Characterizing the Lassa Virus Envelope Glycoprotein Membrane Proximal External Region for Its Role in Fusogenicity

The membrane-proximal external region(MPER) of Lassa virus(LASV) glycoprotein complex(GPC) is critical in modulating its functionality. Till now, the high-resolution structure of the intact GPC, including MPER is not available. In this study, we used alanine substitution to scan all 16 residues located in LASV MPER. Western blotting and quantification fusion assay showed that the residues located at the C terminus of the HR2(M414 and L415) and N terminus of the MPER(K417 and Y419) are critical for GPC-mediated membrane fusion function. Furthermore, cell surface biotinylation experiments revealed that M414 A, K417 A and Y419 A expressed similar levels as WT, whereas L415 A mutant led to a reduction of mature GPC on the cell surface. Moreover, substitution of these residues with the similar residue such as M414 L, L415 I, K417 R and Y419 F would partly compensate the loss of the fusion activity caused by the alanine mutant in these sites. Results from this study showed that several key residues in the MPER region are indispensable to promote the conformational changes that drive fusion events and shed light on the structure analysis of LASV GPC and anti-LASV therapeutics.     

Molecular characterization of the processing of arenavirus envelope glycoprotein precursors by subtilisin kexin isozyme-1/site-1 protease

A crucial step in the life cycle of arenaviruses is the biosynthesis of the mature fusion-active viral envelope glycoprotein (GP) that is essential for virus-host cell attachment and entry. The maturation of the arenavirus GP precursor (GPC) critically depends on proteolytic processing by the cellular proprotein convertase (PC) subtilisin kexin isozyme-1 (SKI-1)/site-1 protease (S1P). Here we undertook a molecular characterization of the SKI-1/S1P processing of the GPCs of the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) and the pathogenic Lassa virus (LASV). Previous studies showed that the GPC of LASV undergoes processing in the endoplasmic reticulum (ER)/cis-Golgi compartment, whereas the LCMV GPC is cleaved in a late Golgi compartment. Herein we confirm these findings and provide evidence that the SKI-1/S1P recognition site RRLL, present in the SKI-1/S1P prodomain and LASV GPC, but not in the LCMV GPC, is crucial for the processing of the LASV GPC in the ER/cis-Golgi compartment. Our structure-function analysis revealed that the cleavage of arenavirus GPCs, but not cellular substrates, critically depends on the autoprocessing of SKI-1/S1P, suggesting differences in the processing of cellular and viral substrates. Deletion mutagenesis showed that the transmembrane and intracellular domains of SKI-1/S1P are dispensable for arenavirus GPC processing. The expression of a soluble form of the protease in SKI-I/S1P-deficient cells resulted in the efficient processing of arenavirus GPCs and rescued productive virus infection. However, exogenous soluble SKI-1/S1P was unable to process LCMV and LASV GPCs displayed at the surface of SKI-I/S1P-deficient cells, indicating that GPC processing occurs in an intracellular compartment. In sum, our study reveals important differences in the SKI-1/S1P processing of viral and cellular substrates.     

Identification of protective Lassa virus epitopes that are restricted by HLA-A2

Recovery from Lassa virus (LASV) infection usually precedes the appearance of neutralizing antibodies, indicating that cellular immunity plays a primary role in viral clearance. To date, the role of LASV-specific CD8(+) T cells has not been evaluated in humans. To facilitate such studies, we utilized a predictive algorithm to identify candidate HLA-A2 supertype epitopes from the LASV nucleoprotein and glycoprotein precursor (GPC) genes. We identified three peptides (GPC(42-50), GLVGLVTFL; GPC(60-68), SLYKGVYEL; and GPC(441-449), YLISIFLHL) that displayed high-affinity binding (< or =98 nM) to HLA-A*0201, induced CD8(+) T-cell responses of high functional avidity in HLA-A*0201 transgenic mice, and were naturally processed from native LASV GPC in human HLA-A*0201-positive target cells. HLA-A*0201 mice immunized with either GPC(42-50) or GPC(60-68) were protected against challenge with a recombinant vaccinia virus that expressed LASV GPC. The epitopes identified in this study represent potential diagnostic reagents and candidates for inclusion in epitope-based vaccine constructs. Our approach is applicable to any pathogen with existing sequence data, does not require manipulation of the actual pathogen or access to immune human donors, and should therefore be generally applicable to category A through C agents and other emerging pathogens.     

Deletion of specific glycan chains affects differentially the stability, local structures, and activity of lecithin-cholesterol acyltransferase

The enzymatic and interfacial binding activity of lecithin-cholesterol acyltransferase (LCAT) is affected differentially by the location and extent of its glycosylation. Two LCAT glycosylation-deficient mutants, N84Q and N384Q, were constructed, permanently expressed in Chinese hamster ovary cells, and purified to determine the effects of deleting individual glycan chains on its stability, structure, and function. These purified mutants were studied by spectroscopic structural methods and enzymatic and binding assays to develop a molecular rationale for the relationship between LCAT glycosylation and activity. The N84Q LCAT mutant did not possess measurable enzymatic activity or interfacial binding affinity for reconstituted high-density lipoproteins. In addition, in thermal and chemical denaturation studies, N84Q LCAT was found to be significantly less stable than wild-type LCAT. The N384Q variant was initially more enzymatically active than wild-type LCAT, but gradually lost activity within months; however, it retained full interfacial binding activity. Significant changes were detected over time by circular dichroism in the alpha-helical content of N384Q LCAT and in the beta-sheet content of N84Q LCAT, compared with wild-type LCAT. Fluorescence measurements with the probe 1-anilinonapthalene-8-sulfonate suggested an alteration of the active site cavity in both mutants. In conclusion, both mutants lost catalytic activity, N84Q shortly after purification and N384Q more gradually, and were destabilized, probably because the deletion of the glycan chains altered local structural elements near the active site cavity and/or the interfacial binding regions.     

Heparan N-sulfatase: in vitro mutagenesis of potential N-glycosylation sites

Heparan N-sulfatase cDNA contains five potential N-glycosylation sites at Asn positions 41, 142, 151, 264, and 413. We used site-directed mutagenesis, substituting the codon of asparagine for glutamine, to eliminate selected glycosylation sites and then performed expression studies in COS-7 cells to determine the influence on the catalytic activity, lysosomal targeting, and glycosylation-phosphorylation of the enzyme. Elimination of site 5 did not affect significantly enzyme activity; elimination of sites 2 and 4 gave a partial reduction, while elimination of sites 1 and 3 resulted in drastic reduction of catalytic activity (25 and 14%, respectively, of normal values), indicating that glycosylation of asparagine 41 and asparagine 151 is essential for catalysis and/or enzyme stability. Wild type enzyme produced in the presence of tunicamycin was also inactive, indicating that glycosylation is required for acquisition of enzyme activity and/or for enzyme stability. Metabolic labeling of each mutant cDNA, transiently transfected into COS cells, showed that enzyme from mutants N142Q, N264Q, and N413Q appeared to be properly folded, as judged by its ability to be proteolytically processed to a lower molecular weight form, while enzyme from mutants N41Q and N151Q did not reach lysosomes. These studies confirm that the five glycosylation sites of heparan N-sulfatase are all functional and show that Asn 41 and Asn 151 have a role in protein folding and/or stability.     

Definitive evidence that a single N-glycan among three glycans on inducible costimulator is required for proper protein trafficking and ligand binding

Glycosylation is a widespread post-translational modification found in glycoproteins. Glycans play key roles in protein folding, quality control in the endoplasmic reticulum (ER) and protein trafficking within cells. However, it remains unclear whether all positions of protein glycosylation are involved in glycan functions, or if specific positions have individual roles. Here we demonstrate the integral involvement of a specific N-glycan from amongst the three glycans present on inducible costimulator (ICOS), a T-cell costimulatory molecule, in proper protein folding and intracellular trafficking to the cell surface membrane. We found that glycosylation-defective mutant proteins lacking N-glycan at amino-acid position 89 (N89), but not proteins lacking either N23 or N110, were retained within the cell and were not detected on the cell surface membrane. Additional evidence suggested that N89 glycosylation was indirectly involved in ICOS ligand binding. These data suggest that amongst the three putative ICOS glycosylation sites, N89 is required for proper ICOS protein folding in the ER, intracellular trafficking and ligand binding activity. This study represents a substantial contribution to the current mechanistic understanding of the necessity and potential functions of a specific N-glycan among the multiple glycans of glycoproteins.     

Antiviral activity of a small-molecule inhibitor of arenavirus glycoprotein processing by the cellular site 1 protease

Arenaviruses merit interest as clinically important human pathogens and include several causative agents, chiefly Lassa virus (LASV), of hemorrhagic fever disease in humans. There are no licensed LASV vaccines, and current antiarenavirus therapy is limited to the use of ribavirin, which is only partially effective and is associated with significant side effects. The arenavirus glycoprotein (GP) precursor GPC is processed by the cellular site 1 protease (S1P) to generate the peripheral virion attachment protein GP1 and the fusion-active transmembrane protein GP2, which is critical for production of infectious progeny and virus propagation. Therefore, S1P-mediated processing of arenavirus GPC is a promising target for therapeutic intervention. To this end, we have evaluated the antiarenaviral activity of PF-429242, a recently described small-molecule inhibitor of S1P. PF-429242 efficiently prevented the processing of GPC from the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) and LASV, which correlated with the compound's potent antiviral activity against LCMV and LASV in cultured cells. In contrast, a recombinant LCMV expressing a GPC whose processing into GP1 and GP2 was mediated by furin, instead of S1P, was highly resistant to PF-429242 treatment. PF-429242 did not affect virus RNA replication or budding but had a modest effect on virus cell entry, indicating that the antiarenaviral activity of PF-429242 was mostly related to its ability to inhibit S1P-mediated processing of arenavirus GPC. Our findings support the feasibility of using small-molecule inhibitors of S1P-mediated processing of arenavirus GPC as a novel antiviral strategy.     

Lassa virus glycoprotein complex review: insights into its unique fusion machinery

Lassa virus (LASV), an arenavirus endemic to West Africa, causes Lassa fever—a lethal hemorrhagic fever. Entry of LASV into the host cell is mediated by the glycoprotein complex (GPC), which is the only protein located on the viral surface and comprises three subunits: glycoprotein 1 (GP1), glycoprotein 2 (GP2), and a stable signal peptide (SSP). The LASV GPC is a class one viral fusion protein, akin to those found in viruses such as human immunodeficiency virus (HIV), influenza, Ebola virus (EBOV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). These viruses are enveloped and utilize membrane fusion to deliver their genetic material to the host cell. Like other class one fusion proteins, LASV-mediated membrane fusion occurs through an orchestrated sequence of conformational changes in its GPC. The receptor-binding subunit, GP1, first engages with a host cell receptor then undergoes a unique receptor switch upon delivery to the late endosome. The acidic pH and change in receptor result in the dissociation of GP1, exposing the fusion subunit, GP2, such that fusion can occur. These events ultimately lead to the formation of a fusion pore so that the LASV genetic material is released into the host cell. Interestingly, the mature GPC retains its SSP as a third subunit—a feature that is unique to arenaviruses. Additionally, the fusion domain contains two separate fusion peptides, instead of a standard singular fusion peptide. Here, we give a comprehensive review of the LASV GPC components and their unusual features.     

Effect of N-glycosylation on ligand binding affinity of rat V1a vasopressin receptor

A rat Vla vasopressin (rVla) receptor has two putative N-glycosylation sites at 14th and 27th amino acid asparagine in the extracellular N-terminus. In the present study, we examined the possible roles of N-glycosylation of the N-terminus in the receptor function. Three point mutants for deglycosylated rVla receptor were generated in which the 14th and/or the 27th asparagine was replaced with glutamine, namely N14Q, N27Q, and N14:27Q, each tagged with an enhanced green fluorescent protein (EGFP) at their C-termini, and transfected to COS-7 or HEK292 cells. The two single mutants and a double mutant have progressively smaller molecular mass compared to the wild-type receptor as determined by immunoblot analysis, indicating that the two sites are effectively glycosylated in vivo. The maximal ligand binding capacities of three mutant receptors were comparable to that of wild-type (17.02 +/- 1.32 pmol/g protein) with modest changes in ligand binding affinities: N27Q and N14:27Q had decreased binding affinities compared to N14Q and wild-type receptors. The reduced binding affinities of the deglycosylated mutants are not likely due to the impaired intracellular transport since their traffickings were indistinguishable from one another. Taken together, these results suggest that the N-glycosylation at the two sites of the N-terminus of rV1a receptor minimally affects the surface expression and trafficking of the receptor.     

Reverse genetics generation of chimeric infectious Junin/Lassa virus is dependent on interaction of homologous glycoprotein stable signal peptide and G2 cytoplasmic domains

The Arenaviridae are a diverse and globally distributed collection of viruses that are maintained primarily by rodent reservoirs. Junin virus (JUNV) and Lassa virus (LASV) can both cause significant outbreaks of severe and often fatal human disease throughout their respective areas of endemicity. In an effort to improve upon the existing live attenuated JUNV Candid1 vaccine, we generated a genetically homogenous stock of this virus from cDNA copies of the virus S and L segments by using a reverse genetics system. Further, these cDNAs were used in combination with LASV cDNAs to successfully generate two recombinant Candid1 JUNV/LASV chimeric viruses (via envelope glycoprotein [GPC] exchange). It was found that while the GPC extravirion domains were readily exchangeable, homologous stable signal peptide (SSP) and G2 transmembrane and cytoplasmic tail domains were essential for correct GPC maturation and production of infectious chimeric viruses. The switching of the JUNV and LASV G1/G2 ectodomains within the Candid1 vaccine background did not alter the attenuated phenotype of the vaccine strain in a lethal mouse model. These recombinant chimeric viruses shed light on the fundamental requirements of arenavirus GPC maturation and may serve as a strategy for the development of bivalent JUNV and LASV vaccine candidates.     

The late endosome-resident lipid bis(monoacylglycero)phosphate is a cofactor for Lassa virus fusion

Arenavirus entry into host cells occurs through a low pH-dependent fusion with late endosomes that is mediated by the viral glycoprotein complex (GPC). The mechanisms of GPC-mediated membrane fusion and of virus targeting to late endosomes are not well understood. To gain insights into arenavirus fusion, we examined cell-cell fusion induced by the Old World Lassa virus (LASV) GPC complex. LASV GPC-mediated cell fusion is more efficient and occurs at higher pH with target cells expressing human LAMP1 compared to cells lacking this cognate receptor. However, human LAMP1 is not absolutely required for cell-cell fusion or LASV entry. We found that GPC-induced fusion progresses through the same lipid intermediates as fusion mediated by other viral glycoproteins–a lipid curvature-sensitive intermediate upstream of hemifusion and a hemifusion intermediate downstream of acid-dependent steps that can be arrested in the cold. Importantly, GPC-mediated fusion and LASV pseudovirus entry are specifically augmented by an anionic lipid, bis(monoacylglycero)phosphate (BMP), which is highly enriched in late endosomes. This lipid also specifically promotes cell fusion mediated by Junin virus GPC, an unrelated New World arenavirus. We show that BMP promotes late steps of LASV fusion downstream of hemifusion–the formation and enlargement of fusion pores. The BMP-dependence of post-hemifusion stages of arenavirus fusion suggests that these viruses evolved to use this lipid as a cofactor to selectively fuse with late endosomes.     

Two N-linked glycans are required to maintain the transport activity of the bile salt export pump (ABCB11) in MDCK II cells

The aim of this study was to determine the role of N-linked glycosylation in protein stability, intracellular trafficking, and bile acid transport activity of the bile salt export pump [Bsep (ATP-binding cassette B11)]. Rat Bsep was fused with yellow fluorescent protein, and the following mutants, in which Asn residues of putative glycosylation sites (Asn(109), Asn(116), Asn(122), and Asn(125)) were sequentially replaced with Gln, were constructed by site-directed mutagenesis: single N109Q, double N109Q + N116Q, triple N109Q + N116Q + N122Q, and quadruple N109Q + N116Q + N122Q + N125Q. Immunoblot and glycosidase cleavage analysis demonstrated that each site was glycosylated. Removal of glycans decreased taurocholate transport activity as determined in polarized MDCK II cells. This decrease resulted from rapid decay of the mutant Bsep protein; biochemical half-lives were 3.76, 3.65, 3.24, 1.35, and 0.52 h in wild-type, single-mutant, double-mutant, triple-mutant, and quadruple-mutant cells, respectively. Wild-type and single- and double-mutant proteins were distributed exclusively along the apical membranes, whereas triple- and quadruple-mutant proteins remained intracellular. MG-132 but not bafilomycin A(1) extended the half-life, suggesting a role for the proteasome in Bsep degradation. To determine whether a specific glycosylation site or the number of glycans was critical for protein stability, we studied the protein expression of combinations of N-glycan-deficient mutants and observed that Bsep with one glycan was considerably unstable compared with Bsep harboring two or more glycans. In conclusion, at least two N-linked glycans are required for Bsep protein stability, intracellular trafficking, and function in the apical membrane.     

Action at a Distance: MUTATIONS OF PERIPHERAL RESIDUES TRANSFORM RAPID REVERSIBLE INHIBITORS TO SLOW,TIGHT BINDERS OF CYCLOOXYGENASE-2*

Cyclooxygenase enzymes (COX-1 and COX-2) catalyze the conversion of arachidonic acid to prostaglandin G₂. The inhibitory activity of rapid, reversible COX inhibitors (ibuprofen, naproxen, mefenamic acid, and lumiracoxib) demonstrated a significant increase in potency and time dependence of inhibition against double tryptophan murine COX-2 mutants at the 89/90 and 89/119 positions. In contrast, the slow, time-dependent COX inhibitors (diclofenac, indomethacin, and flurbiprofen) were unaffected by those mutations. Further mutagenesis studies suggested that mutation at position 89 was principally responsible for the changes in inhibitory potency of rapid, reversible inhibitors, whereas mutation at position 90 may exert some effect on the potency of COX-2-selective diarylheterocycle inhibitors; no effect was observed with mutation at position 119. Several crystal structures with or without NSAIDs indicated that placement of a bulky residue at position 89 caused a closure of a gap at the lobby, and alteration of histidine to tryptophan at position 90 changed the electrostatic profile of the side pocket of COX-2. Thus, these two residues, especially Val-89 at the lobby region, are crucial for the entrance and exit of some NSAIDs from the COX active site.     

Involvement of asparagine 118 in the nucleotide specificity of the catalytic subunit of protein kinase CK2

Protein kinase CK2 is a heteromeric enzyme with catalytic (alpha) and regulatory (beta) subunits which form an alpha2beta2 holoenzyme and utilizes both ATP and GTP as nucleotide substrate. Site-directed mutagenesis of CK2alpha subunit was used to study this capacity to use GTP. Deletion of asparagine 118 (alpha(deltaN118)) or the mutant alphaN118E gives a 5-6-fold increase in apparent Km for GTP with little effect on the affinity for ATP. Mutants alphaN118A and alphaD120N did not alter significantly the Km for either nucleotide. CK2alphaN118 has an apparent Ki for inosine 5' triphosphate 5-fold higher than wild-type and is very heat labile. These studies complement recent crystallographic data indicating a role for CK2alpha asparagine 118 in binding the guanine base.     

Effect of Asp69 and Arg310 on the pK of His68, a key catalytic residue of adenylosuccinate lyase

Adenylosuccinate lyase (ASL) of Bacillus subtilis contains three conserved histidines, His(68), His(89), and His(141), identified by affinity labeling and site-directed mutagenesis as critical to the intersubunit catalytic site. The pH-V(max) profile for wild-type ASL is bell-shaped (pK (1) = 6.74 and pK (2) = 8.28). Only the alkaline side changes with temperature, characteristic of histidine pKs. To identify determinants of pK (2) in the enzyme-substrate complex, we replaced residues at two positions close to His(68) (but not to His(89) or His(141)) in the structure. Compared with the specific activity of 1.75 mumol adenylosuccinate reacting/min/mg of wild-type enzyme at pH 7.0, mutant enzymes D69E, D69N, R310Q, and R310K exhibit specific activities of 0.40, 0.04, 0.00083, and 0.10, respectively. While D69E has a K (m) for adenylosuccinate similar to that of wild-type ASL, D69N and R310K exhibit modest increases in K (m), and R310Q has an 11-fold increase in K (m). The mutant enzymes show no significant change in molecular weight or secondary structure. The major change is in the pH-V(max) profile: pK (2) is 8.48 for the D69E mutant and is decreased to 7.83 in D69N, suggesting a proximal negative charge is needed to maintain the high pK of 8.28 observed for wild-type enzyme and attributed to His(68). Similarly, R310Q exhibits a decrease in its pK (2) (7.33), whereas R310K shows little change in pK (2) (8.24). These results suggest that Asp(69) interacts with His(68), that Arg(310) interacts with and orients the beta-carboxylate of Asp(69), and that His(68) must be protonated for ASL to be active.     

Solution structure of the low-molecular-weight protein tyrosine phosphatase from Tritrichomonas foetus reveals a flexible phosphate binding loop

Eukaryotic low-molecular-weight protein tyrosine phosphatases (LMW PTPs) contain a conserved serine, a histidine with an elevated pKa, and an active site asparagine that together form a highly conserved hydrogen bonding network. This network stabilizes the active site phosphate binding loop for optimal substrate binding and catalysis. In the phosphatase from the bovine parasite Tritrichomonas foetus (TPTP), both the conserved serine (S37) and asparagine (N14) are present, but the conserved histidine has been replaced by a glutamine residue (Q67). Site-directed mutagenesis, kinetic, and spectroscopic experiments suggest that Q67 is located near the active site and is important for optimal catalytic activity. Kinetic experiments also suggest that S37 participates in the active site/hydrogen bonding network. Nuclear magnetic resonance spectroscopy was used to determine the three-dimensional structure of the TPTP enzyme and to further examine the roles of S37 and Q67. The backbone conformation of the TPTP phosphate binding loop is nearly superimposable with that of other tyrosine phosphatases, with N14 existing in a strained, left-handed conformation that is a hallmark of the active site hydrogen bonding network in the LMW PTPs. As expected, both S37 and Q67 are located at the active site, but in the consensus structure they are not within hydrogen bonding distance of N14. The hydrogen bond interactions that are observed in X-ray structures of LMW PTPs may in fact be transient in solution. Protein dynamics within the active site hydrogen bonding network appear to be affected by the presence of substrate or bound inhibitors such as inorganic phosphate.     

N-Linked Glycosylation of the Hemagglutinin Protein Influences Virulence and Antigenicity of the 1918 Pandemic and Seasonal H1N1 Influenza A Viruses

The hemagglutinin (HA) protein is a major virulence determinant for the 1918 pandemic influenza virus; however, it encodes no known virulence-associated determinants. In comparison to seasonal influenza viruses of lesser virulence, the 1918 H1N1 virus has fewer glycosylation sequons on the HA globular head region. Using site-directed mutagenesis, we found that a 1918 HA recombinant virus, of high virulence, could be significantly attenuated in mice by adding two additional glycosylation sites (asparagine [Asn] 71 and Asn 286) on the side of the HA head. The 1918 HA recombinant virus was further attenuated by introducing two additional glycosylation sites on the top of the HA head at Asn 142 and Asn 172. In a reciprocal experimental approach, deletion of HA glycosylation sites (Asn 142 and Asn 177, but not Asn 71 and Asn 104) from a seasonal influenza H1N1 virus, A/Solomon Islands/2006 (SI/06), led to increased virulence in mice. The addition of glycosylation sites to 1918 HA and removal of glycosylation sites from SI/06 HA imposed constraints on the theoretical structure surrounding the glycan receptor binding sites, which in turn led to distinct glycan receptor binding properties. The modification of glycosylation sites for the 1918 and SI/06 viruses also caused changes in viral antigenicity based on cross-reactive hemagglutinin inhibition antibody titers with antisera from mice infected with wild-type or glycan mutant viruses. These results demonstrate that glycosylation patterns of the 1918 and seasonal H1N1 viruses directly contribute to differences in virulence and are partially responsible for their distinct antigenicity.     

Role of N-linked glycans on bunyamwera virus glycoproteins in intracellular trafficking, protein folding, and virus infectivity

The membrane glycoproteins (Gn and Gc) of Bunyamwera virus (BUN, family Bunyaviridae) contain three potential sites for the attachment of N-linked glycans: one site (N60) on Gn and two (N624 and N1169) on Gc. We determined that all three sites are glycosylated. Digestion of the glycoproteins with endo-beta-N-acetylglucosaminidase H (endo H) or peptide:N-glycosidase F revealed that Gn and Gc differ significantly in their glycan status and that late in infection Gc glycans remain endo H sensitive. The roles of the N-glycans in intracellular trafficking of the glycoproteins to the Golgi, protein folding, and virus replication were investigated by mutational analysis and confocal immunofluorescence. Elimination of the glycan on Gn, by changing N60 to a Q residue, resulted in the protein misfolding and failure of both Gn and Gc proteins to traffic to the Golgi complex. We were unable to rescue a viable virus by reverse genetics from a cDNA containing the N60Q mutation. In contrast, mutant Gc proteins lacking glycans on either N624 or N1169, or both sites, were able to target to the Golgi. Gc proteins containing mutations N624Q and N1169Q acquired endo H resistance. Three viable N glycosylation-site-deficient viruses, lacking glycans on one site or both sites on Gc, were created by reverse genetics. The viability of these recombinant viruses and analysis of growth kinetics indicates that the glycans on Gc are not essential for BUN replication, but they do contribute to the efficiency of virus infection.     

Single-point amino acid substitutions at the 119th residue of thermolysin and their pressure-induced activation

The effect of amino acid substitution at the 119th site of thermolysin (TLN) on the pressure activation behavior of this enzyme was studied for four mutants at pressures < 300 MPa. For Q119Q, Q119N and Q119R, the highest activation was observed to be over 30 times that at atmospheric pressure and the activation volumes (deltaV++) were about -75 ml/mol. However, we obtained only 10 times higher activation for Q119E and Q119D (deltaV++ approximately -60 ml/mol). The intrinsic fluorescence of TLN changed at pressures > 300 MPa, and the latter two mutants showed a smaller deltaGapp and deltaVapp of transition than the wild type. These results are discussed with respect to the hydration change in the enzyme protein around the substituted region.     

N-Linked glycosylation of the human ABC transporter ABCG2 on asparagine 596 is not essential for expression, transport activity, or trafficking to the plasma membrane

The human ATP-binding cassette half-transporter ABCG2 is a 72 kDa plasma membrane protein that can confer multidrug resistance to cells in culture when overexpressed. Both transiently and stably expressed ABCG2 are glycosylated, and treatment with peptide N-glycosidase F reduces the apparent molecular mass on SDS-PAGE gels to approximately 60 kDa. Sequence analysis revealed three potential N-linked glycosylation sites in human ABCG2 at amino acids 418, 557, and 596. Site-directed mutagenesis experiments, in which each Asn was changed to Gln independently, revealed that only asparagine 596 is N-linked glycosylated. These data provide the first direct identification of the modified residue in ABCG2 and evidence for the localization of loop 5 to the extracellular space, previously only predicted from hydropathy analysis. Immunoblot and pulse-chase analyses revealed that the glycosylation-deficient ABCG2 (N596Q) variant and the glycosylated parent transporter are expressed equivalently at steady state and have similar half-lives. Cell surface analysis of ABCG2 expression showed comparable amounts of the N596Q variant present at the plasma membrane compared to the glycosylated ABCG2 protein. The ABCG2 (N596Q) variant is also functional, demonstrating rhodamine 123 transport in intact cells comparable to that in cells expressing glycosylated ABCG2. Furthermore, in crude membrane preparations, neither the basal nor the prazosin-stimulated ( approximately 2-fold) ATPase activities of ABCG2 (N596Q) were affected compared to glycosylated ABCG2. Although subtle defects in transporter trafficking and function may exist, these data taken together suggest that N-glycosylation at arginine 596 is not essential for the expression, trafficking to the plasma membrane, or the overall function of ABCG2.