Endoplasmic reticulum (ER)-associated degradation (ERAD) is a universally important process among eukaryotic cells. ERAD is necessary to preserve cell integrity since the accumulation of defective proteins results in diseases associated with neurological dysfunction, cancer, and infections. This process involves recognition of misfolded or misassembled proteins that have been translated in association with ER membranes. Recognition of ERAD substrates leads to their extraction through the ER membrane (retrotranslocation or dislocation), ubiquitination, and destruction by cytosolic proteasomes. This review focuses on ERAD and its components as well as how viruses use this process to promote their replication and to avoid the immune response.

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Mouse mammary tumor virus (MMTV) encodes a Rem precursor protein that specifies both regulatory and accessory functions. Rem is cleaved at the ER membrane into a functional N-terminal signal peptide (SP) and the C-terminus (Rem-CT). Rem-CT lacks a membrane-spanning domain and a known ER retention signal, yet was not detectably secreted into cell supernatants. Inhibition of intracellular trafficking by the drug Brefeldin A (BFA), which interferes with the ER to Golgi secretory pathway, resulted in dramatically reduced intracellular Rem-CT levels that were not rescued by proteasomal or lysosomal inhibitors. A Rem mutant lacking glycosylation was cleaved into SP and Rem-CT, but was insensitive to BFA, suggesting that unglycosylated Rem-CT does not reach this BFA-dependent compartment. Treatment with Endoglycosidase H indicated that Rem-CT does not traffic through the Golgi. Analysis of wild-type Rem-CT and its glycosylation mutant by confocal microscopy revealed that both were primarily localized to the ER lumen. A small fraction of wild-type Rem-CT, but not the unglycosylated mutant, was co-localized with Rab5+ early endosomes. Expression of a dominant-negative (DN) form of ADP ribosylation factor 1 (Arf1) (T31N) mimicked the effects of BFA by reducing Rem-CT levels and increased Rem-CT association with early and late endosomes. Inhibition of the AAA ATPase, p97/VCP, rescued Rem-CT in the presence of BFA or DN Arf1 and prevented localization to Rab5+ endosomes. Thus, Rem-CT uses an unconventional p97-mediated scheme for trafficking to early endosomes. IMPORTANCE Mouse mammary tumor virus is a complex retrovirus that encodes a regulatory/accessory protein, Rem. Rem is a precursor protein that is processed at the endoplasmic reticulum (ER) membrane by signal peptidase. The N-terminal SP uses the p97/VCP ATPase to elude ER-associated degradation to traffic to the nucleus and serve a human immunodeficiency virus Rev-like function. In contrast, the function of the C-terminal glycosylated cleavage product (Rem-CT) is unknown. Since localization is critical for protein function, we used mutants, inhibitors, and confocal microscopy to localize Rem-CT. Surprisingly, Rem-CT, which lacks a transmembrane domain or an ER retention signal, was detected primarily within the ER and required glycosylation and the p97 ATPase for early endosome trafficking without passage through the Golgi. Thus, Rem-CT uses a novel intracellular trafficking pathway, potentially impacting host anti-viral immunity.

Retroviruses cause immunodeficiency and cancer but also are used as vectors for the expression of heterologous genes. Nevertheless, optimal translation of introduced genes often is not achieved. Here we show that transfection into mammalian cells of lentiviral or gammaretroviral vectors, including those with specific shRNAs, increased expression of a cotransfected gene relative to standard plasmid vectors. Levels of most endogenous cellular proteins were unchanged. Transfer of lentiviral vector sequences into a standard plasmid conferred the ability to give increased expression of cotransfected genes (superinduction). Superinduction by the retroviral vector was not dependent on the cell type or species, the type of reporter gene, or the method of transfection. No differences were detected in the IFN, unfolded protein, or stress responses in the presence of retroviral vectors. RT-PCRs revealed that RNA levels of cotransfected genes were unchanged during superinduction, yet Western blotting, pulse labeling, and the use of bicistronic vectors showed increased cap-dependent translation of cointroduced genes. Expression of the mammalian target of rapamycin (mTOR) kinase target 4E-BP1, but not the mTOR inhibitor Torin 1, preferentially inhibited superinduction relative to basal protein expression. Furthermore, transcription of lentiviral vector sequences from a doxycycline-inducible promoter eliminated superinduction, consistent with a DNA-triggered event. Thus, retroviral DNA increased translation of cointroduced genes in trans by an mTORindependent signaling mechanism. Our experiments have broad applications for the design of retroviral vectors for transfections, DNA vaccines, and gene therapy.ranslational control is critical for mammalian cells and the viruses that infect them. For example, picornaviruses and some flaviviruses have an internal ribosomal entry site (IRES) (1) that allows cap-independent translation of viral RNAs and provides preferential translation over most host cell mRNAs (2). Structural modifications at the 5′ ends of viral RNAs, such as differences in cap methylation or absence of a cap, often distinguish viral mRNAs from their cellular counterparts (3). Multiple cellular proteins recognize foreign RNAs or DNAs, which trigger specific signaling pathways that lead to general translational arrest and/or selective viral RNA degradation (4). Many viral RNAs trigger the IFN signaling pathway and translational inhibition, but viruses encode various proteins or RNAs that mute this response (5). Recognition of foreign nucleic acids by cellular surface or cytosolic receptors also leads to signaling events that provide an innate antiviral response (6).Retroviruses are positive-sense RNA viruses that have been used extensively for introduction of genes or small hairpin RNAs (shRNAs) into cells, both in culture and for gene therapy. Unlike most RNA viruses, retroviruses replicate through a DNA intermediate and use RNA polymerase II to produce mRNAs with structures that are very similar to those of host mRN...

ABSTRACTMouse mammary tumor virus (MMTV) encodes a Rem precursor protein that specifies both regulatory and accessory functions. Rem is cleaved at the ER membrane into a functional N-terminal signal peptide (SP) and the C-terminus (Rem-CT). Rem-CT lacks a membrane-spanning domain and a known ER retention signal, yet was not detectably secreted into cell supernatants. Inhibition of intracellular trafficking by the drug Brefeldin A (BFA), which interferes with the ER to Golgi secretory pathway, resulted in dramatically reduced intracellular Rem-CT levels. A Rem mutant lacking glycosylation sites was cleaved into SP and Rem-CT, but was insensitive to BFA, suggesting that unglycosylated Rem-CT does not exit the ER or reach a degradative compartment. BFA reduction of Rem-CT levels was not rescued by proteasome or lysosomal inhibitors. Rem-CT has simple glycans, which are necessary for Rem-CT stability and trafficking, but indicate that Rem-CT does not traffic through the Golgi. Analysis of wild-type Rem-CT and its glycosylation mutant by confocal microscopy revealed that both were primarily localized to the ER lumen. A small fraction of wild-type Rem-CT, but not the unglycosylated mutant, were co-localized with Rab5+ endosomes. Expression of a dominant-negative (DN) form of ADP ribosylation factor 1 (Arf1) (T31N) mimicked the effects of BFA by reducing Rem-CT levels, suggesting that Arf1 prevents Rem-CT localization to a degradative compartment. A DN form of the AAA ATPase, p97/VCP, rescued Rem-CT in the presence of BFA or DN Arf1. Thus, Rem-CT uses an unconventional trafficking scheme, perhaps to thwart innate immunity to MMTV infection.IMPORTANCEMouse mammary tumor virus is a complex retrovirus that encodes a regulatory/accessory protein, Rem. Rem is a precursor protein that is processed at the endoplasmic reticulum (ER) membrane by signal peptidase. The N-terminal SP eludes ER-associated degradation to traffic to the nucleus and serve a human immunodeficiency virus Rev-like function. In contrast, the function of the C-terminal glycosylated cleavage product (Rem-CT) is unknown. Since localization is critical for protein function, we used multiple methods to localize Rem-CT. Surprisingly, Rem-CT, which lacks a transmembrane domain or an ER retention signal, was detected primarily within the ER and required glycosylation for trafficking to endosomes. Blocking of retrograde trafficking through Arf1 reduced Rem-CT levels, but was not restored by lysosomal or proteasomal inhibitors. The unique trafficking of Rem-CT suggests a novel intracellular trafficking pathway, potentially impacting host anti-viral immunity.