AN INTERNALIZATION SIGNAL IN THE SIMIAN IMMUNODEFICIENCY VIRUS TRANSMEMBRANE PROTEIN CYTOPLASMIC DOMAIN MODULATES EXPRESSION OF ENVELOPE GLYCOPROTEINS ON THE CELL-SURFACE

A Tyr to Cys mutation at amino acid position 723 in the cytoplasmic do main of the simian immunodeficiency virus (SIV) transmembrane (TM) mol ecule has been shown to increase expression of envelope glycoproteins on the surface of infected cells. Here we show that Tyr-723 contribute s to a sorting signal that directs the rapid endocytosis of viral glyc oproteins from the plasma membrane via coated pits. On cells infected by SIVs with a Tyr at position 723, envelope glycoproteins were transi ently expressed on the cell surface and then rapidly endocytosed. Simi lar findings were noted for envelope molecules expressed in the absenc e of other viral proteins. Immunoelectron microscopy demonstrated that these molecules were localized in patches on the cell surface and wer e frequently associated with coated pits. In contrast, envelope glycop roteins containing a Y723C mutation were diffusely distributed over th e entire plasma membrane. To determine if an internalization signal wa s present in the SIV TM, chimeric molecules were constructed that cont ained the CD4 external and membrane spanning domains and a SIV TM cyto plasmic tail with a Tyr or other amino acids at SIV position 723. In H ela cells stably expressing these molecules, chimeras with a Tyr-723 w ere rapidly endocytosed, while chimeras containing other amino acids a t position 723, including a Phe, were internalized at rates only sligh tly faster than a CD4 molecule that lacked a cytoplasmic domain. In ad dition, the biological effects of the internalization signal were eval uated on infectious viruses. A mutation that disrupted the signal and as a result, increased the level of viral envelope glycoprotein on inf ected cells, was associated with accelerated infection kinetics and in creased cell fusion during viral replication. These results demonstrat e that a Tyr-dependent motif in the SIV TM cytoplasmic domain can func tion as an internalization signal that can modulate expression of the viral envelope molecules on the cell surface and affect the biological properties of infectious viruses. The conservation of an analogous Ty r in all human and simian immunodeficiency viruses suggests that this signal may be present in other primate lentiviruses and could be impor tant in the pathogenesis of these viruses in vivo.