Structural and functional dissection of an MHC class I antigen-binding adenovirus glycoprotein

An interaction between β'-COP and the ArfGAP, Glo3, maintains post-Golgi cargo recycling

The essential COPI coat mediates retrieval of transmembrane proteins at the Golgi and endosomes following recruitment by the small GTPase, Arf1. ArfGAP proteins regulate COPI coats, but molecular details for COPI recognition by ArfGAPs remain elusive. Biochemical and biophysical data reveal how β'-COP propeller domains directly engage the yeast ArfGAP, Glo3, with a low micromolar binding affinity. Calorimetry data demonstrate that both β'-COP propeller domains are required to bind Glo3. An acidic patch on β'-COP (D437/D450) interacts with Glo3 lysine residues located within the BoCCS (binding of coatomer, cargo, and SNAREs) region. Targeted point mutations in either Glo3 BoCCS or β'-COP abrogate the interaction in vitro, and loss of the β'-COP/Glo3 interaction drives Ste2 missorting to the vacuole and aberrant Golgi morphology in budding yeast. These data suggest that cells require the β'-COP/Glo3 interaction for cargo recycling via endosomes and the TGN, where β'-COP serves as a molecular platform to coordinate binding to multiple proteins, including Glo3, Arf1, and the COPI F-subcomplex.

Investigation of the Endoplasmic Reticulum Localization of UDP-Glucuronosyltransferase 2B7 with Systematic Deletion Mutants

UDP-Glucuronosyltransferase (UGT) plays an important role in the metabolism of endogenous and exogenous compounds. UGT is a type I membrane protein, and has a dilysine motif (KKXX/KXKXX) in its C-terminal cytoplasmic domain. Although a dilysine motif is defined as an endoplasmic reticulum (ER) retrieval signal, it remains a matter of debate whether this motif functions in the ER localization of UGT. To address this issue, we generated systematic deletion mutants of UGT2B7, a major human isoform, and compared their subcellular localizations with that of an ER marker protein calnexin (CNX), using subcellular fractionation and immunofluorescent microscopy. We found that although the dilysine motif functioned as the ER retention signal in a chimera that replaced the cytoplasmic domain of CD4 with that of UGT2B7, UGT2B7 truncated mutants lacking this motif extensively colocalized with CNX, indicating dilysine motif-independent ER retention of UGT2B7. Moreover, deletion of the C-terminal transmembrane and cytoplasmic domains did not affect ER localization of UGT2B7, suggesting that the signal necessary for ER retention of UGT2B7 is present in its luminal domain. Serial deletions of the luminal domain, however, did not affect the ER retention of the mutants. Further, a cytoplasmic and transmembrane domain-deleted mutant of UGT2B7 was localized to the ER without being secreted. These results suggest that UGT2B7 could localize to the ER without any retention signal, and lead to the conclusion that the static localization of UGT results from lack of a signal for export from the ER.

Structure and mechanism of COPI vesicle biogenesis

Distinct trafficking pathways within the secretory and endocytic systems ensure prompt and precise delivery of specific cargo molecules to different cellular compartments via small vesicular (50-150nm) and tubular carriers. The COPI vesicular coat is required for retrograde trafficking from the cis-Golgi back to the ER and within the Golgi stack. Recent structural data have been obtained from X-ray crystallographic studies on COPI coat components alone and on COPI subunits in complex with either cargo motifs or Arf1, and from reconstructions of COPI coated vesicles by electron tomography. These studies provide important molecular information and indicate key differences in COPI coat assembly as compared with clathrin-based and COPII-based coats.

The transmembrane domain of the adenovirus E3/19K protein acts as an endoplasmic reticulum retention signal and contributes to intracellular sequestration of major histocompatibility complex class I molecules

The human adenovirus E3/19K protein is a type I transmembrane glycoprotein of the endoplasmic reticulum (ER) that abrogates cell surface transport of major histocompatibility complex class I (MHC-I) and MHC-I-related chain A and B (MICA/B) molecules. Previous data suggested that E3/19K comprises two functional modules: a luminal domain for interaction with MHC-I and MICA/B molecules and a dilysine motif in the cytoplasmic tail that confers retrieval from the Golgi apparatus back to the ER. This study was prompted by the unexpected phenotype of an E3/19K molecule that was largely retained intracellularly despite having a mutated ER retrieval motif. To identify additional structural determinants responsible for ER localization, chimeric molecules were generated containing the luminal E3/19K domain and the cytoplasmic and/or transmembrane domain (TMD) of the cell surface protein MHC-I K(d). These chimeras were analyzed for transport, cell surface expression, and impact on MHC-I and MICA/B downregulation. As with the retrieval mutant, replacement of the cytoplasmic tail of E3/19K allowed only limited transport of the chimera to the cell surface. Efficient cell surface expression was achieved only by additionally replacing the TMD of E3/19K with that of MHC-I, suggesting that the E3/19K TMD may confer static ER retention. This was verified by ER retention of an MHC-I K(d) molecule with the TMD replaced by that of E3/19K. Thus, we have identified the E3/19K TMD as a novel functional element that mediates static ER retention, thereby increasing the concentration of E3/19K in the ER. Remarkably, the ER retrieval signal alone, without the E3/19K TMD, did not mediate efficient HLA downregulation, even in the context of infection. This suggests that the TMD is required together with the ER retrieval function to ensure efficient ER localization and transport inhibition of MHC-I and MICA/B molecules.

Molecular basis for recognition of dilysine trafficking motifs by COPI

COPI mediates retrograde trafficking from the Golgi to the endoplasmic reticulum (ER) and within the Golgi stack, sorting transmembrane proteins bearing C-terminal KKxx or KxKxx motifs. The structure of KxKxx motifs bound to the N-terminal WD-repeat domain of β'-COP identifies electrostatic contacts between the motif and complementary patches at the center of the β'-COP propeller. An absolute requirement of a two-residue spacing between the terminal carboxylate group and first lysine residue results from interactions of carbonyl groups in the motif backbone with basic side chains of β'-COP. Similar interactions are proposed to mediate binding of KKxx motifs by the homologous α-COP domain. Mutation of key interacting residues in either domain or in their cognate motifs abolishes in vitro binding and results in mistrafficking of dilysine-containing cargo in yeast without compromising cell viability. Flexibility between β'-COP WD-repeat domains and the location of cargo binding have implications for COPI coat assembly.

Correlative light and electron microscopy of GFP

The correlation of light and electron microscopy (EM) is a powerful tool as it combines the investigation of dynamic processes in vivo with the resolution power of the electron microscope. The green fluorescent proteins (GFPs) and its derivatives revolutionized live-cell light microscopy. Hence, this review outlines correlative microscopy of GFP through photo-oxidation, a method that allows for the direct ultrastructural visualization of fluorophores upon illumination. Oxygen radicals generated during the GFP bleaching process photo-oxidize diaminobenzidine (DAB) into an electron dense precipitate that can be visualized both by routine EM of thin sections and by electron tomography for 3D analysis. There are different levels of correlative microscopy, i.e. the correlation of certain areas, cells, or organelles from light to EM, where photo-oxidation of DAB through GFP allows the highest possible degree--the correlation of specific molecules.

Determinants of the endoplasmic reticulum (ER) lumenal-domain of the adenovirus serotype 2 E3-19K protein for association with and ER-retention of major histocompatibility complex class I molecules

The E3-19K immunomodulatory protein from adenoviruses (Ads) inhibits antigen presentation by major histocompatibility complex (MHC) class I molecules. As a result, the ability of Ad-specific cytotoxic T lymphocytes (CTLs) to lyse infected cells is suppressed. The ER-lumenal domain of E3-19K is subdivided into a variable (residues 1 to ∼78/81) and conserved (residues ∼79/82 to 98) region followed by a linker (residues 99-107). Using molecular and cellular approaches, we characterized in detail the properties of the ER-lumenal domain of E3-19K that enable it to target MHC class I molecules. Proteolysis of recombinant serotype 2 E3-19K (residues 1-100) (with six His residues) generated a large N-terminal (residues 1-88) and a small C-terminal fragment (residues 94-100) in solution. Neither of these fragments associates with HLA-A*1101 as shown by a native gel band-shift assay. In contrast, the N-terminal 1-93 residues of Ad2 E3-19K exhibited the same binding affinity to HLA-A*1101 as E3-19K. Using a site-directed mutational analysis and flow cytometry, we show that Tyr(93), but not Tyr(88), critically modulates the cell-surface expression of MHC class I molecules. Taken together, these results indicate that the sequence comprising residues 89-93 (M(89)SKQY(93)), and in particular Tyr(93), in the conserved region of E3-19K is critical for its immunomodulatory function. Residues 89-93 likely form a linker or loop in E3-19K. Overall, our data provide novel insights into the structure of E3-19K and identify key determinants for association with and ER-retention of its cellular target protein. This knowledge is important for our understanding of the molecular basis of Ad pathogenesis.

Conserved amino acids within the adenovirus 2 E3/19K protein differentially affect downregulation of MHC class I and MICA/B proteins

Successful establishment and persistence of adenovirus (Ad) infections are facilitated by immunosubversive functions encoded in the early transcription unit 3 (E3). The E3/19K protein has a dual role, preventing cell surface transport of MHC class I/HLA class I (MHC-I/HLA-I) Ags and the MHC-I-like molecules (MHC-I chain-related chain A and B [MICA/B]), thereby inhibiting both recognition by CD8 T cells and NK cells. Although some crucial functional elements in E3/19K have been identified, a systematic analysis of the functional importance of individual amino acids is missing. We now have substituted alanine for each of 21 aas in the luminal domain of Ad2 E3/19K conserved among Ads and investigated the effects on HLA-I downregulation by coimmunoprecipitation, pulse-chase analysis, and/or flow cytometry. Potential structural alterations were monitored using conformation-dependent E3/19K-specific mAbs. The results revealed that only a small number of mutations abrogated HLA-I complex formation (e.g., substitutions W52, M87, and W96). Mutants M87 and W96 were particularly interesting as they exhibited only minimal structural changes suggesting that these amino acids make direct contacts with HLA-I. The considerable number of substitutions with little functional defects implied that E3/19K may have additional cellular target molecules. Indeed, when assessing MICA/B cell-surface expression we found that mutation of T14 and M82 selectively compromised MICA/B downregulation with essentially no effect on HLA-I modulation. In general, downregulation of HLA-I was more severely affected than that of MICA/B; for example, substitutions W52, M87, and W96 essentially abrogated HLA-I modulation while largely retaining the ability to sequester MICA/B. Thus, distinct conserved amino acids seem preferentially important for a particular functional activity of E3/19K.

Correlative microscopy and electron tomography of GFP through photooxidation

We have developed a simple correlative photooxidation method that allows for the direct ultrastructural visualization of the green fluorescent protein (GFP) upon illumination. The method, termed GRAB for GFP recognition after bleaching, uses oxygen radicals generated during the GFP bleaching process to photooxidize 3,3'-diaminobenzidine (DAB) into an electron-dense precipitate that can be visualized by routine electron microscopy and electron tomography. The amount of DAB product produced by the GRAB method appears to be linear with the initial fluorescence, and the resulting images are of sufficient quality to reveal detailed spatial information. This is exemplified by the observed intra-Golgi stack and intracisternal distribution of a human Golgi resident glycosylation enzyme, N-acetylgalactosaminyltransferase-2 fused either to enhanced GFP or CFP.

Modulation of the antigen transport machinery TAP by friends and enemies

The transporter associated with antigen processing (TAP) is a key factor of the major histocompatibility complex (MHC) class I antigen presentation pathway. This ABC transporter translocates peptides derived mainly from proteasomal degradation from the cytosol into the ER lumen for loading onto MHC class I molecules. Manifold mechanisms have evolved to regulate TAP activity. During infection, TAP expression is upregulated by interferon-gamma. Furthermore, the assembly and stability of the transport complex is promoted by various auxiliary factors. However, tumors and viruses have developed sophisticated strategies to escape the immune surveillance by suppressing TAP function. The activity of TAP can be impaired on the transcriptional or translational level, by enhanced degradation or by inhibition of peptide translocation. In this review, we briefly summarize existing data concerning the regulation of the TAP complex.

The endoplasmic reticulum lumenal domain of the adenovirus type 2 E3-19K protein binds to peptide-filled and peptide-deficient HLA-A*1101 molecules

E3-19K is a type I membrane glycoprotein expressed by adenoviruses (Ads) to modulate host antiviral immune responses. We have developed an expression system for the endoplasmic reticulum lumenal domain (residues 1 to 100) of Ad type 2 E3-19K tagged with a C-terminal His6 sequence in baculovirus-infected insect cells. In this system, recombinant E3-19K is secreted into the culture medium. A characterization of soluble E3-19K by analytical ultracentrifugation and circular dichroism showed that the protein is monomeric and adopts a stable and correctly folded tertiary structure. Using a gel mobility shift assay and analytical ultracentrifugation, we showed that soluble E3-19K associates with soluble peptide-filled and peptide-deficient HLA-A*1101 molecules. This is the first example of a viral immunomodulatory protein that interacts with conformationally distinct forms of class I major histocompatibility complex molecules. The E3-19K/HLA-A*1101 complexes formed in a 1:1 stoichiometry with equilibrium dissociation constants (Kd) of 50 +/- 10 nM for peptide-filled molecules and of about 10 microM for peptide-deficient molecules. A temperature-dependent proteolysis study revealed that the association of E3-19K with peptide-deficient HLA-A*1101 molecules stabilizes the binding groove. Importantly, our studies showed that peptide-deficient HLA-A*1101 molecules sequestered by E3-19K are capable of binding antigenic peptides and maturing into peptide-filled molecules. This firmly establishes that E3-19K does not block binding of antigenic peptides. Together, our results suggest that Ads have evolved to exploit the late and early stages of the class I antigen presentation pathway.

Functions and mechanisms of action of the adenovirus E3 proteins

In the evolutionary battle between viruses and their hosts, viruses have armed themselves with weapons to defeat the host's attacks on infected cells. Various proteins encoded in the adenovirus (Ad) E3 transcription unit protect cells from killing mediated by cytotoxic T cells and death-inducing cytokines such as tumor necrosis factor (TNF), Fas ligand, and TNF-related apoptosis-inducing ligand (TRAIL). The viral protein E3-gp19 K blocks MHC class-I-restricted antigen presentation, which diminishes killing by cytotoxic T cells. The receptor internalization and degradation (RID) complex (formerly E3-10.4 K/14.5 K) stimulates the clearance from the cell surface and subsequent degradation of the receptors for Fas ligand and TRAIL, thereby preventing the action of these important immune mediators. RID also downmodulates the epidermal growth factor receptor (EGFR), although what role, if any, this function has in immune regulation is uncertain. In addition, RID antagonizes TNF-mediated apoptosis and inflammation through a mechanism that does not primarily involve receptor downregulation. E3-6.7 K functions together with RID in downregulating some TRAIL receptors and may block apoptosis independently of other E3 proteins. Furthermore, E3-14.7 K functions as a general inhibitor of TNF-mediated apoptosis and blocks TRAIL-induced apoptosis. Finally, after expending great effort to maintain cell viability during the early part of the virus replication cycle, Ads lyse the cell to allow efficient virus release and dissemination. To perform this task subgroup C Ads synthesize a protein late in infection named ADP (formerly E3-11.6 K) that is required for efficient virus release. This review focuses on recent experiments aimed at discovering the mechanism of action of these critically important viral proteins.

Immune modulation by flaviviruses

Flaviviruses cause pleomorphic disease with significant morbidity and mortality worldwide. Interestingly, in contrast to most viruses, which subvert or avoid host immune systems, members of the neurotropic Japanese encephalitis serocomplex cause functional changes associated with increased efficacy of the immune response. These viruses induce increased cell surface expression of immune recognition molecules, including class I and II major histocompatibility complex (MHC) and various adhesion molecules. Increases are functional: infected cells are significantly more susceptible to both virus- and MHC-specific cytotoxic T cell lysis. Induced changes are modulated positively or negatively by Th1 and Th2 cytokines, as well as by cell cycle position and adherence status at infection. Infection also increases costimulatory molecule expression on Langerhans cells in the skin. Local interleukin-1 beta production causes accelerated migration of phenotypically altered Langerhans cells to local draining lymph nodes, where initiation of antiviral immune responses occur. The exact mechanism(s) of upregulation is unclear, but changes are associated with NF-kappa B activation and increased MHC and ICAM-1 gene transcription, independently of interferon (IFN) or other proinflammatory cytokines. Increased MHC and adhesion molecule expression may contribute to the pathogenesis of flavivirus encephalitis. Results from a murine model of flavivirus encephalitis developed in this laboratory suggest that fatal disease is immunopathological in nature, with IFN-gamma playing a crucial role. We hypothesize that these viruses may decoy the adaptive immune system into generating low-affinity T cells, which clear virus poorly, as part of their survival strategy. This may enable viral growth and immune escape in cycling cells, which do not significantly upregulate cell surface molecules.

Immune Evasion by Adenovirus E3 Proteins: Exploitation of Intracellular Trafficking Pathways

Adenoviruses (Ads) are nonenveloped viruses which replicate and assemble in the nucleus. Therefore, viral membrane proteins are not directly required for their multiplication. Yet, all human Ads encode integral membrane proteins in the early transcription unit 3 (E3). Previous studies on subgenus C Ads demonstrated that most E3 proteins exhibit immunomodulatory functions. In this review we focus on the E3 membrane proteins, which appear to be primarily devoted to remove critical recognition structures for the host immune system from the cell surface. The molecular mechanism for removal depends on the E3 protein involved: E3/19K prevents expression of newly synthesized MHC molecules by inhibition of ER export, whereas E3/10.4-14.5K down-regulate apoptosis receptors by rerouting them into lysosomes. The viral proteins mediating these processes contain typical transport motifs, such as KKXX, YXXphi, or LL. E3/49K, another recently discovered E3 protein, may require such motifs to reach a processing compartment essential for its presumed immunomodulatory activity. Thus, E3 membrane proteins exploit the intracellular trafficking machinery for immune evasion. Conspicuously, many E3 membrane proteins from Ads other than subgenus C also contain putative transport motifs. Close inspection of surrounding amino acids suggests that many of these are likely to be functional. Therefore, Ads might harbor more E3 proteins that exploit intracellular trafficking pathways as a means to manipulate immunologically important key molecules. Differential expression of such functions by Ads of different subgenera may contribute to their differential pathogenesis. Thus, an unexpected link emerges between viral manipulation of intracellular transport pathways and immune evasion.

Interaction of flaviviruses with cells of the vertebrate host and decoy of the immune response

Flaviviruses cause endemic and epidemic disease with significant morbidity and mortality throughout the world. In contrast to viruses that avoid the host immune response by down-regulating cell surface major histocompatibility complex expression, infection by members of the neurotropic Japanese encephalitis serogroup induce virus-directed functional increases in expression of class I and II major histocompatibility complex and various adhesion molecules, resulting in increased susceptibility to both virus- and major histocompatibility complex-specific cytotoxic T lymphocyte lysis. These changes are comodulated by T1 and T2 cytokines, as well as by cell cycle position and adherence status at infection. Infected skin dendritic (Langerhans) cells also show increased costimulatory molecule expression and local interleukin-1beta production causes accelerated migration of Langerhans cells to local draining lymph nodes, where initiation of antiviral immune responses occur. The exact mechanism(s) of up-regulation is unclear, but changes are associated with NF-kappaB activation and increased MHC and ICAM-1 gene transcription, independently of interferon or other pro-inflammatory cytokines. We hypothesize that these viruses may decoy the adaptive immune system into generating low-affinity, self-reactive T cells which clear virus poorly, as part of their survival strategy. This may enable viral growth and immune escape in cycling cells, which do not significantly up-regulate cell surface molecules. A possible side-effect of this might be immunopathology, caused by 'autoimmune' cross-reactive damage of uninfected high major histocompatibility complex and adhesion molecule-expressing cells, with consequent exacerbation of encephalitic disease. Results from a murine model of flavivirus encephalitis developed in this laboratory further suggest that interferon-gamma plays a crucial role in fatal immunopathology.

Subversion of host defense mechanisms by adenoviruses

Adenoviruses (Ads) cause acute and persistent infections. Alike the much more complex herpesviruses, Ads encode numerous immunomodulatory functions. About a third of the viral genome is devoted to counteract both the innate and the adaptive antiviral immune response. Immediately upon infection, E1A blocks interferon-induced gene expression and the VA-RNA inhibits interferon-induced PKR activity. At the same time, E1A reprograms the cell for DNA synthesis and induces the intrinsic cellular apoptosis program that is interrupted by E1B/19K and E1B/55K proteins, the latter inhibits p53-mediated apoptosis. Most other viral stealth functions are encoded by a separate transcription units, E3. Several E3 products prevent death receptor-mediated apoptosis. E3/14.7K seems to interfere with the cytolytic and pro-inflammatory activities of TNF while E3/10.4K and 14.5K proteins remove Fas and TRAIL receptors from the cell surface by inducing their degradation in lysosomes. These and other functions that may afect granule-mediated cell death might drastically limit lysis by NK cells and cytotoxic T cells (CTL). Moreover, Ads interfere with recognition of infected cell by CTL. The paradigmatic E3/19K protein subverts antigen presentation by MHC class I molecules by inhibiting their transport to the cell surface. In concert, these viral countermeasures ensure prolonged survival in the infected host and, as a consequence, facilitate transmission. Elucidating the molecular mechanisms of Ad-mediated immune evasion has stimulated corresponding research on other viruses. This knowledge will also be instrumental for designing better vectors for gene therapy and vaccination, and may lead to a more rational treatment of life-threatening Ad infections, e.g. in transplantation patients.

Quantitative ER <--> Golgi transport kinetics and protein separation upon Golgi exit revealed by vesicular integral membrane protein 36 dynamics in live cells

To quantitatively investigate the trafficking of the transmembrane lectin VIP36 and its relation to cargo-containing transport carriers (TCs), we analyzed a C-terminal fluorescent-protein (FP) fusion, VIP36-SP-FP. When expressed at moderate levels, VIP36-SP-FP localized to the endoplasmic reticulum, Golgi apparatus, and intermediate transport structures, and colocalized with epitope-tagged VIP36. Temperature shift and pharmacological experiments indicated VIP36-SP-FP recycled in the early secretory pathway, exhibiting trafficking representative of a class of transmembrane cargo receptors, including the closely related lectin ERGIC53. VIP36-SP-FP trafficking structures comprised tubules and globular elements, which translocated in a saltatory manner. Simultaneous visualization of anterograde secretory cargo and VIP36-SP-FP indicated that the globular structures were pre-Golgi carriers, and that VIP36-SP-FP segregated from cargo within the Golgi and was not included in post-Golgi TCs. Organelle-specific bleach experiments directly measured the exchange of VIP36-SP-FP between the Golgi and endoplasmic reticulum (ER). Fitting a two-compartment model to the recovery data predicted first order rate constants of 1.22 +/- 0.44%/min for ER --> Golgi, and 7.68 +/- 1.94%/min for Golgi --> ER transport, revealing a half-time of 113 +/- 70 min for leaving the ER and 1.67 +/- 0.45 min for leaving the Golgi, and accounting for the measured steady-state distribution of VIP36-SP-FP (13% Golgi/87% ER). Perturbing transport with AlF(4)(-) treatment altered VIP36-SP-GFP distribution and changed the rate constants. The parameters of the model suggest that relatively small differences in the first order rate constants, perhaps manifested in subtle differences in the tendency to enter distinct TCs, result in large differences in the steady-state localization of secretory components.

Immunomodulatory functions encoded by the E3 transcription unit of adenoviruses

Persistent viruses have evolved multiple strategies to escape the host immune system. One important prerequisite for efficient viral reproduction in the face of an ongoing immune response is prevention of premature lysis of infected cells. A number of viruses achieve this goal by interfering with antigen presentation and recognition of infected cells by cytotoxic T cells (CTL). Another viral strategy aims to block apoptosis triggered by host defense mechanisms. Both types of strategies seem to be realized by human adenoviruses (Ads). The early transcription unit E3 of Ads encodes proteins that inhibit antigen presentation by MHC class I molecules as well as apoptosis induced by tumor necrosis factor alpha (TNF-alpha) and Fas ligand (FasL). Here, we will describe the organization of the E3 regions of different Ad subgroups and compare the structure and functions of the known immunomodulatory E3 proteins.

Control of MHC class I traffic from the endoplasmic reticulum by cellular chaperones and viral anti-chaperones

MHC class I molecules assemble with peptides in the endoplasmic reticulum (ER). To ensure that only peptide-loaded MHC molecules leave the ER, empty molecules are retained by ER-resident chaperones, most notably the MHC-specific tapasin. ER exit of class I MHC is also controlled by viruses, but for the opposite purpose of preventing peptide presentation to T cells. Interestingly, some viral proteins are able to retain MHC class I molecules in the ER despite being transported. By contrast, other viral proteins exit the ER only upon binding to class I MHC, thereby rerouting newly synthesized class I molecules to intracellular sites of proteolysis. Thus, immune escape can be achieved by reversing, inhibiting or redirecting the chaperone-assisted MHC class I folding, assembly and intracellular transport.

The luminal part of the murine cytomegalovirus glycoprotein gp40 catalyzes the retention of MHC class I molecules

Murine cytomegalovirus (MCMV) interferes with the MHC class I pathway of antigen presentation. The type I transmembrane glycoprotein gp40, encoded by the gene m152, retains major histocompatibility complex (MHC) class I complexes in the endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC)/cis-Golgi. These MHC class I complexes are stable, show an extended half-life and do not exchange beta(2)-microglobulin, whereas gp40 reaches an endosomal/lysosomal compartment where it subsequently is degraded. The analysis of regions within the viral protein that are essential for MHC class I retention revealed that a secreted form of gp40, lacking the cytoplasmic tail and the transmembrane region, still has the capacity to retain MHC class I complexes. Continuous expression of gp40 is not required for MHC class I retention. Our data indicate that the retention of MHC class I complexes in the ERGIC/cis-Golgi is triggered by gp40 and does not require the further presence of the viral protein.

The amyloid precursor-like protein 2 associates with the major histocompatibility complex class I molecule K(d)

Amyloid precursor-like protein 2 (APLP2) is a member of a protein family related to the amyloid precursor protein, which is implicated in Alzheimer's disease. Little is known about the physiological function of this protein family. The adenovirus E3/19K protein binds to major histocompatibility complex (MHC) class I antigens in the endoplasmic reticulum, thereby preventing their transport to the cell surface. In cells coexpressing E3/19K and the MHC K(d) molecule, K(d) is associated with E3/19K and two cellular protein species with masses of 100 and 110 kDa, termed p100/110. Interestingly, p100/110 are released from the complex upon the addition of K(d)-binding peptides, suggesting a role for these proteins in peptide transfer to MHC molecules. Here we demonstrate by microsequencing, reactivity with APLP2-specific antibodies, and comparison of biochemical parameters that p100/110 is identical to human APLP2. We further show that the APLP2/K(d) association does not require the physical presence of E3/19K. Thus, APLP2 exhibits an intrinsic affinity for the MHC K(d) molecule. Similar to the binding of MHC molecules to the transporter associated with antigen processing, complex formation between APLP2 and K(d) strictly depends upon the presence of beta(2)-microglobulin. Conditions that prolong the residency of K(d) in the endoplasmic reticulum lead to a profound increase of the association and a drastic reduction of APLP2 transport. Therefore, this unexpected interplay between these unrelated molecules may have implications for both MHC antigen and APLP2 function.

Rab6 coordinates a novel Golgi to ER retrograde transport pathway in live cells

We visualized a fluorescent-protein (FP) fusion to Rab6, a Golgi-associated GTPase, in conjunction with fluorescent secretory pathway markers. FP-Rab6 defined highly dynamic transport carriers (TCs) translocating from the Golgi to the cell periphery. FP-Rab6 TCs specifically accumulated a retrograde cargo, the wild-type Shiga toxin B-fragment (STB), during STB transport from the Golgi to the endoplasmic reticulum (ER). FP-Rab6 TCs associated intimately with the ER, and STB entered the ER via specialized peripheral regions that accumulated FP-Rab6. Microinjection of antibodies that block coatomer protein I (COPI) function inhibited trafficking of a KDEL-receptor FP-fusion, but not FP-Rab6. Additionally, markers of COPI-dependent recycling were excluded from FP-Rab6/STB TCs. Overexpression of Rab6:GDP (T27N mutant) using T7 vaccinia inhibited toxicity of Shiga holotoxin, but did not alter STB transport to the Golgi or Golgi morphology. Taken together, our results indicate Rab6 regulates a novel Golgi to ER transport pathway.

A comparison of viral immune escape strategies targeting the MHC class I assembly pathway

Peptide fragments from proteins of intracellular pathogens such as viruses are displayed at the cell surface by MHC class I molecules thus enabling surveillance by cytotoxic T cells. Peptides are produced in the cytosol by proteasomal degradation and translocated into the endoplasmic reticulum by the peptide transporter TAP. Empty MHC class I molecules associate with TAP prior to their acquisition of peptides, a process which is assisted and controlled by a series of chaperones. The first part of this review summarizes our current knowledge of this assembly pathway and describes recent observations that tapasin functions as an endoplasmic reticulum retention molecule for empty MHC class I molecules. To defeat the presentation of virus-derived peptides, several DNA viruses have devised strategies to interfere with MHC class I assembly. Although these evasion strategies have evolved independently and differ mechanistically they often target the same step in this pathway. We compare escape mechanisms of different viruses with particular emphasis on the retention of newly synthesized MHC class I molecules in the endoplasmic reticulum and the inhibition of peptide transport by viral proteins.

Mechanisms of viral interference with MHC class I antigen processing and presentation

Viruses are ubiquitous and dangerous obligate intracellular parasites. To facilitate recognition of virus-infected cells by the immune system, vertebrates evolved a system that displays oligopeptides derived from viral proteins on the surface of cells in association with class I molecules of the major histocompatibility complex. Here we review the mechanisms counter-evolved by viruses to interfere with the generation of viral peptides, their intracellular trafficking, or the cell surface expression of class I molecules bearing viral peptides. This topic is important in its own right because the viruses that encode these proteins represent medically important pathogens, are potential vectors for vaccines or gene therapy, and provide strategies and tools for blocking immune recognition in transplantation, autoimmunity, and gene therapy. In addition, studies on viral interference provide unique insights into unfettered antigen processing and normal cellular functions that are exploited and exaggerated by viruses.

Recycling of golgi-resident glycosyltransferases through the ER reveals a novel pathway and provides an explanation for nocodazole-induced Golgi scattering

During microtubule depolymerization, the central, juxtanuclear Golgi apparatus scatters to multiple peripheral sites. We have tested here whether such scattering is due to a fragmentation process and subsequent outward tracking of Golgi units or if peripheral Golgi elements reform through a novel recycling pathway. To mark the Golgi in HeLa cells, we stably expressed the Golgi stack enzyme N-acetylgalactosaminyltransferase-2 (GalNAc-T2) fused to the green fluorescent protein (GFP) or to an 11-amino acid epitope, VSV-G (VSV), and the trans/TGN enzyme beta1,4-galactosyltransferase (GalT) fused to GFP. After nocodazole addition, time-lapse microscopy of GalNAc-T2-GFP and GalT-GFP revealed that scattered Golgi elements appeared abruptly and that no Golgi fragments tracked outward from the compact, juxtanuclear Golgi complex. Once formed, the scattered structures were relatively stable in fluorescence intensity for tens of minutes. During the entire process of dispersal, immunogold labeling for GalNAc-T2-VSV and GalT showed that these were continuously concentrated over stacked Golgi cisternae and tubulovesicular Golgi structures similar to untreated cells, suggesting that polarized Golgi stacks reform rapidly at scattered sites. In fluorescence recovery after photobleaching over a narrow (FRAP) or wide area (FRAP-W) experiments, peripheral Golgi stacks continuously exchanged resident proteins with each other through what appeared to be an ER intermediate. That Golgi enzymes cycle through the ER was confirmed by microinjecting the dominant-negative mutant of Sar1 (Sar1pdn) blocking ER export. Sar1pdn was either microinjected into untreated or nocodazole-treated cells in the presence of protein synthesis inhibitors. In both cases, this caused a gradual accumulation of GalNAc-T2-VSV in the ER. Few to no peripheral Golgi elements were seen in the nocodazole-treated cells microinjected with Sar1pdn. In conclusion, we have shown that Golgi-resident glycosylation enzymes recycle through the ER and that this novel pathway is the likely explanation for the nocodazole-induced Golgi scattering observed in interphase cells.

gp25L/emp24/p24 protein family members of the cis-Golgi network bind both COP I and II coatomer

Abstract. Five mammalian members of the gp25L/ emp24/p24 family have been identified as major constituents of the cis-Golgi network of rat liver and HeLa cells. Two of these were also found in membranes of higher density (corresponding to the ER), and this correlated with their ability to bind COP I in vitro. This binding was mediated by a K(X)KXX-like retrieval motif present in the cytoplasmic domain of these two members. A second motif, double phenylalanine (FF), present in the cytoplasmic domain of all five members, was shown to participate in the binding of Sec23 (COP II). This motif is part of a larger one, similar to the F/YXXXXF/Y strong endocytosis and putative AP2 binding motif. In vivo mutational analysis confirmed the roles of both motifs so that when COP I binding was expected to be impaired, cell surface expression was observed, whereas mutation of the Sec23 binding motif resulted in a redistribution to the ER. Surprisingly, upon expression of mutated members, steady-state distribution of unmutated ones shifted as well, presumably as a consequence of their observed oligomeric properties.

Localization of three human polypeptide GalNAc-transferases in HeLa cells suggests initiation of O-linked glycosylation throughout the Golgi apparatus

O-glycosylation of proteins is initiated by a family of UDP-N-acetylgalactosamine:polypeptide N-acetylgalactos-aminyltransferases (GalNAc-T). In this study, we have localized endogenous and epitope-tagged human GalNAc-T1, -T2 and -T3 to the Golgi apparatus in HeLa cells by subcellular fractionation, immunofluorescence and immunoelectron microscopy. We show that all three GalNAc-transferases are concentrated about tenfold in Golgi stacks over Golgi associated tubular-vesicular membrane structures. Surprisingly, we find that GalNAc-T1, -T2 and -T3 are present throughout the Golgi stack suggesting that initiation of O-glycosylation may not be restricted to the cis Golgi, but occur at multiple sites within the Golgi apparatus. GalNAc-T1 distributes evenly across the Golgi stack whereas GalNAc-T2 and -T3 reside preferentially on the trans side and in the medial part of the Golgi stack, respectively. Moreover, we have investigated the possibility of O-glycan initiation in pre-Golgi compartments such as the ER. We could not detect endogenous polypeptide GalNAc-transferase activity in the ER of HeLa cells, neither by subcellular fractionation nor by situ glycosylation of an ER-retained form of CD8 (CD8/E19). However, upon relocation of chimeric GalNAc-T1 or -T2 to the ER, CD8/E19 is glycosylated with different efficiencies indicating that all components required for initiation of O-glycosylation are present in the ER except for polypeptide GalNAc-transferases.

Exit of the pre-TCR from the ER/cis-Golgi is necessary for signaling differentiation, proliferation, and allelic exclusion in immature thymocytes

A major issue is whether surface expression of the pre-TCR is necessary for signaling the development of immature thymocytes. To address this question, we generated transgenic mice expressing a TCRbeta chain that had a strong endoplasmic reticulum (ER) retrieval signal (TCRbetaER) and that was expressed intracellularly but failed to reach the cell surface. In TCRbetaER transgenic mice, there was a failure of allelic exclusion. Also, the transgene failed to rescue the developmental defects observed in TCRbeta-null mice. In contrast, TCRbeta transgenes with a mutant ER retrieval sequence or lacking this sequence signaled efficient allelic exclusion and suppressed the TCRbeta-/- defect. These data show that exit of the pre-TCR from the ER/cis-Golgi is required for progression through the double-negative thymocyte checkpoint.

A sorting motif localizes the foamy virus glycoprotein to the endoplasmic reticulum

We recently identified an endoplasmic reticulum (ER) retrieval signal-the dilysine motif-in the glycoproteins of all five foamy viruses (FVs) for which sequences were available (P. A. Goepfert, G. Wang, and M. J. Mulligan, Cell 82:543-544, 1995). In the present study, expression of recombinant human FV (HFV) glycoprotein and analyses of oligosaccharide modifications and precursor cleavage indicated that the protein was localized to the ER. HFV glycoproteins encoding seven different dilysine motif mutations were then expressed. The results indicated that disruptions of the dilysine motif resulted in higher levels of forward transport of the HFV glycoprotein from the ER through the Golgi apparatus to the plasma membrane. We conclude that the dilysine motif is responsible for ER sorting of the FV glycoprotein. Signal-mediated ER localization has not previously been described for a retroviral glycoprotein.

Molecular basis of immune evasion strategies by adenoviruses

Human adenoviruses have provided valuable insights into virus-host interactions at the clinical and experimental levels. In addition to the medical importance of adenoviruses in acute infections and the ability of the virus to persist in the host, adenovirus-based recombinants are being developed as potential vaccine vectors. It is now clear that adenoviruses employ various strategies to modulate the innate and the adaptive host immune defences. Adenovirus genome-coded products that interact with the immune response of the host have been identified, and to a large extent the molecular mechanisms of their functions have been revealed. Such knowledge will no doubt influence our approach to the areas of viral pathogenesis, vaccine development and immune modulation for disease management.

Molecular mechanisms of class I major histocompatibility complex antigen processing and presentation

The presentation of antigenic peptides by class I major histocompatibility complex molecules plays a central role in the cellular immune response, since immune surveillance for detection of viral infections or malignant transformations is achieved by CD8+ T lymphocytes which inspect peptides, derived from intracellular proteins, bind to class I molecules on the surface of most cells. The transporter associated with antigen processing selectively translocates cytoplasmically derived peptides of appropriate sequence and length into the lumen of the endoplasmic reticulum where they associate with newly synthesized class I molecules. The translocated peptides are generated by multicatalytic and multisubunit proteasomes which degrade cytoplasmic proteins in a ATP-ubiquitin-dependent manner. This review discusses our current molecular understanding of class I antigen processing and presentation.

The role of the membrane-spanning domain and stalk region of N-acetylglucosaminyltransferase I in retention, kin recognition and structural maintenance of the Golgi apparatus in HeLa cells

Using a series of chimeric and truncated N-acetylglucosaminyltransferase I (NAGT I) molecules we have shown that part of the lumenal stalk region is both necessary and sufficient for kin recognition of mannosidase II and retention in the Golgi stack. The membrane-spanning domain was not required for retention, but replacing part or all of this domain with leucine residues did have a dramatic effect on Golgi morphology. In stable cell lines, stacked cisternae were replaced by tubulo-vesicular clusters containing the mutated NAGT I. The loss of stacked cisternae was proportional to the number of leucines used to replace the membrane-spanning domain.

The role of a 21-kDa viral membrane protein in the assembly of vaccinia virus from the intermediate compartment

We have recently provided morphological evidence that a key event in the assembly of vaccinia virus is the formation of a novel cisternal domain of the intermediate compartment (IC) between the endoplasmic reticulum and the Golgi complex (Sodeik, B., Doms, R. W., Ericsson, M., Hiller, G., Machamer, C. E., van't Hof, W., van Meer, G., Moss, B., and Griffiths, G. (1993) J. Cell Biol. 121, 521-541). This tightly apposed cisternal domain incompletely surrounds the spherical immature virus that matures into the first of the two distinct infectious forms of vaccinia, the intracellular mature virus (IMV). In this study we describe the characterization of an abundant membrane protein of the IMV, the gene product of A17L, a 21-kDa protein that has recently been shown to be essential for the formation of the viral membranes (Rodriguez, D., Esteban, M., and Rodriguez, J. R. (1995) J. Virol. 69, 4640-4648). Upon translation in vitro, p21 associated with rough microsomal membranes in a co-translational manner. Using NH2- and COOH-terminal specific antibodies, we show that both in vitro as well as in vivo, p21 adopts a topology where the NH2 and COOH termini are cytoplasmically orientated. Immunocytochemical experiments demonstrated that p21 is a component of the inner of the two cisternal membranes of the immature virus as well as of membranes of the IC, identified using antibodies against Rab1. Taken together, these data provide the first molecular evidence in support of our assembly model; they show that an essential membrane protein of the IMV inserts into the rough endoplasmic reticulum, but gets efficiently targeted to the IC and membranes of the viral factory.

One step ahead of the game: viral immunomodulatory molecules

For decades cell biologists have relied on viruses to facilitate the study of complex cellular function. More recently, the tragedy of the AIDS epidemic has focused considerable human and financial resources on both virology and immunology, resulting in the generation of new information relating these disciplines. As the miracle of the mammalian immune system unfolds in the laboratory, the elegance of the mechanisms used by co-evolving viruses to circumvent detection and destruction by the host becomes inescapably obvious. Although many observation of virus-induced phenomena that likely contribute to the virus's escape of immune surveillance are still empirical, many other such phenomena have now been defined at the molecular level and confirmed in in vivo models. Immune modulators encoded within viral genomes include proteins that regulate antigen presentation, function as cytokines or cytokine antagonists, inhibit apoptosis, and interrupt the complement cascade. The identification of such gene products and the elucidation of their function have substantially strengthened our understanding of specific virus-host interactions and, unexpectedly, have contributed to the recognition of potent synergy between viruses, which can result in an unpredictable exacerbation of disease in co-infected individuals. Because many viral immune modulators clearly have host counterparts, viruses provide a valuable method for studying normal immune mechanisms. It is conceivable that an improved understanding of virus-encoded immunomodulators will enhance our ability to design reagents for use in therapeutic intervention in disease and in vaccine development.

The cytoplasmic carboxy-terminal amino acid specifies cleavage of membrane TGF alpha into soluble growth factor

Membrane-anchored transforming growth factor alpha (proTGF alpha) belongs to a group of transmembrane proteins whose extracellular domains are selectively cleaved and released into the medium. We demonstrate that the carboxy-terminal valine in the cytoplasmic tail of proTGF alpha is required for cleavage of the growth factor ectodomain in response to various activators. This cleavage process occurs outside Golgi or lysosomal locations, affects cell surface proTGF alpha, and requires little or no membrane traffic. We propose that cleavage and release of proTGF alpha ectodomain involve a specialized proteolytic system and depend on the recognition of a simple and specific determinant located in the proTGF alpha cytoplasmic tail.

Anti-HLA antigen class I heavy chain monoclonal antibodies inhibit human immunodeficiency virus production by peripheral blood mononuclear cells

We have examined the capacity of monoclonal antibodies (mAb) specific for HLA class I heavy chain to interfere with the human immunodeficiency virus (HIV) replicative cycle in human T cells. Among six anti-HLA class I heavy chain-specific mAb assayed, two mAb, RL4-24-6 and W6/32, were able to delay HIV1 and HIV2 cytopathic effect on MT4 cells, a human T cell leukemia virus type I (HTLVI) immortalized T cell line, mAb RL4-24-6, chosen for further studies, also inhibited HIV1 production by peripheral blood mononuclear cells (PBMC), and this inhibition was dose dependent. However, no effect was observed when mAb treatment was performed with either the CEM or Jurkat T cell lines. Our investigation of how RL4-24-6 interferes with the HIV replicative cycle revealed that: (a) incubation of PBMC with RL4-24-6 prior to HIV exposure did not change the susceptibility of these cells to HIV infection, (b) syncytia formation between CD4+ MT4 cells and HIV chronically infected PBMC was not affected by RL4-24-6 and (c) treatment of freshly infected PBMC with RL4-24-6, however, inhibited viral production. These data, together with those we previously reported using anti-beta 2-microglobulin (beta 2m) mAb, suggest that anti-HLA class I/beta 2m complex mAb can modify an early step of the HIV replicative cycle without affecting the viral entry.

Intracellular transport of class II MHC molecules directed by invariant chain

Three structural motifs in the invariant chain (li) control the intracellular transport of class II major histocompatibility complex molecules. An endoplasmic reticulum retention signal in the full-length li suggests a role for li in the alpha-beta heterodimer assembly. Another signal motif directs a truncated li, alone or associated with individual class II chains, to a degradation compartment by a pathway circumventing the Golgi. When this truncated li binds alpha-beta dimers, a third signal dominates, directing the complex by way of the Golgi to vesicles in the cell periphery, which may represent a subcompartment of recycling endosomes.

Intracellular interaction of human immunodeficiency virus type 1 (ARV-2) envelope glycoprotein gp160 with CD4 blocks the movement and maturation of CD4 to the plasma membrane

The envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1) plays a major role in the down-regulation of its receptor, CD4. Using a transient-expression system, we investigated the interaction of the HIV-1 envelope glycoprotein with CD4 during their movement through the intracellular membrane traffic. In singly transfected cells, the envelope glyprotein gp160 was synthesized, glycosylated, and localized predominantly in the endoplasmic reticulum. Only a minor fraction of gp160 was proteolytically cleaved, producing gp120 and gp41, and gp120 was secreted into the medium. On the other hand, the CD4 molecule, when expressed alone, was properly glycosylated and transported efficiently to the cell surface. However, when gp160 and CD4 were coexpressed in the same cell, the cell surface delivery of CD4 was greatly reduced. In coexpressing cells, CD4 formed a specific intracellular complex with gp160 as both proteins could be immunoprecipitated by antibodies against either the gp160 or CD4 (OKT4) but not by OKT4A, a blocking antibody against CD4. The specific gp160-CD4 complex was localized predominantly in the endoplasmic reticulum, and the CD4 in the complex did not acquire endoglycosidase H resistance. The present studies demonstrated that a specific intracellular interaction between gp160 and CD4 was responsible for the cell surface down-regulation of CD4 in cells expressing both the envelope glycoprotein of HIV-1 and its receptor, CD4.

The regulation and expression of MHC class I genes

The expression of mouse MHC class I genes and their products in vivo reveals complex patterns of regulation. Different promoter elements, which are required for gene activation or modulation in response to various external stimuli, have now been characterized as well as the proteins that bind to them. As described here by Brigitte David-Watine and colleagues, the picture that has gradually emerged from these in vitro studies is of an intricate interplay of transacting factors that ultimately lead to the fine tuning of MHC class I expression in vivo.

Antigen presentation requires transport of MHC class I molecules from the endoplasmic reticulum

The role of exocytosis of major histocompatibility complex (MHC) class I molecules in the presentation of antigens to mouse cytotoxic T lymphocytes (CTLs) was examined by use of a recombinant vaccinia virus that expresses the E19 glycoprotein from adenovirus. E19 blocked the presentation of vaccinia and influenza virus proteins to CTLs in a MHC class I allele-specific manner identical to its inhibition of MHC class I transport from the endoplasmic reticulum. This finding indicates that (i) the relevant parameter for antigen presentation is the rate of MHC class I molecule exocytosis, not the level of class I cell surface expression, and (ii) association of class I molecules with antigen is likely to occur within the endoplasmic reticulum.

Amino acids bracketing the predicted transmembrane domains of membrane proteins

The cell membrane is a complex mixture of several classes of biomolecules but amino acids and lipids are the main constituents. For this reason we are establishing a data base of transmembrane proteins with the intent of using the data base to identify interfacial peptide sequences useful for studying protein-lipid interactions at membrane interfaces. Our present intention is to characterize transmembrane peptides and amino acids found near the membrane interface. A data base containing only signal peptides is available (G. von Heijne, Prot. Seq. Data Anal. 1:41-42, 1987).

Short cytoplasmic sequences serve as retention signals for transmembrane proteins in the endoplasmic reticulum

The adenoviral transmembrane E3/19K glycoprotein is a resident of the endoplasmic reticulum. Here we show that the last six amino acid residues of the 15-membered cytoplasmic tail are necessary and sufficient for the ER retention. These residues can be transplanted onto the cytoplasmic tail of other membrane-bound proteins such that ER residency is conferred. Deletion analysis demonstrated that no single amino acid residue is responsible for the retention. The identified structural motif must occupy the extreme COOH-terminal position to be functional. An endogenous transmembrane ER protein, UDP-glucuronosyltransferase, also contains a retention signal in its cytoplasmic tail. We suggest that short linear sequences occupying the extreme COOH-terminal position of transmembrane ER proteins serve as retention signals.