Structural requirements for recognition of major histocompatibility complex class II by membrane-associated RING-CH (MARCH) protein E3 ligases

Expression profile, subcellular localization of MARCH4 and transcriptome analysis of its potential regulatory signaling pathway in large yellow croaker (Larimichthys crocea)

Membrane-associated RING-CH (MARCH) family, as Ring-type E3 ligases, have attracted extensive attention to their immune functions. MARCH4 plays an essential role in regulating immune response in mammal. In the present study, it is the first to report on MARCH4 characteristics and signal pathway in fish. MARCH4 in large yellow croaker Larimichthys crocea (named as LcMARCH4) encodes a RING-CH domain and two TM domains, as well as other function domains, including an N-terminal proline rich domain, an AxxxG-motif in TM1, a tyrosine-based YXXØ motif, and a C-terminal PDZ-binding domain. LcMARCH4 is a tissue-specific protein with highly significant expression in brain. The mRNA transcripts of LcMARCH4 were significantly induced in the main organs (skin, gill, spleen, and head-kidney) by C. irritans infection. Consistently, significant increase was observed in spleen and head-kidney after LPS, Poly I:C stimulation and V. parahaemolyticus infection. Subcellular localization analysis showed that LcMARCH4 was localized in the cytoplasm and membrane. Moreover, we found 46 DEGs in a comparative transcriptome analysis between the LcMARCH4 overexpression group and control vector group. The analysis showed that HSPA6, HSPA1B and DNAJB1 might play important regulatory roles to MARCH4 in fish. Notably, two noncoding RNA, both RN7SL1 and RN7SL2, the expression levels went up in MARCH4 overexpression cells. Taken together, this study will provide new insights into finfish MARCH4 and its potential regulatory signaling pathway as well.

Immunogenomic pan-cancer landscape reveals immune escape mechanisms and immunoediting histories

Immune reactions in the tumor microenvironment are an important hallmark of cancer, and emerging immune therapies have been proven effective against several types of cancers. To investigate cancer genome-immune interactions and the role of immunoediting or immune escape mechanisms in cancer development, we analyzed 2834 whole genome and RNA sequencing datasets across 31 distinct tumor types with respect to key immunogenomic aspects and provided comprehensive immunogenomic profiles of pan-cancers. We found that selective copy number changes in immune-related genes may contribute to immune escape. Furthermore, we developed an index of the immunoediting history of each tumor sample based on the information of mutations in exonic regions and pseudogenes and evaluated the immunoediting history of each tumor. Our immuno-genomic analyses of pan-cancers have the potential to identify a subset of tumors with immunogenicity and diverse backgrounds or intrinsic pathways associated with their immune status and immunoediting history.

Human and viral membrane-associated E3 ubiquitin ligases MARCH1 and MIR2 recognize different features of CD86 to downregulate surface expression

Immune-stimulatory ligands, such as major histocompatibility complex molecules and the T-cell costimulatory ligand CD86, are central to productive immunity. Endogenous mammalian membrane-associated RING-CHs (MARCH) act on these and other targets to regulate antigen presentation and activation of adaptive immunity, whereas virus-encoded homologs target the same molecules to evade immune responses. Substrate specificity is encoded in or near the membrane-embedded domains of MARCHs and the proteins they regulate, but the exact sequences that distinguish substrates from nonsubstrates are poorly understood. Here, we examined the requirements for recognition of the costimulatory ligand CD86 by two different MARCH-family proteins, human MARCH1 and Kaposi's sarcoma herpesvirus modulator of immune recognition 2 (MIR2), using deep mutational scanning. We identified a highly specific recognition surface in the hydrophobic core of the CD86 transmembrane (TM) domain (TMD) that is required for recognition by MARCH1 and prominently features a proline at position 254. In contrast, MIR2 requires no specific sequences in the CD86 TMD but relies primarily on an aspartic acid at position 244 in the CD86 extracellular juxtamembrane region. Surprisingly, MIR2 recognized CD86 with a TMD composed entirely of valine, whereas many different single amino acid substitutions in the context of the native TM sequence conferred MIR2 resistance. These results show that the human and viral proteins evolved completely different recognition modes for the same substrate. That some TM sequences are incompatible with MIR2 activity, even when no specific recognition motif is required, suggests a more complicated mechanism of immune modulation via CD86 than was previously appreciated.

Elucidating the Antiviral Mechanism of Different MARCH Factors

The membrane-associated RING-CH (MARCH) proteins belong to a family of E3 ubiquitin ligases, whose main function is to remove transmembrane proteins from the plasma membrane. Recent work has shown that the human MARCH1, 2, and 8 are antiretroviral factors that target the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins by reducing their incorporation in the budding virions. Nevertheless, the dearth of information regarding the antiviral mechanism of this family of proteins necessitates further examination. In this study, using both the human MARCH proteins and their mouse homologues, we provide a comprehensive analysis of the antiretroviral mechanism of this family of proteins. Moreover, we show that human MARCH proteins restrict to various degrees the envelope glycoproteins of a diverse number of viruses. This report sheds light on the important antiviral function of MARCH proteins and their significance in cell intrinsic immunity.IMPORTANCE This study examines the mechanism utilized by different MARCH proteins to restrict retrovirus infection. MARCH proteins block the incorporation of envelope glycoproteins to the budding virions. In this report, by comparing the human and mouse MARCH genes and using murine leukemia virus (MLV) and HIV-1, we identify differences in the mechanism of restriction among MARCH proteins. Furthermore, we perform a comprehensive analysis on a number of envelope glycoproteins and show that MARCH proteins have broad antiviral functions.

Membrane-associated RING-CH (MARCH) proteins down-regulate cell surface expression of the interleukin-6 receptor alpha chain (IL6Rα)

Interleukin 6 (IL6) is a cytokine that regulates a number of important immune and inflammatory pathways. We used the ability of IL6 to inhibit the clonal proliferation of the mouse M1 myeloid leukemia cell line in agar to positively screen a cDNA expression library for proteins that inhibited IL6 activity. We found three clones completely resistant to IL6 that contained the cDNA for the Membrane-Associated RING-CH E3 ubiquitin ligase MARCH2. MARCH2 is a member of a family of membrane-bound E3 ubiquitin ligases that target cell surface receptors for degradation. MARCH2 overexpressing M1 clones retained responsiveness to the related cytokines leukemia inhibitory factor and oncostatin M and we showed that its inhibitory effect was a result of selective down-regulation of the IL6 receptor alpha chain and not the shared receptor subunit, gp130 or other signalling molecules. This activity of MARCH2 was also shared with related proteins MARCH4, MARCH9 and an isoform of MARCH3. The transmembrane domains and C-terminal domains, as well as a functional RING domain, of MARCH proteins were all required for substrate recognition and down-regulation. Genetic deletion of individual MARCH proteins in mice had no or little effect on IL6Rα levels but combined deletions of MARCH2,3 and 4 displayed elevated steady-state levels of IL6Rα in selected haemopoietic cell subsets including CD8+ and CD4+ T cells. These studies extend the potential immunosuppressive roles of MARCH proteins to include down-regulation of IL6 inflammatory responses.

The E3 Ubiquitin Ligase MARCH8 Regulates TNF-α-Induced Apoptosis in Hippocampal Neurons by Targeting Myosin Light Chain 2 for Degradation

Tumor necrosis factor-α (TNF-α) is an important inflammatory cytokine that plays a key role in neuronal damage. Elevated expression of TNF-α is associated with numerous neurodegenerative diseases including Alzheimer's Disease and Parkinson's Disease. However, the specific mechanism of the signaling events that trigger TNF-α-mediated neurotoxicity remain unknown. In this study, we report that intracerebroventricular injection of TNF-α in rat hippocampal neurons down-regulates MLC2 and up-regulates MARCH8, an essential light chain and regulatory myosin light chain of NM Myosin II, respectively. MARCH8 overexpression attenuates the degradation of MLC2 by promoting its ubiquitination and degradation. Inhibition of MARCH8 by siRNA blocks caspase-3 activation and apoptosis signaling, suggesting that TNF-α-induced apoptosis of neurons is partially dependent on the accumulation of MARCH8 and the ubiquitination of MLC2. Taken together, our data not only clarify the function of MARCH8 in TNF-α-induced neurotoxicity, but also demonstrates that TNF-α promotes the MARCH8-MLC2 mediated apoptosis of hippocampal neurons. Anat Rec, 302:2271-2278, 2019. © 2019 American Association for Anatomy.

MARCH ligases in immunity

Membrane associated RING-CH (MARCH) ubiquitin ligases control the stability, trafficking and function of important immunoreceptors, including MHC molecules and costimulatory molecule CD86. Regulation of the critical antigen presenting molecule MHC II by MARCH1 and the control of MARCH1 expression by inflammatory stimuli is a key step in the function of antigen presenting cells. MHC II ubiquitination by MARCH8 and CD83 plays a critical role in T cell thymic selection. Recent studies reveal new immune functions of MARCH ligases in innate immunity, regulation of FcγR expression and Treg development. In addition, we review the importance of MARCH in immunomodulation at the host-pathogen interface. Both bacterial and viral pathogens manipulate MARCH function, while MARCH ligases act as an important host anti-viral defence mechanism. Here, we review the role of membrane-bound MARCH ligases in immune function and provide an update on new substrates and concepts. Understanding the increasingly complex roles of MARCH E3 ligases will be vital to develop therapeutic strategies for their regulation.

Membrane-associated RING-CH (MARCH) 1 and 2 are MARCH family members that inhibit HIV-1 infection

Membrane-associated RING-CH 8 (MARCH8) is one of 11 members of the MARCH family of RING finger E3 ubiquitin ligases and down-regulates several membrane proteins (e.g. major histocompatibility complex II [MHC-II], CD86, and transferrin receptor). We recently reported that MARCH8 also targets HIV-1 envelope glycoproteins and acts as an antiviral factor. However, it remains unclear whether other family members might have antiviral functions similar to those of MARCH8. Here we show that MARCH1 and MARCH2 are MARCH family members that reduce virion incorporation of envelope glycoproteins. Infectivity assays revealed that MARCH1 and MARCH2 dose-dependently suppress viral infection. Treatment with type I interferon enhanced endogenous expression levels of MARCH1 and MARCH2 in monocyte-derived macrophages. Expression of these proteins in virus-producing cells decreased the efficiency of viral entry and down-regulated HIV-1 envelope glycoproteins from the cell surface, resulting in reduced incorporation of envelope glycoproteins into virions, as observed in MARCH8 expression. With the demonstration that MARCH1 and MARCH2 are antiviral MARCH family members as presented here, these two proteins join a growing list of host factors that inhibit HIV-1 infection.

A serine in the first transmembrane domain of the human E3 ubiquitin ligase MARCH9 is critical for down-regulation of its protein substrates

The membrane-associated RING-CH (MARCH) family of membrane-bound E3 ubiquitin ligases regulates the levels of cell-surface membrane proteins, many of which are involved in immune responses. Although their role in ubiquitin-dependent endocytosis and degradation of cell-surface proteins is extensively documented, the features of MARCH proteins and their substrates that drive the molecular recognition events leading to ubiquitin transfer remain poorly defined. In this study, we sought to determine the features of human MARCH9 that are required for regulating the surface levels of its substrate proteins. Consistent with previous studies of other MARCH proteins, we found that susceptibility to MARCH9 activity is encoded in the transmembrane (TM) domains of its substrates. Accordingly, substitutions at specific residues and motifs within MARCH9's TM domains resulted in varying degrees of functional impairment. Most notably, a single serine-to-alanine substitution in the first of its two TM domains rendered MARCH9 completely unable to alter the surface levels of two different substrates: the major histocompatibility class I molecule HLA-A2 and the T-cell co-receptor CD4. Solution NMR analysis of a MARCH9 fragment encompassing the two TM domains and extracellular connecting loop revealed that the residues contributing most to MARCH9 activity are located in the α-helical portions of TM1 and TM2 that are closest to the extracellular face of the lipid bilayer. This observation defines a key region required for substrate regulation. In summary, our biochemical and structural findings demonstrate that specific sequences in the α-helical MARCH9 TM domains make crucial contributions to its ability to down-regulate its protein substrates.

MARCH2 is upregulated in HIV-1 infection and inhibits HIV-1 production through envelope protein translocation or degradation

MARCH2 is one of the MARCH family E3 ligases, which contains eleven members that play pivotal roles in controlling the turn-over of membrane proteins, such as MHC class I, MHC class II, and cell surface receptors. In this study, we found the expression of MARCH2 to be upregulated upon HIV-1 infection. MARCH2 inhibits the production and infection of HIV-1 through ligase activity-dependent envelope protein degradation and/or intracellular retention, a mechanism shared by MARCH8 that also leads to the inhibition of HIV-1 infection. Nevertheless, unlike MARCH8 and other MARCH proteins whose transcription levels are unrelated to viral infection, the expression level of MARCH2 is markedly upregulated upon HIV-1 infection, conferring MARCH2 a unique role in monitoring and regulating the HIV-1 infection-associated process.

Identification of an antiviral host transmembrane protein MARCH8

Membrane-associated RING-CH 8 (MARCH8) is one of 11 members of the recently discovered MARCH family of RING-finger E3 ubiquitin ligases. MARCH8 downregulates several host transmembrane proteins; however, its physiological roles remain unknown. Here we identify MARCH8 as a novel antiviral factor. The overexpression of MARCH8 in virus producing cells did not affect levels of lentivirus production, but markedly reduced viral infectivity. MARCH8 blocked the incorporation of HIV-1 envelope glycoprotein into virions by downregulating it from the cell surface, probably through their interaction, resulting in reduced viral entry efficiency. The inhibitory effect of MARCH8 on vesicular stomatitis virus G-glycoprotein was even more remarkable, suggesting a broad-spectrum inhibition of enveloped viruses by MARCH8. Importantly, the endogenous expression of MARCH8 was high in monocyte-derived macrophages and dendritic cells, and MARCH8 depletion in macrophages significantly increased the infectivity of virions produced from these cells. Our findings thus indicate that MARCH8, which is highly expressed in terminally differentiated myeloid cells, is a potent antiviral host transmembrane protein that reduces virion incorporation of viral envelope glycoproteins.

Molecular mechanism and cellular function of MHCII ubiquitination

The major histocompatibility complex class II (MHCII) is ubiquitinated via the evolutionarily conserved lysine in the cytoplasmic tail of the β chain in dendritic cells (DCs) and B cells. The ubiquitination is mediated by the membrane-associated RING-CH1 (MARCH1) ubiquitin ligase although it can be also mediated by the homologous ligase MARCH8 in model cell lines. The ubiquitination promotes MHCII endocytosis and lysosomal sorting that results in a reduction in the level of MHCII at cell surface. Functionally, MHCII ubiquitination serves as a means by which DCs suppress MHCII expression and reduce antigen presentation in response to the immune regulatory cytokine interleukin-10 (IL-10) and regulatory T cells. Recently, additional roles of MHCII ubiquitination have emerged. MHCII ubiquitination promoted DC production of inflammatory cytokines in response to the Toll-like receptor ligands. It also potentiated DC ability to activate antigen-specific naive CD4(+) T cells while limiting the amount of antigens presented at cell surface. Similarly, MHCII ubiquitination promoted DC activation of CD4(+) thymocytes supporting regulatory T-cell development independent of its effect of limiting antigen presentation. Thus, ubiquitination appears to confer MHCII a function independent of presenting antigens by a mechanism yet to be identified.

Immunological Functions of the Membrane Proximal Region of MHC Class II Molecules

Major histocompatibility complex (MHC) class II molecules present exogenously derived antigen peptides to CD4 T cells, driving activation of naïve T cells and supporting CD4-driven immune functions. However, MHC class II molecules are not inert protein pedestals that simply bind and present peptides. These molecules also serve as multi-functional signaling molecules delivering activation, differentiation, or death signals (or a combination of these) to B cells, macrophages, as well as MHC class II-expressing T cells and tumor cells. Although multiple proteins are known to associate with MHC class II, interaction with STING (stimulator of interferon genes) and CD79 is essential for signaling. In addition, alternative transmembrane domain pairing between class II α and β chains influences association with membrane lipid sub-domains, impacting both signaling and antigen presentation. In contrast to the membrane-distal region of the class II molecule responsible for peptide binding and T-cell receptor engagement, the membrane-proximal region (composed of the connecting peptide, transmembrane domain, and cytoplasmic tail) mediates these "non-traditional" class II functions. Here, we review the literature on the function of the membrane-proximal region of the MHC class II molecule and discuss the impact of this aspect of class II immunobiology on immune regulation and human disease.

MARCH8 inhibits HIV-1 infection by reducing virion incorporation of envelope glycoproteins

Membrane-associated RING-CH 8 (MARCH8) is one of 11 members of the recently discovered MARCH family of RING (really interesting new gene)-finger E3 ubiquitin ligases. MARCH8 downregulates several host transmembrane proteins, including major histocompatibility complex (MHC)-II, CD86, interleukin (IL)-1 receptor accessory protein, TNF-related apoptosis-inducing ligand (TRAIL) receptor 1 and the transferrin receptor. However, its physiological roles remain largely unknown. Here we identify MARCH8 as a novel antiviral factor. The ectopic expression of MARCH8 in virus-producing cells does not affect levels of lentivirus production, but it does markedly reduce viral infectivity. MARCH8 blocks the incorporation of HIV-1 envelope glycoprotein into virus particles by downregulating it from the cell surface, probably through their interaction, resulting in a substantial reduction in the efficiency of viral entry. The inhibitory effect of MARCH8 on vesicular stomatitis virus G-glycoprotein is even more remarkable, suggesting a broad-spectrum inhibition of enveloped viruses by MARCH8. Notably, the endogenous expression of MARCH8 is high in monocyte-derived macrophages and dendritic cells, and MARCH8 knockdown or knockout in macrophages significantly increases the infectivity of virions produced by these cells. Our findings thus indicate that MARCH8 is highly expressed in terminally differentiated myeloid cells, and that it is a potent antiviral protein that targets viral envelope glycoproteins and reduces their incorporation into virions.

The Membrane Associated RING-CH Proteins: A Family of E3 Ligases with Diverse Roles through the Cell

Since the discovery that conjugation of ubiquitin to proteins can drive proteolytic degradation, ubiquitination has been shown to perform a diverse range of functions in the cell. It plays an important role in endocytosis, signal transduction, trafficking of vesicles inside the cell, and even DNA repair. The process of ubiquitination-mediated control has turned out to be remarkably complex, involving a diverse array of proteins and many levels of control. This review focuses on a family of structurally related E3 ligases termed the membrane-associated RING-CH (MARCH) ubiquitin ligases, which were originally discovered as structural homologs to the virals E3s, K3, and K5 from Kaposi's sarcoma-associated herpesvirus (KSHV). These proteins contain a catalytic RING-CH finger and are typically membrane-bound, with some having up to 14 putative transmembrane domains. Despite several lines of evidence showing that the MARCH proteins play a complex and essential role in several cellular processes, this family remains understudied.

Cell adhesion in zebrafish embryos is modulated by March 8

March8 is a member of a family of transmembrane E3 ubiquitin ligases that have been studied mostly for their role in the immune system. We find that March8 is expressed in the zebrafish egg and early embryo, suggesting a role in development. Both knock-down and overexpression of March8 leads to abnormal development. The phenotype of zebrafish embryos and Xenopus animal explants overexpressing March8 implicates impairment of cell adhesion as a cause of the effect. In zebrafish embryos and in cultured cells, overexpression of March8 leads to a reduction in the surface levels of E-cadherin, a major cell-cell adhesion molecule. Experiments in cell culture further show that E-cadherin can be ubiquitinated by March8. On the basis of these observations we suggest that March8 functions in the embryo to modulate the strength of cell adhesion by regulating the localization of E-cadherin.

Ubiquitin-dependent sorting in endocytosis

When ubiquitin (Ub) is attached to membrane proteins on the plasma membrane, it directs them through a series of sorting steps that culminate in their delivery to the lumen of the lysosome where they undergo complete proteolysis. Ubiquitin is recognized by a series of complexes that operate at a number of vesicle transport steps. Ubiquitin serves as a sorting signal for internalization at the plasma membrane and is the major signal for incorporation into intraluminal vesicles of multivesicular late endosomes. The sorting machineries that catalyze these steps can bind Ub via a variety of Ub-binding domains. At the same time, many of these complexes are themselves ubiquitinated, thus providing a plethora of potential mechanisms to regulate their activity. Here we provide an overview of how membrane proteins are selected for ubiquitination and deubiquitination within the endocytic pathway and how that ubiquitin signal is interpreted by endocytic sorting machineries.

MHC class II antigen presentation by dendritic cells regulated through endosomal sorting

For the initiation of adaptive immune responses, dendritic cells present antigenic peptides in association with major histocompatibility complex class II (MHCII) to naïve CD4(+) T lymphocytes. In this review, we discuss how antigen presentation is regulated through intracellular processing and trafficking of MHCII. Newly synthesized MHCII is chaperoned by the invariant chain to endosomes, where peptides from endocytosed pathogens can bind. In nonactivated dendritic cells, peptide-loaded MHCII is ubiquitinated and consequently sorted by the ESCRT machinery to intraluminal vesicles of multivesicular bodies, ultimately leading to lysosomal degradation. Ubiquitination of newly synthesized MHCII is blocked when dendritic cells are activated, now allowing its transfer to the cell surface. This mode of regulation for MHCII is a prime example of how molecular processing and sorting at multivesicular bodies can determine the expression of signaling receptors at the plasma membrane.

MHC Class II Protein Turnover In vivo and Its Relevance for Autoimmunity in Non-Obese Diabetic Mice

Major histocompatibility complex class II (MHCII) proteins are loaded with endosomal peptides and reside at the surface of antigen-presenting cells (APCs) for a time before being degraded. In vitro, MHCII protein levels and turnover are affected by peptide loading and by rates of ubiquitin-dependent internalization from the cell surface, which is in turn affected by APC type and activation state. Prior work suggested that fast turnover of disease-associated MHCII alleles may contribute to autoimmunity. We recently developed novel stable isotope tracer techniques to test this hypothesis in vivo. In non-obese diabetic (NOD) mice, a model of type 1 diabetes (T1D), MHCII turnover was affected by APC type, but unaffected by disease-associated structural polymorphism. Differences in MHCII turnover were observed between NOD colonies with high and low T1D incidence, but fast turnover was dispensable for autoimmunity. Moreover, NOD mice with gene knockouts of peptide loading cofactors do not develop T1D. Thus, fast turnover does not appear pathogenic, and conventional antigen presentation is critical for autoimmunity in NOD mice. However, shared environmental factors may underpin colony differences in MHCII protein turnover, immune regulation, and pathogenesis.

Effects of transmembrane region variability on cell surface expression and allorecognition of HLA-DP3

The functional relevance of polymorphisms outside the peptide binding groove of HLA molecules is poorly understood. Here we have addressed this issue by studying HLA-DP3, a common antigen relevant for functional matching algorithms of unrelated hematopoietic stem cell transplantation (HSCT) encoded by two transmembrane (TM) region variants, DPB1(*)03:01 and DPB1(*)104:01. The two HLA-DP3 variants were found at a overall allelic frequency of 10.4% in 201 volunteer stem cell donors, at a ratio of 4.2:1. No significant differences were observed in cell surface expression levels of the two variants on B lymphoblastoid cell lines (BLCL), primary B cells or monocytes. Three different alloreactive T cell lines or clones showed similar levels of activation marker CD107a and/or CD137 upregulation in response to HLA-DP3 encoded by DPB1(*)03:01 and DPB1(*)104:01, either endogenously on BLCL or after lentiveral-vector mediated transfer into the same cellular background. These data provide, for the first time, direct evidence for a limited functional role of a TM region polymorphism on expression and allorecognition of HLA-DP3 and are compatible with the notion that the two variants can be considered as a single functional entity for unrelated stem cell donor selection.

Ubiquitination of HLA-DO by MARCH family E3 ligases

HLA-DO (DO) is a nonclassical MHC class II (MHCII) molecule that negatively regulates the ability of HLA-DM to catalyse the removal of invariant chain-derived CLIP peptides from classical MHCII molecules. Here, we show that DO is posttranslationally modified by ubiquitination. The location of the modified lysine residue is shared with all classical MHCII beta chains, suggesting a conserved function. Three membrane-associated RING-CH (MARCH1, 8 and 9) family E3 ligases that polyubiquitinate MHCII induce similar profiles of polyubiquitination on DOβ. All three MARCH proteins also influenced trafficking of DO indirectly by a mechanism that required the DOβ encoded di-leucine and tyrosine-based endocytosis motifs. This may be the result of MARCH-induced ubiquitination of components of the endocytic machinery. MARCH9 was by far the most efficient at inducing intracellular redistribution of DO but did not target molecules for lysosomal degradation. The specificity of MARCH9 for HLA-DQ and HLA-DO suggests a need for common regulation of these two MHC-encoded molecules.

Ubiquitination by the membrane-associated RING-CH-8 (MARCH-8) ligase controls steady-state cell surface expression of tumor necrosis factor-related apoptosis inducing ligand (TRAIL) receptor 1

The eleven members of the membrane-associated RING-CH (MARCH) ubiquitin ligase family are relatively unexplored. Upon exogenous (over)expression, a number of these ligases can affect the trafficking of membrane molecules. However, only for MARCH-1 endogenous functions have been demonstrated. For the other endogenous MARCH proteins, no functions or substrates are known. We report here that TRAIL-R1 is a physiological substrate of the endogenous MARCH-8 ligase. Human TRAIL-R1 and R2 play a role in immunosurveillance and are targets for cancer therapy, because they selectively induce apoptosis in tumor cells. We demonstrate that TRAIL-R1 is down-regulated from the cell surface, with great preference over TRAIL-R2, by exogenous expression of MARCH ligases that are implicated in endosomal trafficking, such as MARCH-1 and -8. MARCH-8 attenuated TRAIL-R1 cell surface expression and apoptosis signaling by virtue of its ligase activity. This suggested that ubiquitination of TRAIL-R1 was instrumental in its down-regulation by MARCH-8. Indeed, in cells with endogenous MARCH expression, TRAIL-R1 was ubiquitinated at steady-state, with the conserved membrane-proximal lysine 273 as one of the potential acceptor sites. This residue was also essential for the interaction of TRAIL-R1 with MARCH-1 and MARCH-8 and its down-regulation by these ligases. Gene silencing identified MARCH-8 as the endogenous ligase that ubiquitinates TRAIL-R1 and attenuates its cell surface expression. These findings reveal that endogenous MARCH-8 regulates the steady-state cell surface expression of TRAIL-R1.