Incorporation of a charged amino acid into the membrane-spanning domain blocks cell surface transport but not membrane anchoring of a viral glycoprotein

ER Stress-Mediated Signaling: Action Potential and Ca(2+) as Key Players

The proper functioning of the endoplasmic reticulum (ER) is crucial for multiple cellular activities and survival. Disturbances in the normal ER functions lead to the accumulation and aggregation of unfolded proteins, which initiates an adaptive response, the unfolded protein response (UPR), in order to regain normal ER functions. Failure to activate the adaptive response initiates the process of programmed cell death or apoptosis. Apoptosis plays an important role in cell elimination, which is essential for embryogenesis, development, and tissue homeostasis. Impaired apoptosis can lead to the development of various pathological conditions, such as neurodegenerative and autoimmune diseases, cancer, or acquired immune deficiency syndrome (AIDS). Calcium (Ca(2+)) is one of the key regulators of cell survival and it can induce ER stress-mediated apoptosis in response to various conditions. Ca(2+) regulates cell death both at the early and late stages of apoptosis. Severe Ca(2+) dysregulation can promote cell death through apoptosis. Action potential, an electrical signal transmitted along the neurons and muscle fibers, is important for conveying information to, from, and within the brain. Upon the initiation of the action potential, increased levels of cytosolic Ca(2+) (depolarization) lead to the activation of the ER stress response involved in the initiation of apoptosis. In this review, we discuss the involvement of Ca(2+) and action potential in ER stress-mediated apoptosis.

Identification and characterization of endoplasmic reticulum-associated protein, ERp43

Disposal of misfolded proteins from the lumen of the endoplasmic reticulum (ER) is one of the quality control mechanisms present in the protein secretory pathway. Through ER-associated degradation, misfolded substrates are targeted to the cytosol where they are degraded by proteasomes. Here we describe the identification of a human ER-associated 43-kD protein (ERp43) by sequencing of the subtraction suppression hybridization cDNA library from ER stress-treated cells. The ERp43 gene encodes a protein of 383 amino acid residues that contains a potential transmembrane domain. Analysis revealed that ERp43 is primarily located in the ER. Quantitative reverse transcriptase-polymerase chain reaction demonstrated that gene expression was relatively high in the neuronal tissues and in the kidney, with ERp43 protein highly expressed in the spinal cord and in the kidney. In cultured cells, overexpression of ERp43 accelerated cell growth and inhibited ER stress-induced cell death, while down-regulation of ERp43 expression decreased proliferation rate and enhanced this type of cell death. These findings indicate that ERp43 plays important roles in cell growth and ER stress-induced cell death.

ER stress and the unfolded protein response

Conformational diseases are caused by mutations altering the folding pathway or final conformation of a protein. Many conformational diseases are caused by mutations in secretory proteins and reach from metabolic diseases, e.g. diabetes, to developmental and neurological diseases, e.g. Alzheimer's disease. Expression of mutant proteins disrupts protein folding in the endoplasmic reticulum (ER), causes ER stress, and activates a signaling network called the unfolded protein response (UPR). The UPR increases the biosynthetic capacity of the secretory pathway through upregulation of ER chaperone and foldase expression. In addition, the UPR decreases the biosynthetic burden of the secretory pathway by downregulating expression of genes encoding secreted proteins. Here we review our current understanding of how an unfolded protein signal is generated, sensed, transmitted across the ER membrane, and how downstream events in this stress response are regulated. We propose a model in which the activity of UPR signaling pathways reflects the biosynthetic activity of the ER. We summarize data that shows that this information is integrated into control of cellular events, which were previously not considered to be under control of ER signaling pathways, e.g. execution of differentiation and starvation programs.

DAP12 and KAP10 (DAP10)-novel transmembrane adapter proteins of the CD3zeta family

Transmembrane adapter proteins are molecules that associate with receptors and mediate intracellular signals following interaction of the receptor with its ligand. Many such molecules have been characterized in detail, particularly the small TM adapters of the CD3zeta class at the core of the T cell receptor. Recently, two new genetically linked members of this class of transmembrane adapters have been identified called DAP12 (KARAP) and KAP10 (DAP10), respectively. In this review, we discuss this new class of TM adapters using the wealth of knowledge concerning CD3zeta and FcRgamma to highlight similarities and differences with DAP12 and KAP10. In addition, novel receptor families which interact with these TM adapters have also been identified. The role of these receptors and their inhibitory isoforms are discussed.

The HIV-1 Env protein signal sequence retards its cleavage and down-regulates the glycoprotein folding

Secretory proteins and most membrane proteins are synthesized with a signal sequence that is usually cleaved from the nascent polypeptide chain, during its transport, into the lumen of the endoplasmic reticulum (ER). We have analyzed the kinetics of the cleavage of the HIV-1 Env protein signal sequence from gp160 and gp120 in HeLa, BHK, and Jurkat cells. Furthermore, we have determined the effects of this cleavage on the association of the gp160 and gp120 glycoproteins with the ER protein calnexin and the effects of the signal sequence cleavage on protein folding. The cleavage of the HIV-1 Env protein signal sequence on both gp160 and gp120 occurred very slowly in all three cell lines with a t(1/2) of 45-60 min. The core glycosylated and signal-sequence-retained forms of gp160 and gp120 associated with calnexin while the signal-sequence-cleaved forms of gp160 and gp120 had disassociated from calnexin and correctly folded as determined by their ability to associate with the CD4 cellular receptor. Further analysis of the folding state of gp160 and gp120 in nonreducing SDS-PAGE revealed that the signal-sequence-retained and calnexin-associated forms of gp160 and gp120 migrated as broad, diffuse bands, whereas the signal-sequence-cleaved or CD4-associated forms of gp160 and gp120 migrated as single sharper bands. The cause of this retardation in the rate of folding and intracellular transport of HIV-1 glycoproteins was localized to their signal sequences by fusing the vesicular stomatitis virus G protein with the HIV-1 Env protein signal sequence and expressing this chimeric protein in mammalian cells. The HIV-1 Env protein signal sequence on the VSV-G protein also confers a reduced rate of cleavage and slow intracellular transport and folding of the chimeric G protein. These results provide direct evidence that in vivo the HIV-1 glycoprotein signal sequence inhibits the folding of HIV-1 Env protein. Our data also suggest a direct correlation between the rate of the signal sequence cleavage and protein folding.

Functional mapping of the surface residues of human thrombin

Utilizing site-directed mutagenesis, 77 charged and polar residues that are highly exposed on the surface of human thrombin were systematically substituted with alanine. Functional assays using thrombin mutants identified residues that were required for the recognition and cleavage of the procoagulant substrate fibrinogen (Lys21, Trp50, Lys52, Asn53 + Thr55, Lys65, His66, Arg68, Tyr71, Arg73, Lys77, Lys106 + Lys107, Asp193 + Lys196, Glu202, Glu229, Arg233, Asp234) and the anticoagulant substrate protein C (Lys21, Trp50, Lys65, His66, Arg68, Tyr71, Arg73, Lys77, Lys106 + Lys107, Glu229, Arg233), interactions with the cofactor thrombomodulin (Gln24, Arg70) and inhibition by the thrombin aptamer, an oligonucleotide-based thrombin inhibitor (Lys65, His66, Arg70, Tyr71, Arg73). Although there is considerable overlap between the functional epitopes, distinct and specific residues with unique functions were identified. When the functional residues were mapped on the surface of thrombin, they were located on a single hemisphere of thrombin that included both the active site cleft and the highly basic exosite 1. No functional residues were located on the opposite face of thrombin. Residues with procoagulant or anticoagulant functions were not spatially separated but interdigitated with residues of opposite or shared function. Thus thrombin utilizes the same general surface for substrate recognition regardless of substrate function although the critical contact residues may vary.

The rat neutrophil low-affinity Fc receptor for IgG: molecular cloning and functional characterization

A complementary DNA (cDNA) clone encoding rat Fc gamma receptor II (Fc gamma RII) was isolated from rat neutrophils and characterized. The cDNA encodes a type I transmembrane protein with 285 amino acids having an extracellular domain consisting of two immunoglobulin-like domains (179 amino acids), a transmembrane domain (26 amino acids), and a cytoplasmic domain (47 amino acids). The nucleotide sequences are identical to that of recently cloned Fc gamma RII from rat mast cells. This protein was expressed on FcR-negative Chinese hamster ovary (CHO) cells. The characterization of cDNA-transfected CHO cells clearly indicated that the protein encoded by the cDNA clone binds guinea-pig IgG1 and IgG2 complexes and unexpectedly binds monomeric rat IgG1, but not IgG2. Furthermore, the affinity for immune complexes was significantly augmented by protease treatment of transfectants. In addition, endocytosis of immune complex was noted in transfectants.

Influenza virus M2 integral membrane protein is a homotetramer stabilized by formation of disulfide bonds

The oligomeric structure of the influenza A virus M2 integral membrane protein was determined. On SDS-polyacrylamide gels under nonreducing conditions, the influenza A/Udorn/72 virus M2 forms disulfide-linked dimers (30 kDa) and tetramers (60 kDa). Sucrose gradient analysis and chemical cross-linking analysis indicated that the oligomeric form of M2 is a tetramer consisting of either a pair of disulfide-linked dimers or disulfide-linked tetramers. In addition, a small amount of a cross-linked species of 150-180,000 kDa, which the available data suggest contains only M2 polypeptides, was observed. The role of M2 cysteine residues in disulfide bond formation and their role in forming oligomers were examined by converting each of the two extracellular and single cytoplasmic cysteine residues to serine residues and expressing the altered M2 proteins in eukaryotic cells. Removal of either one of the N-terminal cysteines at residues 17 or 19 indicated that tetramers formed that consisted of a pair of noncovalently associated disulfide-linked dimers, suggesting that each of the cysteine residues is equally competent for forming disulfide bonds. When both cysteine residues were removed from the M2 N-terminal domain, no disulfide-linked forms were observed. When solubilized in detergent this double-cysteine mutant lost reactivity with a M2-specific mAb and exhibited an altered sedimentation pattern on sucrose gradients. However, chemical cross-linking of this double-cysteine mutant in membranes indicated that it can form tetramers. Taken together, these data suggest that disulfide bond formation, although not essential for oligomeric assembly, stabilizes the M2 tetramer from disruption by detergent solubilization.

Topology of eukaryotic type II membrane proteins: importance of N-terminal positively charged residues flanking the hydrophobic domain

We have tested the role of different charged residues flanking the sides of the signal/anchor (S/A) domain of a eukaryotic type II (N(cyt)C(exo)) integral membrane protein in determining its topology. The removal of positively charged residues on the N-terminal side of the S/A yields proteins with an inverted topology, while the addition of positively charged residues to only the C-terminal side has very little effect on orientation. Expression of chimeric proteins composed of domains from a type II protein (HN) and the oppositely oriented membrane protein M2 indicates that the HN N-terminal domain is sufficient to confer a type II topology and that the M2 N-terminal ectodomain can direct a type II topology when modified by adding positively charged residues. These data suggest that eukaryotic membrane protein topology is governed by the presence or absence of an N-terminal signal for retention in the cytoplasm that is composed in part of positive charges.

Equilibrium binding of thrombin to recombinant human thrombomodulin: effect of hirudin, fibrinogen, factor Va, and peptide analogues

Thrombomodulin is an endothelial cell surface receptor for thrombin that acts as a physiological anticoagulant. The properties of recombinant human thrombomodulin were studied in COS-7, CHO, CV-1, and K562 cell lines. Thrombomodulin was expressed on the cell surface as shown by the acquisition of thrombin-dependent protein C activation. Like native thrombomodulin, recombinant thrombomodulin contained N-linked oligosaccharides, had Mr approximately 100,000, and was inhibited or immunoprecipitated by anti-thrombomodulin antibodies. Binding studies demonstrated that nonrecombinant thrombomodulin expressed by A549 carcinoma cells and recombinant thrombomodulin expressed by CV-1 and K562 cells had similar Kd's for thrombin of 1.3 nM, 3.3 nM, and 4.7 nM, respectively. The Kd for DIP-thrombin binding to recombinant thrombomodulin on CV-1(18A) cells was identical with that of thrombin. Increasing concentrations of hirudin or fibrinogen progressively inhibited the binding of 125I-DIP-thrombin, while factor Va did not inhibit binding. Three synthetic peptides were tested for ability to inhibit DIP-thrombin binding. Both the hirudin peptide Hir53-64 and the thrombomodulin fifth-EGF-domain peptide Tm426-444 displaced DIP-thrombin from thrombomodulin, but the factor V peptide FacV30-43 which is similar in composition and charge to Hir53-64 showed no binding inhibition. The data exclude the significant formation of a ternary complex consisting of thrombin, thrombomodulin, and hirudin. These studies are consistent with a model in which thrombomodulin, hirudin, and fibrinogen compete for binding to DIP-thrombin at the same site.

Folding and oligomerization properties of a soluble and secreted form of the paramyxovirus hemagglutinin-neuraminidase glycoprotein

The paramyxovirus SV5 hemagglutinin-neuraminidase (HN) glycoprotein (a type II integral membrane protein) was converted into a soluble and secreted form (HN-F) by replacing the HN signal/anchor domain with a hydrophobic domain that can act as a cleavable signal sequence. Approximately 40% of the HN-F synthesized was secreted from cells (t1/2 approximately 2.5-3 hr). The extracellular HN-F molecules were identified as disulfide-linked dimers and the majority of the population of molecules were resistant to endoglycosidase H digestion. Examination of the oligomeric form of the secreted HN-F, by sucrose density gradient sedimentation, indicated that under conditions where HN was a tetramer, HN-F was found to be a dimer, and no extracellular HN-F monomeric species could be detected. Secreted HN-F was fully reactive with conformation-specific monoclonal antibodies and was enzymatically active as shown by HN-F having neuraminidase activity. Examination of the intracellular HN-F species indicated that HN-F monomers were slowly converted to the disulfide-linked form and that under the sucrose density gradient sedimentation conditions used the HN-F monomers aggregated. Some of the HN-F monomers were degraded intracellularly. These data are discussed in relationship to the seemingly different folding and oligomerization requirements for the intracellular transport of soluble and membrane bound forms of a glycoprotein. The soluble and biologically active form of HN may be suitable for further structural and enzymatic studies.

The carboxy terminus of human immunodeficiency virus type 1 gp160 limits its proteolytic processing and transport in transfected cell lines

Mutagenesis of the transmembrane domain and cytoplasmic tail of human immunodeficiency virus type 1 envelope glycoprotein gp160 revealed that its intracellular transport and processing in transfected cell lines were modulated by a functional domain included in the carboxy-terminal sequence consisting of residues 751 to 856.

Comparative studies of hepatitis B virus precore and core particles

Hepatitis B virus core antigen gene expresses two cocarboxy-terminal proteins, termed precore and core proteins. Both precore and core proteins can form nucleocapsid-like particles. In order to understand the mechanism that leads to the formation of the nucleocapsid, we have expressed precore and core protein sequences in COS cells, a monkey kidney cell line, and compared the properties of these two particles. Our results show that core protein can form particles with various densities and they are present mostly in the cytosol. Precore protein, on the other hand, forms particles with one predominant density, and a majority of these particles are present in the lumen of the endoplasmic reticulum (ER). Furthermore, our results show that, when coexpressed in the same cells, core protein and the ER-associated surface antigens (envelope protein) show colocalization, indicating interaction between these two viral structural proteins.

Fc receptor isoforms exhibit distinct abilities for coated pit localization as a result of cytoplasmic domain heterogeneity

Mouse macrophages and lymphocytes express two distinct isoforms of a single class of Fc receptor for IgG. The macrophage isoform (FcRII-B2) is identical to the lymphocyte isoform (FcRII-B1) except for an inframe insertion in the cytoplasmic tail of FcRII-B1 that increases its length from 47 to 94 amino acids. To determine the functional significance of this cytoplasmic domain variation, presumably the result of alternative mRNA splicing, we expressed both isoforms in receptor-negative fibroblasts. While FcRII-B2 mediated the efficient ligand internalization and delivery to lysosomes, endocytosis via FcRII-B1--and via a tailminus mutant--was relatively inefficient. This difference reflected the inability of FcRII-B1 (and the tailminus mutant) to accumulate in clathrin-coated pits. Thus, the FcRII-B2 cytoplasmic tail contains a domain needed for accumulation in coated pits, and this domain is disrupted by the 47 amino acid insertion in FcRII-B1.

Impact of altered protein structures on the intracellular traffic of a mutated vasopressin precursor from Brattleboro rats

The rat vasopressin precursor, synthesized in the reticulocyte lysate system under the direction of in vitro transcribed mRNA, is processed and correctly delivered to the lumen of added microsomal vesicles. Translation of mRNA for the mutant (Brattleboro) vasopressin precursor which lacks a translational stop codon as a consequence of a frame-shift mutation, gives rise to a mutated protein (B-mutant precursor) with a C-terminal poly(lysine) sequence encoded by the poly(A) tail. Upon addition of microsomal membranes, the mutated precursor has access to the lumen of the vesicles as indicated by removal of the signal peptide; however, the C-terminal part with the poly(lysine) tail remains outside the vesicles as shown by its sensitivity to proteinase K. When a modified RNA, including a stop codon located similarly to that found in the cDNA encoding the normal precursor, is translated in the presence of microsomal membranes, the resulting product (S-mutant precursor) is refractory to proteolysis by exogenously added proteinase K. Analysis of the microsomal membranes indicates, however, that the C-terminus of the S-mutant precursor is still anchored within membranes. For studying the intracellular transport of the mutated precursor Xenopus laevis oocytes were injected with various RNA constructs. To monitor the transport steps from the endoplasmic reticulum to the Golgi compartment an RNA encoding a glycosylation site within the S-mutant precursor sequence was constructed. The resulting GS-mutant precursor is synthesized in the oocyte but not secreted into the incubation medium, completely in contrast to the normal vasopressin precursor which can be detected in the incubation bath 4 h after injection of the respective RNA. The sensitivity of the GS-mutant precursor carbohydrate side chain to endoglycosidase H treatment suggests that the mutated precursor does not reach the Golgi apparatus.

Structure and expression of the glycoprotein gene of Chandipura virus

A cDNA copy of the mRNA for the glycoprotein G of Chandipura virus, a rhabdovirus, has been cloned, sequenced, and expressed in mammalian cells. The deduced amino acid sequence of G shows that the encoded protein is a typical transmembrane glycoprotein of 524 amino acids containing a cleavable amino-terminal signal peptide, two potential N-linked glycosylation sites, a hydrophobic membrane anchor domain near the carboxy terminus, and a cytoplasmic domain at the carboxy terminus. Somewhat unusual is the appearance of two charged amino acid residues, aspartate and arginine, within the putative membrane anchor sequence. Expression of the G gene in COS cells resulted in production of a glycosylated protein of mol wt 71,000 which was recognized by anti-Chandipura antibodies. Like the viral G protein, the expressed G contained covalently linked palmitic acid. However, unlike its vesicular stomatitis virus (Indiana serotype) counterpart, the Chandipura G protein has no potential palmitate-accepting cysteine residue within its cytoplasmic domain. Thus, the covalent attachment of fatty acid to this molecule may occur at one or both of the cysteines within the membrane anchor domain. The G protein was intracellularly transported to the cell surface and could induce cell fusion at low pH, showing that the expressed G protein was biologically active.

Surface expression of the T cell receptor complex requires charged residues within the alpha chain transmembrane region

The T cell receptor (TcR) complex is a multi-subunit glycoprotein comprising at least five transmembrane polypeptides. An unusual characteristic of each of the transbilayer domains is the presence of charged amino acids. To examine the importance of these residues for the association and consequent surface expression of the components of the complex, a TcR alpha chain containing either charged or neutral residues within its transbilayer segment was introduced into the human T cell line MOLT-4, and the appearance of the TcR complex at the cell surface was assayed. Surface expression was observed only in MOLT-4 cells transfected with the alpha chain containing charged transbilayer residues. Thus, these residues most probably play a crucial role in the assembly process.

Topology of membrane insertion in vitro and plasma membrane assembly in vivo of the yeast arginine permease

We have examined the topology of the yeast arginine permease, a plasma-membrane protein with multiple membrane-spanning domains. Using fusions of the permease with the glycosylatable secreted yeast protein, acid phosphatase, we identified membrane-spanning sequences that can translocate adjacent acid phosphatase across the membrane of the endoplasmic reticulum (ER), as measured by in vitro glycosylation. Examination for the presence or absence of glycosylation in a systematic series of such fusions gave an internally consistent model for the lumenal or cytoplasmic disposition of the acid phosphatase reporter, defining the topology of the permease. The phenotypes of a further set of arginine permease gene fusions with portions of the gene for the secreted protein, killer toxin, suggest that the pathways of export of membrane and secreted proteins need not be functionally distinct.

Processing of the Semliki Forest virus structural polyprotein: role of the capsid protease

The protease activities responsible for the cotranslational processing of the Semliki Forest virus structural polyprotein were investigated by using an in vitro transcription-translation system. Three cleavages released the individual chains from the nascent polyprotein in the order capsid, p62, 6K (a nonstructural peptide), and E1. We showed directly that the protease activity responsible for the release of the capsid protein resides in the capsid itself: by progressive truncation of the cDNA used for the SP6 transcription, we showed that a precursor containing as few as 38 residues of the p62 protein left at the C terminus of the capsid was still very efficiently cleaved in vitro. We further tested the possibility that serine-219 of the capsid is involved in autoproteolysis by site-directed in vitro mutagenesis. A change in the sequence Gly-Asp-Ser(219)-Gly, a tetrapeptide conserved among several animal serine proteases, to Gly-Asp-Arg-Ser-Thr was shown to completely abolish in vitro cleavage. This supports the notion that the capsid is a serine protease. The role of the capsid protease in the processing of the 6K junctions was then investigated by translations of a hybrid polyprotein in which the capsid and most of the p62 sequences are replaced by those of the secretory protein lysozyme. The cleavages and concomitant appearance of the 6K peptide occurred efficiently and were shown to require the presence of membranes. This demonstrates that the capsid protease is not required for those cleavages and suggests that a membrane-associated host protease is responsible for the cleavage.

Ability of the hydrophobic fusion-related external domain of a paramyxovirus F protein to act as a membrane anchor

The hydrophobic NH2 terminus of F1 (FRED) of the simian virus 5 fusion (F) protein is implicated in mediating cell fusion, but in the inactive F0 precursor the FRED is translocated across membranes. Hybrid proteins containing the FRED as a potential membrane anchorage domain and a mutant of F0 lacking the preceding five-arginine cleavage/activation site were used to study the effect of position on the FRED. The experiments indicate that the SV5 F protein has evolved an exquisite control system for biological activity: the FRED is close to the threshold of hydrophobicity required to function as a membrane anchor. The FRED is not sufficiently hydrophobic to halt translocation when in an internal position, but on cleavage/activation the threshold of hydrophobicity is effectively lowered, and the FRED, now the NH2 terminus of F1, is able to interact stably with membranes.

Abnormal subcellular distribution of beta-glucuronidase in mice with a genetic alteration in enzyme structure

Liver beta-glucuronidase is structurally altered in inbred strain PAC so that a peptide subunit with a more basic isoelectric point, GUS-SN, is produced. This allele of beta-glucuronidase was transferred to strain C57BL/6J by 12 backcross matings to form the congenic line B6 X PAC-Gus(n). Liver beta-glucuronidase activity was halved in males of the congenic strain compared to normal males. The lowered activity was specifically accounted for by a decrease in the lysosomal component. There was no alteration in the concentration of microsomal activity. This alteration in the subcellular distribution of beta-glucuronidase in Gus(n)/Gus(n) mice was confirmed by two independent gel electrophoretic systems which separate microsomal and lysosomal components. beta-Glucuronidase activity was likewise approximately halved in mutant spleen, lung, and brain, organs which contain exclusively or predominantly lysosomal beta-glucuronidase. The loss of liver lysosomal beta-glucuronidase activity was shown by immunotitration to be due to a decrease in the number of beta-glucuronidase molecules in lysosomes of the congenic strain. The Gus(n) structural alteration likely causes the lowered lysosomal beta-glucuronidase activity since the two traits remain in congenic animals. Heterozygous Gus(n)/Gus(b) animals had intermediate levels of liver beta-glucuronidase. Also, the effect was specific, in that three other lysosomal enzymes were not reproducibly lower in Gus(n)/Gus(n) mice. Gus(n) is, therefore, an unusual example of a mutation which causes a change in the subcellular distribution of a two-site enzyme.

Foreign transmembrane peptides replacing the internal signal sequence of transferrin receptor allow its translocation and membrane binding

Each subunit of the human transferrin receptor (TR) dimer is inserted into the ER membrane as a transmembrane polypeptide having its N-terminus in the cytoplasm. The transmembrane segment of the molecule serves both as a signal for chain translocation and as a membrane anchor. To study which structural features of this segment are required for its dual function, we have essentially replaced the transmembrane peptide with the C-terminal membrane-spanning segment of two proteins having a separate N-terminal translocation signal and with an artificial uncharged peptide. In each case the mutant TR molecules are efficiently translocated in vitro. In contrast, substitution of the transmembrane peptide of TR with a hydrophilic peptide results in no detectable translocation activity of the mutant TR. This suggests that the hydrophobic character of the transmembrane peptide of TR, rather than its actual amino acid sequence, is important for chain translocation and membrane binding.

Domains of virus glycoproteins

This chapter reviews current information about the structure and function of virus glycoproteins. There are few virus glycoproteins that provide prototypes for illustrating important relationships between the functions and glycoprotein structure. The discussion presented in the chapter concentrates on those viral glycoproteins that (1) span the lipid bilayer once, (2) are oriented such that the carboxy terminus comprises the cytoplasmic domain, and (3) contain asparagine-linked oligosaccharides. There are also viral glycoproteins with extensive O-linked glycosylation, some of which are also presented in the discussion. The chapter also focuses on the studies involving directed mutagenesis and construction of chimeric proteins. The effects of altering specific amino acid sequences, of swapping domains, and of adding a new domain to a protein serve to define the functions of a domain and to show that a domain can be independently associated with a specific function. The experiments described have been carried out by inserting the genes of particular viral glycoproteins—such as cDNAs—into expression vectors and transcribing the cDNAs from the promoter provided by the expression vector. This approach established that localization and functions such as the fusogenic activity are properties of the viral glycoprotein per se and do not require other viral-coded components.

The properties and sequence of glycoprotein H of herpes simplex virus type 1

The map position of the coding sequence of glycoprotein H of herpes simplex virus type 1 was determined by marker transfer studies in which DNA fragments cloned from a virus resistant to neutralisation by an anti-gH monoclonal antibody were used to transfer antibody resistance to wild type virus DNA following cotransfection. The gH coding sequence was mapped to the BglII "m" fragment of HSV-1 DNA (map coordinates 0.27-0.312), confirming the map position previously determined by intertypic recombinant analysis (Buckmaster et al., 1984). The complete nucleotide sequence of the BglII "m" fragment revealed two large open reading frames in addition to the thymidine kinase gene. The open reading frame lying immediately 3' of the thymidine kinase gene has a predicted translation product with the features of a large glycoprotein. This open reading frame translates to an amino acid sequence of 90,323 mol wt with a signal peptide, a membrane anchor sequence, a large external domain containing potential N-glycosylation sites, and a charged C- terminal cytoplasmic domain. We suppose that this amino acid sequence corresponds to gH of HSV-1, and A. Davison (personal communication) has noted the existence of homologous glycoproteins predicted from the nucleotide sequences of Varicella-zoster virus and Epstein-Barr virus. The properties of monoclonal antibody LP11, directed against gH show remarkable similarities to the properties for gD antibodies. LP11 efficiently neutralizes virus infectivity, blocks cell fusion by syncytial virus strains, and inhibits the formation of plaques when added to cell monolayers after infection. These similarities in antibody activity imply functional relatedness between gH and gD of herpes simplex virus.

In vitro mutagenesis of the putative membrane-binding domain of polyomavirus middle-T antigen

Polyomavirus middle-T antigen contains a contiguous sequence of 22 hydrophobic amino acids near the carboxyl terminus, which is the putative membrane-binding domain of the protein. The DNA encoding this region was mutated to form a series of deletions, insertions, and substitutions called RX mutants. The phenotypes of these mutants fall into three groups based on the transforming and biochemical properties of their encoded proteins. The first group, with deletions outside but proximal to the hydrophobic domain, displayed an essentially wild-type phenotype. A second group, with extensive deletions within the region encoding the hydrophobic domain, expressed middle-T species which did not fractionate with cellular membranes or associate with pp60c-src and which were defective in their ability to transform. A third group of mutants with more subtle predicted alterations in the hydrophobic domain were wild type for the biochemical parameters investigated but were unable to transform cultured rodent cells. These observations are consistent with previous findings that membrane association plays an important role in transformation by middle-T and that, whereas association between middle-T and pp60c-src is a necessary correlate of transformation, it is not sufficient. A comparison of murine polyomavirus middle-T and a newly described hamster papovavirus putative middle-T revealed a strong homology between their respective hydrophobic-domain amino acid sequences. This homology is not observed in the anchorage domains of other model proteins, and this may imply that the middle-T hydrophobic domain is important in transformation for reasons other than simple membrane association.

Structural requirements of a membrane-spanning domain for protein anchoring and cell surface transport

The membrane-spanning domain of the vesicular stomatitis virus glycoprotein (G) contains 20 uncharged and mostly hydrophobic amino acids. We created DNAs specifying G proteins with shortened transmembrane domains, by oligonucleotide-directed mutagenesis. Expression of these DNAs showed that G proteins containing 18, 16, or 14 amino acids of the original transmembrane domain assumed a transmembrane configuration and were transported to the cell surface. G proteins containing only 12 or 8 amino acids of this domain also spanned intracellular membranes, but their transport was blocked within a Golgi-like region in the cell. A G protein completely lacking the membrane-spanning domain accumulated in the endoplasmic reticulum and was secreted slowly. These experiments indicate that the size of the transmembrane domain is critical not only for membrane anchoring, but also for normal cell surface transport.