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 domain of the simian immunodeficiency virus (SIV) transmembrane (TM) molecule has been shown to increase expression of envelope glycoproteins on the surface of infected cells. Here we show that Tyr-723 contributes to a sorting signal that directs the rapid endocytosis of viral glycoproteins from the plasma membrane via coated pits. On cells infected by SIVs with a Tyr at position 723, envelope glycoproteins were transiently expressed on the cell surface and then rapidly endocytosed. Similar findings were noted for envelope molecules expressed in the absence of other viral proteins. Immunoelectron microscopy demonstrated that these molecules were localized in patches on the cell surface and were frequently associated with coated pits. In contrast, envelope glycoproteins containing a Y723C mutation were diffusely distributed over the entire plasma membrane. To determine if an internalization signal was present in the SIV TM, chimeric molecules were constructed that contained the CD4 external and membrane spanning domains and a SIV TM cytoplasmic tail with a Tyr or other amino acids at SIV position 723. In Hela cells stably expressing these molecules, chimeras with a Tyr-723 were rapidly endocytosed, while chimeras containing other amino acids at position 723, including a Phe, were internalized at rates only slightly faster than a CD4 molecule that lacked a cytoplasmic domain. In addition, the biological effects of the internalization signal were evaluated in infectious viruses. A mutation that disrupted the signal and as a result, increased the level of viral envelope glycoprotein on infected cells, was associated with accelerated infection kinetics and increased cell fusion during viral replication. These results demonstrate that a Tyr-dependent motif in the SIV TM cytoplasmic domain can function 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 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 in vivo.
Sauter
M M
MM
Hematology-Oncology Division, University of Pennsylvania, Philadelphia, 19104, USA.
Pelchen-Matthews
A
A
Bron
R
R
Marsh
M
M
LaBranche
C C
CC
Vance
P J
PJ
Romano
J
J
Haggarty
B S
BS
Hart
T K
TK
Lee
W M
WM
Hoxie
J A
JA
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001692:Biological Transport; D015704:CD4 Antigens / Q000235:genetics / Q000378:metabolism; D002451:Cell Compartmentation; D002462:Cell Membrane / Q000378:metabolism* / Q000648:ultrastructure; D002478:Cells, Cultured; D002966:Clathrin; D003034:Coated Pits, Cell-Membrane; D004705:Endocytosis; D015967:Gene Expression Regulation, Viral; D015686:Gene Products, env / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D015682:Retroviridae Proteins, Oncogenic / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D015302:Simian Immunodeficiency Virus / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D013329:Structure-Activity Relationship; D014760:Viral Fusion Proteins / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure
D015704:CD4 Antigens; D002966:Clathrin; D015686:Gene Products, env; D011993:Recombinant Fusion Proteins; D015682:Retroviridae Proteins, Oncogenic; D014760:Viral Fusion Proteins; C059580:transmembrane protein, Simian immunodeficiency virus