused cell fusion. Striking biochemical and morphologic differences included a high level of envelope expression on virions and the infected cell surface, a high binding affinity of envelope for CD4, and an increased stability in the association of surface (SU) and transmembrane (TM) envelope glycoproteins for the CP-MAC virus. Sequence analysis of the env genes revealed eight amino acid differences between NC-MAC and CP-MAC, two in SU and six in TM. These mutations were shown to confer the full biological phenotype of CP-MAC. Remarkably, increased expression of viral envelope on infected cells and virions was mapped to a single amino acid mutation, 723Y $\to$ C in the TM cytoplasmic tail. Additional studies have shown that 723Y likely stabilizes a tight turn that mediates constitutive endocytosis of SIV envelope, and we hypothesize that the marked increase in envelope expression for CP-MAC results from a decrease in endocytosis. The implications of this finding for the pathogenesis of HIV and SIV will be discussed. Future studies will continue to identify structure/function correlates in the envelope using this SIV system and will likely lead to other hypotheses regarding the mechanisms by which this SIV and perhaps other HIV viruses mediate cytopathicity and pathogenesis