A viral protease-mediated cleavage of the transmembrane glycoprotein of Mason-Pfizer monkey virus can be suppressed by mutations within the matrix protein.

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

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confidence: 70%

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confidence: 99%

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

Cao

Micoli

Bauerová

et al. 2005

“…It has been suggested that the virus has adopted this strategy of regulating Env fusogenicity in order to limit the expression of a potentially cytotoxic molecule on the cell surface (53,54). Similar processing of Env cytoplasmic tails has been observed in the MasonPfizer monkey virus (M-PMV), the gibbon ape leukemia virus (GaLV), and the equine infectious anemia virus (4,7,56); in addition, truncated M-PMV and GaLV Env proteins are more fusogenic than their full-length counterparts (3,7). Truncations of the long cytoplasmic domains of lentiviral Env proteins occur under certain culture conditions (5,31,32,59), and an increase in Env fusogenicity has been reported for truncated versions of simian immunodeficiency virus (SIV), human immunodeficiency virus type 1 (HIV-1), and HIV-2 Envs (18,45,63,64,68,80).…”

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confidence: 84%

Cytoplasmic Tail of Moloney Murine Leukemia Virus Envelope Protein Influences the Conformation of the Extracellular Domain: Implications for Mechanism of Action of the R Peptide

Aguilar

Anderson

Cannon

2003

The envelope (Env) protein of Moloney murine leukemia virus (MoMuLV) is a homotrimeric complex whose monomers consist of linked surface (SU) and transmembrane (TM) proteins cleaved from a precursor protein by a cellular protease. In addition, a significant fraction of virion-associated TM is further processed by the viral protease to remove the C-terminal 16 amino acids of the cytoplasmic domain, the R peptide. This cleavage greatly enhances the fusogenicity of the protein and is necessary for the formation of a fully functional Env protein complex. We have previously proposed that R peptide cleavage enhances fusogenicity by altering the conformation of the ectodomain of the protein (Y. Zhao et al., J. Virol. 72:5392-5398, 1998). Using a series of truncation and point mutants of MoMuLV Env, we now provide direct biochemical and immunological evidence that the cytoplasmic tail and the membrane-spanning region of Env can influence the overall structure of the ectodomain of the protein and alter the strength of the SU-TM interaction. The R-peptidetruncated form of the protein, in particular, exhibits a markedly different conformation than the full-length protein.Retroviral envelope (Env) glycoproteins are expressed on the surface of infected cells and are incorporated into the viral lipid envelope during budding of a virion from a cell. Following binding to a specific cell surface receptor, Env promotes fusion between the viral and host cell membranes and thereby initiates a new infection. For the murine leukemia viruses (MuLVs), Env is initially translated as a precursor protein, Pr85, which oligomerizes in the endoplasmic reticulum, probably as a homotrimer (17). This precursor is cleaved by a cellular protease into two subunits, the surface (SU) protein gp70 and the transmembrane (TM) protein p15E, which remain associated in a labile interaction which may include a disulfide bond (48,51).MuLV Env also undergoes a second cleavage event in the cytoplasmic tail of TM that is essential for protein function. The C-terminal 16 amino acids of the cytoplasmic tail, the R peptide, is removed from a subset of the virion TM proteins by the viral protease (22,27,60). This results in an Env protein that is inherently more fusogenic than the full-length protein, causing, for example, cell-cell fusion to occur when it is expressed in NIH 3T3 cells (29,53,54). It has been suggested that the virus has adopted this strategy of regulating Env fusogenicity in order to limit the expression of a potentially cytotoxic molecule on the cell surface (53, 54). Similar processing of Env cytoplasmic tails has been observed in the MasonPfizer monkey virus (M-PMV), the gibbon ape leukemia virus (GaLV), and the equine infectious anemia virus (4, 7, 56); in addition, truncated M-PMV and GaLV Env proteins are more fusogenic than their full-length counterparts (3, 7). Truncations of the long cytoplasmic domains of lentiviral Env proteins occur under certain culture conditions (5,31,32,59), and an increase in Env fusogenicity has been reported for...

“…Several viral proteins are known to be cleaved inside retroviral virions by PR, such as HIV-1 Vif (35), HIV-1 and HIV-2 Nef (4,43,50,54), and, in several retroviruses, the cytoplasmic tail of the transmembrane portion of Env (2,19,23). However, up to this point, it has not been generally agreed that Gag proteins can be cleaved in budded virus, with the exception of the CA-p2 intermediate of Rous sarcoma virus Gag (1).…”

Section: Discussionmentioning

confidence: 99%

Phosphorylation and Proteolytic Cleavage of Gag Proteins in Budded Simian Immunodeficiency Virus

Rue

Roos

Tarwater

et al. 2005