Amino Acid Residues in the Cytoplasmic Domain of the Mason-Pfizer Monkey Virus Glycoprotein Critical for Its Incorporation into Virions

Abstract

The envelope (Env) glycoprotein of Mason-Pfizer monkey virus (M-PMV), like those of other retroviruses, is synthesized on the rough endoplasmic reticulum (ER) and is cotranslationally glycosylated and inserted into the lumen of the ER (5-8, 30). Shortly after synthesis, the glycosylated precursor is assembled into trimers, a process which is thought to be required for transport of Env from the ER to the Golgi complex (2,20). It is then cleaved by a cellular protease into two subunits, gp70 (SU) and gp22 (TM), in a late compartment of the Golgi complex (26). The oligomeric, noncovalently associated gp70 and gp22 complexes are then transported to the plasma membrane, where they are incorporated into budding virions (10, 68). The SU glycoprotein is responsible for receptor binding, whereas the TM glycoprotein is responsible for anchoring the SU protein at the surface of infected cells or the viral membrane. The TM glycoprotein also mediates virus-cell membrane fusion during viral entry as well as cell-cell fusion via a fusion peptide and heptad repeat motifs located at the extracellular domain (2,18,35,69,74,76). This fusion process also is influenced by the cytoplasmic domain of the TM glycoprotein as demonstrated previously (9,13,19,36,44,55,68,70).As is observed with murine leukemia virus (MuLV) and Gibbon ape leukemia virus, but unlike most other retroviruses, a viral protease-mediated maturational cleavage of the TM cytoplasmic domain occurs following virus release, which results in conversion of gp22 into gp20 (9,10,13,55,67). Based on cytoplasmic domain truncation mutants, this maturational cleavage of the cytoplasmic domain appears to dramatically increase the fusion activity of the TM proteins and results in the loss of 17 amino acids from the carboxy terminus of the cytoplasmic domain (9, 68).The incorporation of glycoprotein into budding virions is essential for the formation of an infectious virus particle, since retrovirus Env proteins play important roles in receptor binding and membrane fusion. In the case of the alphaviruses, an interaction between the cytoplasmic domain of the spike glycoprotein and the virus nucleocapsid has been demonstrated directly and is absolutely required for virus budding (1,23,72,87). For retroviruses, which do not require glycoprotein expression for virus assembly and release, the nature of capsidenvelope interactions is less well defined. In M-PMV, the glycoprotein appears to play an important role in intracellular transport of assembled capsids to the plasma membrane, and mutations that interfere with Env incorporation also decrease the efficiency of virus release (64,65,68). In contrast, Rous sarcoma virus, which encodes a glycoprotein lacking a cytoplasmic domain, can efficiently assemble and infect cells (53). In the case of Moloney MuLV, some deletion mutations in the cytoplasmic domain of the TM protein decrease infectivity without reducing glycoprotein incorporation (33). Evidence derived from Env and Gag mutagenesis and pseudotyping

Regulation of APOBEC3 Proteins by a Novel YXXL Motif in Human Immunodeficiency Virus Type 1 Vif and Simian Immunodeficiency Virus SIVagm Vif

“…The ability of these Vif mutants to support viral infectivity correlates with agmA3G expression in the producer cells; A3G levels were 7%, 14%, and 35% in cells expressing SIVagm Vif Y40L, Y71L, and Y40,71L, respectively, compared to A3G levels in the absence of SIVagm Vif (100%). These results suggest that the Y40,71L double mutant exhibits a net additive effect in reducing A3G degradation, suggesting that the 40 YRHHY 44 and 69 YXXL 72 motifs both contribute to neutralizing A3G (Fig. 3C).…”

“…The 40 YRHHY 44 and 69 YXXL 72 motifs in HIV-1 Vif are both important for counteracting hA3G (29,35). We therefore examined whether there is evidence of cooperative interactions between these motifs.…”

“…In other retroviral proteins, YXXL motifs function to facilitate interactions with cellular proteins (7,32,34,44). For example, the YPXL late domain in equine infectious anemia virus p9 recruits the cellular protein ALIX (also known as AIP1) to facilitate virus budding (4,47).…”

“…We then used site-directed mutagenesis to create point mutations in the 40 YRHHY 44 tested the ability of these mutants to bind recombinant His-A3G in a GST pull-down assay. The Y69A and W70A mutants bound only trace amounts of His-A3G, while the H42,43N mutant bound similar amounts of His-A3G as did the wild type.…”

Mutational Analysis of the Cytoplasmic Tail of Jaagsiekte Sheep Retrovirus Envelope Protein

“…Moreover, if transformation involves interactions of distinct cellular proteins with the TM cytoplasmic tail, it is possible that interactions important for transformation could compete with those involved in replication. In terms of replication, the JSRV TM protein is likely to participate in several processes, by analogy to other retroviruses and, in particular, to the related betaretrovirus Mason-Pfizer monkey virus (M-PMV) that has been well studied (23). The cytoplasmic tail of M-PMV TM has been shown to be important for interaction with viral Gag protein (the MA domain) during viral particle morphogenesis (20), and tyrosine-containing motifs are important for intracellular trafficking (3).…”