An internalization signal in the SIV TM protein modulates the expression of viral glycoproteins on the cell surface

Monica Maria Sauter, University of Pennsylvania


We have derived and characterized two viruses with markedly different biological phenotypes from a single infectious molecular clone of SIV (BK28). The virus CP-MAC infected Sup-T1 cells with rapid kinetics, cell fusion, and CD4 down-modulation. Cells infected with CP-MAC exhibited an increased level of cell surface viral glycoproteins. All properties of the CP-MAC phenotype mapped to mutations in the env gene. A single amino acid change of Tyr to Cys (Y723C) in the TM cytoplasmic domain was the principle determinant for the increased level of envelope glycoproteins on the cell surface. In these studies, the mechanism of this effect and its biological consequences were explored. The Y723C substitution was associated with accelerated infection kinetics and increased cell fusion during viral replication. Introduction of a Tyr to Cys change at the analogous codon (Y721) in the SIVmac239 infectious molecular clone, which encodes a native TM protein, resulted in an increase in the level of cell surface viral glycoproteins. Further mutagenesis of SIVmac239 demonstrated that a structural element, dependent on an aromatic amino acid in the SIV TM cytoplasmic domain, could modulate envelope glycoprotein levels on the surface of infected cells. We have shown that Tyr-723, a component of this structural element, contributes to a sorting signal that directs the rapid endocytosis of viral glycoproteins from the plasma membrane via coated pits. Envelope glycoproteins were expressed transiently and then endocytosed from the surface of SIV-infected or env-expressing cells, whereas the Y723C substitution resulted in a protein that was diffusely distributed over the plasma membrane. Chimeric molecules were constructed that contained the CD4 extracellular and membrane spanning domains and the SIV cytoplasmic domain to demonstrate that the SIV cytoplasmic domain contains a functional endocytosis signal dependent on Y723. Conservation of an analogous Tyr in all human and simian immunodeficiency viruses suggests that this signal may be present in other primate lentiviruses and could be important in the pathogenesis of these viruses, such as HIV, in vivo.

Subject Area

Microbiology|Cellular biology|Molecular biology

Recommended Citation

Sauter, Monica Maria, "An internalization signal in the SIV TM protein modulates the expression of viral glycoproteins on the cell surface" (1996). Dissertations available from ProQuest. AAI9628002.