Interactions of the cytoplasmic domains of human and simian retroviral transmembrane proteins with components of the clathrin adaptor complexes modulate intracellular and cell surface expression of envelope glycoproteins

The cytoplasmic domains of the transmembrane (TM) envelope proteins (TM-CDs ) of most retroviruses have a Tyr-based motif, YXXempty set, in their membr ane-proximal regions. This signal is involved in the trafficking and endocy tosis of membrane receptors via clathrin-associated AP-1 and AP-2 adaptor c omplexes. We have used CD8-TM-CD chimeras to investigate the role of the Ty r-based motif of human immunodeficiency virus type 1 (HIV-1), simian immuno deficiency virus (SIV), and human T-leukemia virus type 1 (HTLV-1) TM-CDs i n the cell surface expression of the envelope glycoprotein. Flow cytometry and confocal microscopy studies showed that this motif is a major determina nt of the cell surface expression of the CD8-HTLV chimera. The YXXempty set motif also plays a key role in subcellular distribution of the envelope of lentiviruses RN-I and SIV, However, these viruses, which encode TM protein s with a long cytoplasmic domain, have additional determinants distal to th e YXXempty set motif that participate in regulating cell surface expression . We have also used the yeast two-hybrid system and in vitro binding assays to demonstrate that all three retroviral YXXempty set motifs interact with the mu 1 and mu 2 subunits of AP complexes and that the C-terminal regions of HIV-1 and SIV TM proteins interact with the beta 2 adaptin subunit. The TM-CDs of HTLV-I, HIV-1, and SIV also interact with the whole AP complexes . These results clearly demonstrate that the cell surface expression of ret roviral envelope glycoproteins is governed by interactions with adaptor com plexes. The YXXempty set-based signal is the major determinant of this inte raction for the HTLV-1 TM, which contains a short cytoplasmic domain, where as the lentiviruses HIV-1 and SIV have additional determinants distal to th is signal that are also involved.