The tyrosine-based YXXØ targeting motif of murine leukemia virus envelope glycoprotein affects pathogenesis

A cellular trafficking signal in the SIV envelope protein cytoplasmic domain is strongly selected for in pathogenic infection

The HIV/SIV envelope glycoprotein (Env) cytoplasmic domain contains a highly conserved Tyr-based trafficking signal that mediates both clathrin-dependent endocytosis and polarized sorting. Despite extensive analysis, the role of these functions in viral infection and pathogenesis is unclear. An SIV molecular clone (SIVmac239) in which this signal is inactivated by deletion of Gly-720 and Tyr-721 (SIVmac239ΔGY), replicates acutely to high levels in pigtail macaques (PTM) but is rapidly controlled. However, we previously reported that rhesus macaques and PTM can progress to AIDS following SIVmac239ΔGY infection in association with novel amino acid changes in the Env cytoplasmic domain. These included an R722G flanking the ΔGY deletion and a nine nucleotide deletion encoding amino acids 734-736 (ΔQTH) that overlaps the rev and tat open reading frames. We show that molecular clones containing these mutations reconstitute signals for both endocytosis and polarized sorting. In one PTM, a novel genotype was selected that generated a new signal for polarized sorting but not endocytosis. This genotype, together with the ΔGY mutation, was conserved in association with high viral loads for several months when introduced into naïve PTMs. For the first time, our findings reveal strong selection pressure for Env endocytosis and particularly for polarized sorting during pathogenic SIV infection in vivo.

Antibody-induced internalisation of retroviral envelope glycoproteins is a signal initiation event

As obligate parasites, viruses highjack, modify and repurpose the cellular machinery for their own replication. Viral proteins have, therefore, evolved biological functions, such as signalling potential, that alter host cell physiology in ways that are still incompletely understood. Retroviral envelope glycoproteins interact with several host proteins, extracellularly with their cellular receptor and anti-envelope antibodies, and intracellularly with proteins of the cytoskeleton or sorting, endocytosis and recirculation pathways. Here, we examined the impact of endogenous retroviral envelope glycoprotein expression and interaction with host proteins, particularly antibodies, on the cell, independently of retroviral infection. We found that in the commonly used C57BL/6 substrains of mice, where murine leukaemia virus (MLV) envelope glycoproteins are expressed by several endogenous MLV proviruses, the highest expressed MLV envelope glycoprotein is under the control of an immune-responsive cellular promoter, thus linking MLV envelope glycoprotein expression with immune activation. We further showed that antibody ligation induces extensive internalisation from the plasma membrane into endocytic compartments of MLV envelope glycoproteins, which are not normally subject to constitutive endocytosis. Importantly, antibody binding and internalisation of MLV envelope glycoproteins initiates signalling cascades in envelope-expressing murine lymphocytic cell lines, leading to cellular activation. Similar effects were observed by MLV envelope glycoprotein ligation by its cellular receptor mCAT-1, and by overexpression in human lymphocytic cells, where it required an intact tyrosine-based YXXΦ motif in the envelope glycoprotein cytoplasmic tail. Together, these results suggest that signalling potential is a general property of retroviral envelope glycoproteins and, therefore, a target for intervention.

The outcome of viral infection depends on the balance between host immunity and the ability of the virus to avoid, evade or subvert it. The envelope glycoproteins of diverse viruses, including retroviruses, are displayed on the surface of virions and of infected cells and thus constitute the major target of the host antibody response. Antibody responses are elicited not only against infectious viruses we acquire during our life-history, but also against the numerous retroviral envelopes encoded by our genome and acquired during our species’ life-history. In turn, viruses have evolved ways to reduce exposure of their envelope glycoproteins to the host immune system, including constitutive endocytosis or antibody-induced internalisation. Using murine leukaemia viruses as models of infectious and endogenous retroviruses, we show that antibody binding to retroviral envelopes induces extensive internalisation of the envelope-antibody complex and initiates signalling cascades, ultimately leading to transcriptional activation of envelope glycoprotein-expressing lymphocytes. We further show that expression of endogenous retroviral envelopes is coupled to physiological lymphocyte activation, integrating them with the immune response. These findings reveal an unexpected layer of interaction between the host antibody response and retroviral envelope glycoproteins, which could be considered immune receptors.

Interaction between the HTLV-1 envelope and cellular proteins: impact on virus infection and restriction

The first human retrovirus, human T-lymphotropic virus 1 (HTLV-1), was discovered 30 years ago. Despite intensive study, the cell surface molecules involved in virus entry have only been identified over the past few years. Three molecules form the receptor complex for HTLV-1: glucose transporter 1, neuropilin 1 and heparan sulfate proteoglycans. Another molecule on the surface of dendritic cells, DC-SIGN, may play a role in dendritic cell-mediated infection of cells. In addition to the cell surface molecules used for entry, the HTLV-1 envelope interacts with cellular proteins, enabling the virus to traffic by exploiting cellular delivery pathways. To facilitate both these steps, HTLV-1 encodes motifs that mimic cellular binding partners for the trafficking system and ligands for the receptors. Here we review the interactions between the HTLV-1 envelope and cellular proteins.

Viral determinants of polarized assembly for the murine leukemia virus

Retrovirus transmission via direct cell-cell contact is more efficient than diffusion through the extracellular milieu. This is believed to be due to the ability of viruses to efficiently coordinate several steps of the retroviral life cycle at cell-cell contact sites (D. C. Johnson et al., J. Virol. 76:1-8, 2002; D. M. Phillips, AIDS 8:719-731, 1994; Q. Sattenau, Nat. Rev. Microbiol. 6:815-826, 2008). Using the murine leukemia virus (MLV) as a model retrovirus, we have previously shown that interaction between viral envelope (Env) and receptor directs viral assembly to cell-cell contact sites to promote efficient viral spreading (J. Jin et al., PLoS Biol. 7:e1000163, 2009). In addressing the underlying mechanism, we observed that Env cytoplasmic tail directs this contact-induced polarized assembly. We present here the viral determinants in the Env cytoplasmic tail and Gag that are important in this process. A tyrosine residue within the cytoplasmic tail of Env was identified, which directs polarized assembly. MLV matrix-mediated membrane targeting is required for Gag recruitment to sites of cell-cell contact. Our results suggest that MLV polarized assembly is mediated by a direct or indirect interaction between both domains, thereby coupling Gag recruitment and virus assembly to Env accumulation at the cell-cell interface. In contrast, HIV Gag that assembles outside of cell-cell interfaces can subsequently be drawn into contact zones mediated by MLV Env and receptor, a finding that is consistent with the previously observed lateral movement of HIV into the virological synapse (W. Hubner et al., Science 323:1743-1747, 2009; D. Rudnicka et al., J. Virol. 83:6234-6246, 2009). As such, we observed two distinct modes of virus cell-to-cell transmission that involve either polarized or nonpolarized assembly, but both result in virus transmission.

Opposing effects of a tyrosine-based sorting motif and a PDZ-binding motif regulate human T-lymphotropic virus type 1 envelope trafficking

Human T-lymphotropic virus type 1 (HTLV-1) envelope (Env) glycoprotein mediates binding of the virus to its receptor on the surface of target cells and subsequent fusion of virus and cell membranes. To better understand the mechanisms that control HTLV-1 Env trafficking and activity, we have examined two protein-protein interaction motifs in the cytoplasmic domain of Env. One is the sequence YSLI, which matches the consensus YXXPhi motifs that are known to interact with various adaptor protein complexes; the other is the sequence ESSL at the C terminus of Env, which matches the consensus PDZ-binding motif. We show here that mutations that destroy the YXXPhi motif increased Env expression on the cell surface and increased cell-cell fusion activity. In contrast, mutation of the PDZ-binding motif greatly diminished Env expression in cells, which could be restored to wild-type levels either by mutating the YXXPhi motif or by silencing AP2 and AP3, suggesting that interactions with PDZ proteins oppose an Env degradation pathway mediated by AP2 and AP3. Silencing of the PDZ protein hDlg1 did not affect Env expression, suggesting that hDlg1 is not a binding partner for Env. Substitution of the YSLI sequence in HTLV-1 Env with YXXPhi elements from other cell or virus membrane-spanning proteins resulted in alterations in Env accumulation in cells, incorporation into virions, and virion infectivity. Env variants containing YXXPhi motifs that are predicted to have high-affinity interaction with AP2 accumulated to lower steady-state levels. Interestingly, mutations that destroy the YXXPhi motif resulted in viruses that were not infectious by cell-free or cell-associated routes of infection. Unlike YXXPhi, the function of the PDZ-binding motif manifests itself only in the producer cells; AP2 silencing restored the incorporation of PDZ-deficient Env into virus-like particles (VLPs) and the infectivity of these VLPs to wild-type levels.

Bluetongue virus outer capsid protein VP5 interacts with membrane lipid rafts via a SNARE domain

Bluetongue virus (BTV) is a nonenveloped double-stranded RNA virus belonging to the family Reoviridae. The two outer capsid proteins, VP2 and VP5, are responsible for virus entry. However, little is known about the roles of these two proteins, particularly VP5, in virus trafficking and assembly. In this study, we used density gradient fractionation and methyl beta cyclodextrin, a cholesterol-sequestering drug, to demonstrate not only that VP5 copurifies with lipid raft domains in both transfected and infected cells, but also that raft domain integrity is required for BTV assembly. Previously, we showed that BTV nonstructural protein 3 (NS3) interacts with VP2 and also with cellular exocytosis and ESCRT pathway proteins, indicating its involvement in virus egress (A. R. Beaton, J. Rodriguez, Y. K. Reddy, and P. Roy, Proc. Natl. Acad. Sci. USA 99:13154-13159, 2002; C. Wirblich, B. Bhattacharya, and P. Roy J. Virol. 80:460-473, 2006). Here, we show by pull-down and confocal analysis that NS3 also interacts with VP5. Further, a conserved membrane-docking domain similar to the motif in synaptotagmin, a protein belonging to the SNARE (soluble N-ethylmaleimide-sensitive fusion attachment protein receptor) family was identified in the VP5 sequence. By site-directed mutagenesis, followed by flotation and confocal analyses, we demonstrated that raft association of VP5 depends on this domain. Together, these results indicate that VP5 possesses an autonomous signal for its membrane targeting and that the interaction of VP5 with membrane-associated NS3 might play an important role in virus assembly.

Glycoprotein targeting signals influence the distribution of measles virus envelope proteins and virus spread in lymphocytes

We previously demonstrated the presence of tyrosine-dependent motifs for specific sorting of two measles virus (MV) glycoproteins, H and F, to the basolateral surface in polarized epithelial cells. Targeted expression of the glycoproteins was found to be required for virus spread in epithelia via cell-to-cell fusion in vitro and in vivo. In the present study, recombinant MVs (rMVs) with substitutions of the critical tyrosines in the H and F cytoplasmic domains were used to determine whether the sorting signals also play a crucial role for MV replication and spread within lymphocytes, the main target cells of acute MV infection. Immunolocalization revealed that only standard glycoproteins are targeted specifically to the uropod of polarized lymphocytes and cluster on the surface of non-polarized lymphocytes. H and F proteins with tyrosine mutations did not accumulate in uropods, but were distributed homogeneously on the surface and did not colocalize markedly with the matrix (M) protein. Due to the defective interaction with the M protein, all mutant rMVs showed an enhanced fusion capacity, but only rMVs harbouring two mutated glycoproteins showed a marked decrease in virus release from infected lymphocytes. These results demonstrate clearly that the tyrosine-based targeting motifs in the MV glycoproteins are not only important in polarized epithelial cells, but are also active in lymphocytes, thus playing an important role in virus propagation in different key target cells during acute MV infection.

The cytoplasmic tail of glycoprotein M (gpUL100) expresses trafficking signals required for human cytomegalovirus assembly and replication

The virion envelope of human cytomegalovirus (HCMV) is complex and consists of an incompletely defined number of glycoproteins. The gM/gN protein complex is the most abundant protein component of the envelope. Studies have indicated that deletion of the viral gene encoding either gM or gN is a lethal mutation. Analysis of the amino acid sequence of gM disclosed a C-terminal acidic cluster of amino acids and a tyrosine-containing trafficking motif, both of which are well-described trafficking/sorting signals in the cellular secretory pathway. To investigate the roles of these signals in the trafficking of the gM/gN complex during virus assembly, we made a series of gM (UL100 open reading frame) mutants in the AD169 strain of HCMV. Mutant viruses that lacked the entire C-terminal cytoplasmic tail of gM were not viable, suggesting that the cytoplasmic tail of gM is essential for virus replication. In addition, the gM mutant protein lacking the cytoplasmic domain exhibited decreased protein stability. Mutant viruses with a deletion of the acidic cluster or alanine substitutions in tyrosine-based motifs were viable but exhibited a replication-impaired phenotype suggestive of a defect in virion assembly. Analysis of these mutant gMs using static immunofluorescence and fluorescence recovery after photobleaching demonstrated delayed kinetics of intracellular localization of the gM/gN protein to the virus assembly compartment compared to the wild-type protein. These data suggest an important role of the glycoprotein gM during virus assembly, particularly in the dynamics of gM trafficking during viral-particle assembly.

Contribution of trafficking signals in the cytoplasmic tail of the infectious bronchitis virus spike protein to virus infection

Coronavirus spike (S) proteins are responsible for binding and fusion with target cells and thus play an essential role in virus infection. Recently, we identified a dilysine endoplasmic reticulum (ER) retrieval signal and a tyrosine-based endocytosis signal in the cytoplasmic tail of the S protein of infectious bronchitis virus (IBV). Here, an infectious cDNA clone of IBV was used to address the importance of the S protein trafficking signals to virus infection. We constructed infectious cDNA clones lacking the ER retrieval signal, the endocytosis signal, or both. The virus lacking the ER retrieval signal was viable. However, this virus had a growth defect at late times postinfection and produced larger plaques than IBV. Further analysis confirmed that the mutant S protein trafficked though the secretory pathway faster than wild-type S protein. A more dramatic phenotype was obtained when the endocytosis signal was mutated. Recombinant viruses lacking the endocytosis signal (in combination with a mutated dilysine signal or alone) could not be recovered, even though transient syncytia were formed in transfected cells. Our results suggest that the endocytosis signal of IBV S is essential for productive virus infection.

Endocytosis of the Nipah virus glycoproteins

Nipah virus (NiV), a highly pathogenic member of the family Paramyxoviridae, encodes the surface glycoproteins F and G. Since internalization of the NiV envelope proteins from the cell surface might be of functional importance for viral pathogenesis either by regulating cytopathogenicity or by modulating recognition of infected cells by the immune system, we analyzed the endocytosis of the NiV F and G proteins. Interestingly, we found both glycoproteins to be internalized in infected and transfected cells. As endocytosis is normally mediated by tyrosine- or dileucine-dependent signals in the cytoplasmic tails of transmembrane proteins, all potential internalization signals in the NiV glycoproteins were mutated. Whereas the G protein appeared to be constitutively internalized with the bulk flow during membrane turnover, uptake of the F protein was found to be signal mediated. F endocytosis clearly depended on a membrane-proximal YXXPhi motif and was found to be of functional importance for the biological activity of the protein.

Sequence analysis of murine leukemia virus envelope gene from inoculated mice

Introduction of amino acids substitutions in murine leukemia virus genome is a powerful method to determine the relative importance of various viral factors in pathogenesis. However, introduction of such amino acids substitution could result in viruses at a selective disadvantage, and eventual selection of revertants. It is thus essential to verify if the mutation is maintained stably in replicating virus and in infected tumor cells. In the present study, viral nucleic acid sequences from diseased animals were determined using different approaches. Small blood samples were found adequate for direct RNA extraction and reverse transcriptase-PCR amplification followed by automated DNA sequencing. Alternatively, replication-competent viruses were recovered specifically by applying blood samples onto permissive cells; viral RNA is then extracted from tissue culture medium and similarly sequenced. Tissue samples were also used to amplify viral sequences from tumors DNA while small pieces of tumors tissues were applied onto permissive cells to isolate replicating viruses. The combined experimental approach was used to show sequence conservation using a mutant altered in the intracytoplasmic region of viral envelope glycoprotein. No difference was observed between viruses recovered directly from the animal and those amplified onto cultured cells.