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.

Mutations within the putative membrane-spanning domain of the simian immunodeficiency virus transmembrane glycoprotein define the minimal requirements for fusion, incorporation, and infectivity

The membrane-spanning domain (MSD) of a number of retroviral transmembrane (TM) glycoproteins, including those from the human and simian immunodeficiency viruses (HIV and SIV), have been predicted to contain a charged arginine residue. The wild-type SIV TM glycoprotein is 354 amino acids long. The entire putative cytoplasmic domain of SIV (amino acids 193 to 354) is dispensable for virus replication in vitro, and such truncation-containing viruses are capable of reaching wild-type titers after a short delay. We show here that further truncation of eight additional amino acids to TM185 results in a protein that lacks fusogenicity but is, nevertheless, efficiently incorporated into budding virions. By analyzing a series of nonsense mutations between amino acids 193 and 185 in Env expression vectors and in the SIVmac239 proviral clone, a region of the SIV TM that contains the minimum requirement for glycoprotein-mediated cell-to-cell fusion and that for virus replication was identified. Virus entry and infectivity were evident in truncations to a minimum of 189 amino acids, whereas cell-cell fusion was observed for a protein of only 187 amino acids. Glycoprotein was efficiently incorporated into budding virions in truncations up to 185 amino acids, indicating that such proteins are membrane anchored and are transported to the cell surface. However, truncation of the TM to 180 amino acids resulted in a protein that displays a transport defect and may be retained in the endoplasmic reticulum. Based on our analyses of these mutants, an alternative model for the MSD of SIV is proposed. Our model suggests that membrane-imbedded charged residues can be neutralized by side-chain interactions with lipid polar head groups. As a consequence, the membrane-spanning region can be reduced by more than a helical turn. This new model accounts for the ability of truncations within the predicted MSD to remain membrane anchored and maintain biological activity.

Structural cues involved in endoplasmic reticulum degradation of G85E and G91R mutant cystic fibrosis transmembrane conductance regulator

Abnormal folding of mutant cystic fibrosis transmembrane conductance regulator (CFTR) and subsequent degradation in the endoplasmic reticulum is the basis for most cases of cystic fibrosis. Structural differences between wild-type (WT) and mutant proteins, however, remain unknown. Here we examine the intracellular trafficking, degradation, and transmembrane topology of two mutant CFTR proteins, G85E and G91R, each of which contains an additional charged residue within the first putative transmembrane helix (TM1). In microinjected Xenopus laevis oocytes, these mutations markedly disrupted CFTR plasma membrane chloride channel activity. G85E and G91R mutants (but not a conservative mutant, G91A) failed to acquire complex N-linked carbohydrates, and were rapidly degraded before reaching the Golgi complex thus exhibiting a trafficking phenotype similar to DeltaF508 CFTR. Topologic analysis revealed that neither G85E nor G91R mutations disrupted CFTR NH2 terminus transmembrane topology. Instead, WT as well as mutant TM1 spanned the membrane in the predicted C-trans (type II) orientation, and residues 85E and 91R were localized within or adjacent to the plane of the lipid bilayer. To understand how these charged residues might provide structural cues for ER degradation, we examined the stability of WT, G85E, and G91R CFTR proteins truncated at codons 188, 393, 589, or 836 (after TM2, TM6, the first nucleotide binding domain, or the R domain, respectively). These results indicated that G85E and G91R mutations affected CFTR folding, not by gross disruption of transmembrane assembly, but rather through insertion of a charged residue within the plane of the bilayer, which in turn influenced higher order tertiary structure.

The nature of topogenic sequences determines the transport competence of topological mutants of neutral endopeptidase-24.11

Type II integral membrane proteins are anchored by a signal-peptide/membrane-anchor domain (SA domain) located near their N-terminus, whereas type I membrane proteins are anchored by stop-transfer sequences usually located near the C-terminus. In this study we have attempted to transform neutral endopeptidase-24.11 (EC 3.4.24.11; NEP), a type II membrane protein, into a type I membrane protein. Three type I mutant proteins were constructed by fusion of topogenic sequences to the C-terminus of SecNEP, a soluble form of NEP. The first two type I mutants, SecNEP-TMC and SecNEP-TMIC, were constructed by fusing in frame the cytosolic and SA domains of NEP to the C-terminus of SecNEP. These two fusion proteins differ only in the orientation of the cytosolic tail. The third type I mutant, SecNEP-ACE, was constructed by fusing in frame the stop-transfer and cytosolic domains of angiotensin I-converting enzyme (EC 3.4.15.1; ACE) to the C-terminus of SecNEP. Our results suggest that: (1) the NEP ectodomain can be anchored with a type I topology in the endoplasmic reticulum (ER) membrane by both NEP and ACE topogenic sequences; (2) SecNEP-TMC and SecNEP-TMIC were transport-incompetent and needed proteolytic cleavage in the C-terminal region to leave the ER, whereas SecNEP-ACE was transported out of the ER as a type I membrane protein. Therefore we concluded that the nature of topogenic sequences determines the transport-competence of topological mutants of neutral endopeptidase-24.11.

The v-sis protein retains biological activity as a type II membrane protein when anchored by various signal-anchor domains, including the hydrophobic domain of the bovine papilloma virus E5 oncoprotein

Membrane-anchored forms of the v-sis oncoprotein have been previously described which are oriented as type I transmembrane proteins and which efficiently induce autocrine transformation. Several examples of naturally occurring membrane-anchored growth factors have been identified, but all exhibit a type I orientation. In this work, we wished to construct and characterize membrane-anchored growth factors with a type II orientation. These experiments were designed to determine whether type II membrane-anchored growth factors would in fact exhibit biological activity. Additionally, we wished to determine whether the hydrophobic domain of the E5 oncoprotein of bovine papilloma virus (BPV) can function as a signal-anchor domain to direct type II membrane insertion. Type II derivatives of the v-sis oncoprotein were constructed, with the NH2 terminus intracellular and the COOH terminus extracellular, by substituting the NH2 terminal signal sequence with the signal-anchor domain of a known type II membrane protein. The signal-anchor domains of neuraminidase (NA), asialoglycoprotein receptor (ASGPR) and transferrin receptor (TR) all yielded biologically active type II derivatives of the v-sis oncoprotein. Although transforming all of the type II signal/anchor-sis proteins exhibited a very short half-life. The short half-life exhibited by the signal/anchor-sis constructs suggests that, in some cases, cellular transformation may result from the synthesis of growth factors so labile that they activate undetectable autocrine loops. The E5 oncoprotein encoded by BPV exhibits amino acid sequence similarity with PDGF, activates the PDGF beta-receptor, and thus resembles a miniature membrane-anchored growth factor with a putative type II orientation. The hydrophobic domain of the E5 oncoprotein, when substituted in place of the signal sequence of v-sis, was indistinguishable compared with the signal-anchor domains of NA, TR, and ASGPR, demonstrating its ability to function as a signal-anchor domain. NIH 3T3 cells transformed by the signal/anchor-sis constructs exhibited morphological reversion upon treatment with suramin, indicating a requirement for ligand/receptor interactions in a suramin-sensitive compartment, most likely the cell surface. In contrast, NIH 3T3 cells transformed by the E5 oncoprotein did not exhibit morphological reversion in response to suramin.

Associations between subunit ectodomains promote T cell antigen receptor assembly and protect against degradation in the ER

The T cell antigen receptor (TCR) is an oligomeric protein complex made from at least six different integral membrane proteins (alpha beta gamma delta epsilon and zeta). The TCR is assembled in the ER of T cells, and correct assembly is required for transport to the cell surface. Single subunits and partial receptor complexes are retained in the ER where TCR alpha, beta, and CD3 delta chains are degraded selectively. The information required for the ER degradation of the TCR beta chain is confined to the membrane anchor of the protein (Wileman et al., 1990c; Bonifacino et al., 1990b). In this study we show that the rapid degradation of the TCR beta chain is inhibited when it assembles with single CD3 gamma, delta, or epsilon subunits in the ER, and have started to define the role played by transmembrane anchors, and receptor ectodomains, in the masking proteolytic targeting information. Acidic residues within the membrane spanning domains of CD3 subunits were essential for binding to the TCR beta chain. TCR beta chains and CD3 subunits therefore interact via transmembrane domains. However, when sites of binding were restricted to the membrane anchor of the TCR beta chain, stabilization by CD3 subunits was markedly reduced. Interactions between membrane spanning domains were not, therefore, sufficient for the protection of the beta chain from ER proteolysis. The presence of the C beta domain, containing the first 150 amino acids of the TCR ectodomain, greatly increased the stability of complexes formed in the ER. For assembly with CD3 epsilon, stability was further enhanced by the V beta amino acids. The results showed that the efficient neutralization of transmembrane proteolytic targeting information required associations between membrane spanning domains and the presence of receptor ectodomains. Interactions between receptor ectodomains may slow the dissociation of CD3 subunits from the beta chain and prolong the masking of transmembrane targeting information. In addition, the close proximity of TCR and CD3 ectodomains within the ER may provide steric protection from the action of proteases within the ER lumen.

von Willebrand disease type B: a missense mutation selectively abolishes ristocetin-induced von Willebrand factor binding to platelet glycoprotein Ib

von Willebrand factor (vWF) is a multimeric glycoprotein that mediates the adhesion of platelets to the subendothelium by binding to platelet glycoprotein Ib. For human vWF, this interaction can be induced in vitro by the antibiotic ristocetin or the snake venom protein botrocetin. A missense mutation, Gly-561-->Ser, was identified within the proposed glycoprotein Ib binding domain of vWF in the proband with von Willebrand disease type B, a unique variant characterized by no ristocetin-induced, but normal botrocetin-induced, binding to glycoprotein Ib. The corresponding mutant recombinant protein, rvWF(G561S), formed normal multimers and exhibited the same functional defect as the patient's plasma vWF, confirming that this mutation causes von Willebrand disease type B. These data show that botrocetin and ristocetin cofactor activities of vWF can be dissociated by a point mutation and confirm that these mediators promote vWF binding to platelets by different mechanisms. The normal botrocetin-induced binding and the defective ristocetin-induced binding of rvWF(G561S) suggest that the primary defect in von Willebrand disease type B may be a failure of normal allosteric regulation of the glycoprotein Ib binding function of vWF.

Fc receptor endocytosis is controlled by a cytoplasmic domain determinant that actively prevents coated pit localization

Macrophages and B-lymphocytes express two major isoforms of Fc receptor (FcRII-B2 and FcRII-B1) that exhibit distinct capacities for endocytosis. This difference in function reflects the presence of an in-frame insertion of 47 amino acids in the cytoplasmic domain of the lymphocyte isoform (FcRII-B1) due to alternative mRNA splicing. By expressing wild type and mutant FcRII cDNAs in fibroblasts, we have now examined the mechanism by which the insertion acts to prevent coated pit localization and endocytosis. We first identified the region of the FcRII-B2 cytoplasmic domain that is required for rapid internalization. Using a biochemical assay for endocytosis and an immuno-EM assay to determine coated pit localization directly, we found that the distal half of the cytoplasmic domain, particularly a region including residues 18-31, as needed for coated pit-mediated endocytosis. Elimination of the tyrosine residues at position 26 and 43, separately or together, had little effect on coated pit localization and a partial effect on endocytosis of ligand. Since the FcRII-B1 insertion occurs in the membrane-proximal region of the cytoplasmic domain (residue 6) not required for internalization, it is unlikely to act by physically disrupting the coated pit localization determinant. In fact, the insertion was found to prevent endocytosis irrespective of its position in the cytoplasmic tail and appeared to selectively exclude the receptor from coated regions. Moreover, receptors bearing the insertion exhibited a temperature- and ligand-dependent association with a detergent-insoluble fraction and with actin filaments, perhaps in part explaining the inability of FcRII-B1 to enter coated pits.

Expression of von Willebrand factor "Normandy": an autosomal mutation that mimics hemophilia A

von Willebrand disease Normandy (vWD Normandy) is a recently described phenotype in which a mutant von Willebrand factor (vWF) appears structurally and functionally normal except that it does not bind to blood coagulation factor VIII. This interaction is required for normal survival of factor VIII in the circulation; consequently, vWD Normandy can present as apparent hemophilia A but with autosomal recessive rather than X chromosome-linked inheritance. A vWF missense mutation, Thr28----Met, was identified in the propositus in or near the factor VIII binding site. The corresponding mutant recombinant vWF(T28M) formed normal multimers and had normal ristocetin cofactor activity. However, vWF(T28M) exhibited the same defect in factor VIII binding as natural vWF Normandy, confirming that this mutation causes the vWD Normandy phenotype. The distinction between hemophilia A and vWD Normandy is clinically important and should be considered in families affected by apparent mild hemophilia A that fail to show strict X chromosome-linked inheritance and, particularly, in potential female carriers with low factor VIII levels attributed to extreme lyonization.

Defective assembly and intracellular transport of mutant paramyxovirus hemagglutinin-neuraminidase proteins containing altered cytoplasmic domains

The hemagglutinin-neuraminidase (HN) integral membrane protein of paramyxoviruses is expressed at the cell surface as a tetramer consisting of a pair of disulfide-linked dimers. HN has a large C-terminal ectodomain, a 19-residue uncleaved signal-anchor domain, and a 17-residue N-terminal cytoplasmic tail. Various mutant HN genes were constructed to examine the role of residues flanking the signal-anchor domain, including the cytoplasmic tail, on assembly and intracellular transport of the HN glycoprotein. Expression of the altered genes showed that by 90 min after synthesis the majority of the mutant HN proteins were in a conformationally mature form as assayed by their reactivity with conformation-specific monoclonal antibodies. However, the mutant proteins showed varied endoplasmic reticulum-to-Golgi apparatus transport rates, ranging from that of wild-type HN (t1/2 approximately 90 min) to slowly transported molecules (t1/2 approximately 5 h) and to molecules in which transport was not detected. Pulse-chase experiments indicated that the altered HN molecules had a specific and transient interaction with the resident endoplasmic reticulum protein GRP78-BiP, and thus the altered HN molecules were not retained in the endoplasmic reticulum by a prolonged interaction with GRP78-BiP. Sucrose density gradient sedimentation analysis of the mutant HN molecules indicated that they all had an oligomeric form that differed from that of wild-type HN; most of the molecules were found as disulfide-linked dimers rather than as tetramers. These data suggest that the HN cytoplasmic tail may function in the assembly of the final transport-competent oligomeric form of HN and that mutant HN molecules with seemingly properly folded ectodomains are retained in the endoplasmic reticulum by an as yet unidentified mechanism. The possible role of the HN cytoplasmic tail as a signal for intracellular transport is discussed.

Conversion of a class II integral membrane protein into a soluble and efficiently secreted protein: multiple intracellular and extracellular oligomeric and conformational forms

The NH2 terminus of the F1 subunit of the paramyxovirus SV5 fusion protein (fusion related external domain; FRED) is a hydrophobic domain that is implicated as being involved in mediating membrane fusion. We have examined the ability of the FRED to function as a combined signal/anchor domain by substituting it for the natural NH2-terminal signal/anchor domain of a model type II integral membrane protein: the hybrid protein (NAF) was expressed in eukaryotic cells. The FRED was shown to act as a signal sequence, targeting NAF to the lumen of the ER, by the fact that NAF acquired N-linked carbohydrate chains. Alkali fractionation of microsomes indicated that NAF is a soluble protein in the lumen of the ER, and the results of NH2-terminal sequence analysis showed that the FRED is cleaved at a site predicted to be recognized by signal peptidase. NAF was found to be efficiently secreted (t1/2 approximately 90 min) from the cell. By using a combination of sedimentation velocity centrifugation and immunoprecipitation assays using polyclonal and conformation-specific monoclonal antibodies it was found that extracellular NAF consisted of a mixture of monomers, disulfide-linked dimers, and tetramers. The majority of the extracellular NAF molecules were not reactive with the conformation-specific monoclonal antibodies, suggesting they were not folded in a native form and that only the NAF tetramers had matured to a native conformation such that they exhibited NA activity. The available data indicate that NAF is transported intracellularly in multiple oligomeric and conformational forms.

Transposition of domains between the M2 and HN viral membrane proteins results in polypeptides which can adopt more than one membrane orientation

The influenza A virus M2 polypeptide is a small integral membrane protein that does not contain a cleaved signal sequence, but is unusual in that it assumes the membrane orientation of a class I integral membrane protein with an NH2-terminal ectodomain and a COOH-terminal cytoplasmic tail. To determine the domains of M2 involved in specifying membrane orientation, hybrid genes were constructed and expressed in which regions of the M2 protein were linked to portions of the paramyxovirus HN and SH proteins, two class II integral membrane proteins that adopt the opposite orientation in membranes from M2. A hybrid protein (MgMH) consisting of the M2 NH2-terminal and membrane-spanning domains linked precisely to the HN COOH-terminal ectodomain was found in cells in two forms: integrated into membranes in the M2 topology or completely translocated across the endoplasmic reticulum membrane and ultimately secreted from the cell. The finding of a soluble form suggested that in this hybrid protein the anchor function of the M2 signal/anchor domain can be overridden. A second hybrid which contained the M2 NH2 terminus linked to the HN signal anchor and ectodomain (MgHH) was found in both the M2 and the HN orientation, suggesting that the M2 NH2 terminus was capable of reversing the topology of a class II membrane protein. The exchange of the M2 signal/anchor domain with that of SH resulted in a hybrid protein which assumed only the M2 topology. Thus, all these data suggest that the NH2-terminal 24 residues to M2 are important for directing the unusual membrane topology of the M2 protein. These data are discussed in relationship to the loop model for insertion of proteins into membranes and the role of charged residues as a factor in determining orientation.

Identification and characterization of a human cytomegalovirus gene coding for a membrane protein that is conserved among human herpesviruses

A rabbit antiserum was raised against envelope material from purified human cytomegalovirus strain AD169. The serum recognized polypeptides 200, 170, 160, 75, 58, and 45 kilodaltons in size. It was used to screen a cDNA library constructed from poly(A)+ RNA from human cytomegalovirus-infected cells in the expression vector lambda gt11. A recombinant bacteriophage expressing cytomegalovirus-specific sequences was identified, and the corresponding gene was mapped to the HindIII R fragment. The gene is transcribed into a late 1.5-kilobase RNA. The nucleotide sequence of the coding region was determined. Computer analysis of the gene product revealed a polypeptide containing multiple potential membrane-spanning domains, representing a type of protein not identified in the envelope of herpesviruses before. The protein shows homology on the amino acid level to hypothetical proteins from reading frames BBRF3 of Epstein-Barr virus, UL10 of herpes simplex virus type 1, and ORF50 of varicella-zoster virus. By using an antiserum raised against procaryote-expressed parts of the cytomegalovirus membrane protein, a 45-kilodalton structural component of the virus was identified as the gene product.

44-amino-acid E5 transforming protein of bovine papillomavirus requires a hydrophobic core and specific carboxyl-terminal amino acids

The 44-amino-acid E5 protein of bovine papillomavirus type 1 is the shortest known protein with transforming activity. To identify the specific amino acids required for in vitro focus formation in mouse C127 cells, we used oligonucleotide-directed saturation mutagenesis to construct an extensive collection of mutants with missense mutations in the E5 gene. Characterization of mutants with amino acid substitutions in the hydrophobic middle third of the E5 protein indicated that efficient transformation requires a stretch of hydrophobic amino acids but not a specific amino acid sequence in this portion of the protein. Many amino acids in the carboxyl-terminal third of the protein can also undergo substitution without impairment of focus-forming activity, but the amino acids at seven positions, including two cysteine residues that mediate dimer formation, appear essential for efficient transforming activity. These essential amino acids are the most well conserved among related fibropapillomaviruses. The small size of the E5 protein, its lack of similarity to other transforming proteins, and its ability to tolerate many amino acid substitutions implies that it transforms cells via a novel mechanism.

44-amino-acid E5 transforming protein of bovine papillomavirus requires a hydrophobic core and specific carboxyl-terminal amino acids

The 44-amino-acid E5 protein of bovine papillomavirus type 1 is the shortest known protein with transforming activity. To identify the specific amino acids required for in vitro focus formation in mouse C127 cells, we used oligonucleotide-directed saturation mutagenesis to construct an extensive collection of mutants with missense mutations in the E5 gene. Characterization of mutants with amino acid substitutions in the hydrophobic middle third of the E5 protein indicated that efficient transformation requires a stretch of hydrophobic amino acids but not a specific amino acid sequence in this portion of the protein. Many amino acids in the carboxyl-terminal third of the protein can also undergo substitution without impairment of focus-forming activity, but the amino acids at seven positions, including two cysteine residues that mediate dimer formation, appear essential for efficient transforming activity. These essential amino acids are the most well conserved among related fibropapillomaviruses. The small size of the E5 protein, its lack of similarity to other transforming proteins, and its ability to tolerate many amino acid substitutions implies that it transforms cells via a novel mechanism.

Evidence for the loop model of signal-sequence insertion into the endoplasmic reticulum

The insertion of proteins into the endoplasmic reticulum is mediated by short hydrophobic domains called signal sequences, which are usually cleaved during insertion. We previously constructed DNAs encoding vesicular stomatitis virus glycoproteins with N-terminal extensions preceding the signal sequence and showed that these extensions allowed normal signal-sequence function and cleavage in vivo. To analyze signal sequence topology during membrane insertion, we generated a point mutation that blocks cleavage of these signal sequences. After expressing these proteins in HeLa cells, we used proteolysis of microsomal membranes to determine that the N terminus of the signal sequence and the C terminus of each protein remain on the cytoplasmic side of the endoplasmic reticulum after insertion. This result indicates that the proteins were inserted in a looped configuration. Extending this finding, we were able to reverse the orientation of such a mutant protein by deleting its normal C-terminal transmembrane and cytoplasmic domains. In addition to demonstrating that a signal sequence can function as a membrane anchor, these findings show that except for the presence of a cleavage site, the cleaved signal sequence of a type I transmembrane protein is structurally and functionally equivalent to the noncleaved signal sequences of type II transmembrane proteins.

Cell surface expression of glycosylated, nonglycosylated, and truncated forms of a cytoplasmic protein pyruvate kinase

The soluble cytoplasmic protein pyruvate kinase (PK) has been expressed at the cell surface in a membrane-anchored form (APK). The hybrid protein contains the NH2-terminal signal/anchor domain of a class II integral membrane protein (hemagglutinin/neuraminidase, of the paramyxovirus SV5) fused to the PK NH2 terminus. APK contains a cryptic site that is used for N-linked glycosylation but elimination of this site by site-specific mutagenesis does not prevent cell surface localization. Truncated forms of the APK molecule, with up to 80% of the PK region of APK removed, can also be expressed at the cell surface. These data suggest that neither the complete PK molecule nor its glycosylation are necessary for intracellular transport of PK to the cell surface, and it is possible that specific signals may not be needed in the ectodomain of this hybrid protein to specify cell surface localization.

Differential effects of mutations in three domains on folding, quaternary structure, and intracellular transport of vesicular stomatitis virus G protein

The vesicular stomatitis virus glycoprotein (G protein) is an integral membrane protein which assembles into noncovalently associated trimers before transport from the endoplasmic reticulum. In this study we have examined the folding and oligomeric assembly of twelve mutant G proteins with alterations in the cytoplasmic, transmembrane, or ectodomains. Through the use of conformation-specific antibodies, we found that newly synthesized G protein folded into a conformation similar to the mature form within 1-3 min of synthesis and before trimer formation. Mutant proteins not capable of undergoing correct initial folding did not trimerize, were not transported, and were found in large aggregates. They had, as a rule, mutations in the ectodomain, including several with altered glycosylation patterns. In contrast, mutations in the cytoplasmic domain generally had little effect on folding and trimerization. These mutant proteins, whose ectodomains were identical to the wild-type by several assays, were either transported to the cell surface slowly or not at all. We concluded that while correct ectodomain folding and trimer formation are prerequisites for transport, they alone are not sufficient. The results suggest that the cytoplasmic domain of the wild-type protein may facilitate rapid, efficient transport from the ER, which can be easily affected or eliminated by tail mutations that do not detectably affect the ectodomain.

Cell surface expression and orientation in membranes of the 44-amino-acid SH protein of simian virus 5

Antiserum was raised against a synthetic peptide containing the N-terminal hydrophilic domain of the small hydrophobic protein (SH) of simian virus 5 (SV5) and used to characterize properties of the SH protein. SH demonstrated properties of an integral membrane protein. Indirect immunofluorescence experiments showed that the protein is involved in the exocytotic pathway, and isolation of plasma membranes from SV5-infected cells showed an enrichment of SH, indicating that SH is transported to the infected-cell surface. Biochemical analysis of the orientation of SH in membranes by proteolysis of intact SV5-infected cell surfaces and intracellular microsomal vesicles indicated that SH is oriented in membranes with its N-terminal hydrophilic domain exposed on the cytoplasmic face of the plasma membrane and the C terminus of approximately five amino acid residues exposed at the cell surface. These data are discussed with respect to positive-acting signals being necessary in the ectodomain of SH for cell surface expression.

Signal and membrane anchor functions overlap in the type II membrane protein I gamma CAT

I gamma CAT is a hybrid protein that inserts into the membrane of the endoplasmic reticulum as a type II membrane protein. These proteins span the membrane once and expose the NH2-terminal end on the cytoplasmic side and the COOH terminus on the exoplasmic side. I gamma CAT has a single hydrophobic segment of 30 amino acid residues that functions as a signal for membrane insertion and anchoring. The signal-anchor region in I gamma CAT was analyzed by deletion mutagenesis from its COOH-terminal end (delta C mutants). The results show that the 13 amino acid residues on the amino-terminal side of the hydrophobic segment are not sufficient for membrane insertion and translocation. Mutant proteins with at least 16 of the hydrophobic residues are inserted into the membrane, glycosylated, and partially proteolytically processed by a microsomal protease (signal peptidase). The degree of processing varies between different delta C mutants. Mutant proteins retaining 20 or more of the hydrophobic amino acid residues can span the membrane like the parent I gamma CAT protein and are not proteolytically processed. Our data suggest that in the type II membrane protein I gamma CAT, the signals for membrane insertion and anchoring are overlapping and that hydrophilic amino acid residues at the COOH-terminal end of the hydrophobic segment can influence cleavage by signal peptidase. From this and previous work, we conclude that the function of the signal-anchor sequence in I gamma CAT is determined by three segments: a positively charged NH2 terminus, a hydrophobic core of at least 16 amino acid residues, and the COOH-terminal flanking hydrophilic segment.

A charged amino acid substitution within the transmembrane anchor of the Rous sarcoma virus envelope glycoprotein affects surface expression but not intracellular transport

Two point mutations were introduced by oligonucleotide-directed mutagenesis into the region of the Rous sarcoma virus envelope gene that encodes the hydrophobic transmembrane anchor of the receptor glycoprotein. Single-nucleotide substitutions ultimately converted a hydrophobic leucine, located centrally within the membrane-spanning domain, to either a similarly hydrophobic methionine or a positively charged arginine. The altered coding region was reinserted into an intact copy of the envelope gene, cloned into simian virus 40 late-replacement vector and expressed in primate cells. Analysis of envelope gene expression in CV-1 monkey cells revealed normal levels of synthesis of a membrane-spanning precursor for both the mutants; however, the arginine-containing mutant [mu 26(arg)] exhibited greatly reduced cell surface expression of mature protein, as determined by indirect immunofluorescence and 125I labeling of surface proteins. In experiments in which cells producing the mu 26(arg) polypeptide were pulsed with radioactive leucine and then chased for 5 h, no intracellular accumulation or extracellular secretion of mature products (gp85 and gp37) could be detected. Treatment of mu 26(arg)-infected cells with lysosomal enzyme inhibitors (chloroquine and leupeptin) resulted in the accumulation of gp85 and gp37, indicating that they were being degraded rapidly in lysosomes. The fact that terminally glycosylated and proteolytically cleaved env gene products were observed under these conditions showed that modifications associated with passage through the trans compartment of the Golgi apparatus occurred normally on the mutant polypeptide; thus insertion of a highly charged amino acid into the transmembrane hydrophobic region of gp37 results in the postGolgi transport to lysosomes. It is proposed that the insertion of this mutation into the transmembrane anchor of the envelope glycoprotein does not affect membrane association, orientation with respect to the membrane, or intracellular transport at early stages during maturation. At a step late in the transport pathway, however, the presence of the charged side chain alters the protein in such a manner that the molecules are transported to the lysosomes and degraded. It seems likely that transport of the protein from the trans-Golgi to the cell surface is either directly blocked, or that after expression on the cell surface the mature glycoprotein complex is unstable and rapidly endocytosed.

Effects of mutations in three domains of the vesicular stomatitis viral glycoprotein on its lateral diffusion in the plasma membrane

The lateral mobility of the vesicular stomatitis virus spike glycoprotein (G protein) and various mutant G proteins produced by site-directed mutagenesis of the G cDNA has been measured. Fluorescence recovery after photobleaching results for the wild type G protein in transfected COS-1 cells yielded a mean diffusion coefficient (D) of 8.5 (+/- 1.3) X 10(-11) cm2/s and a mean mobile fraction of 75% (+/- 3%). Eight mutant proteins were also examined: dTM14, lacking six amino acids from the transmembrane domain; TA2, lacking an oligosaccharide in the extracellular domain; QN2, possessing an extra N-linked oligosaccharide in the extracellular domain; CS2, possessing a serine instead of a cysteine at residue 489 in the cytoplasmic domain, preventing palmitate addition to the glycoprotein; TMR-stop, lacking the entire cytoplasmic domain except an arginine at residue 483; and three chimeric proteins, G mu, G23, and GHA, containing in place of the 29 amino acid wild type cytoplasmic domain the cytoplasmic domains from the surface IgM from the spike protein of the infectious bronchitis virus or from the hemagglutinin protein of the influenza virus, respectively. The mean D for the mutant proteins varied over a relatively small range, with the slowest mutant, G23, exhibiting a value of 11.3 (+/- 1.4) X 10(-11) cm2/s and the fastest mutant, GHA, having a D of 28.6 (+/- 4.5) X 10(-11) cm2/s. The mean mobile fraction similarly varied over a small range, extending from 55 to 68%. None of the mutations resulted in the more rapid diffusion characteristic of membrane proteins embedded in artificial bilayers. Therefore, it appears that the cytoplasmic and transmembrane domains themselves contribute little to restraining the lateral mobility of this integral membrane protein when expressed in transfected cells.

Nonpolarized secretion of truncated forms of the influenza hemagglutinin and the vesicular stomatitus virus G protein from MDCK cells

The demonstration that the envelope glycoproteins G of vesicular stomatitus virus and hemagglutinin of influenza virus synthesized in polarized epithelial cells transfected with the corresponding genes are effectively segregated to the basolateral or apical plasma membrane domains, respectively, implies that the information determining this segregation resides within the structures of the proteins themselves. To localize the sorting information within these proteins, the polarity of secretion of truncated hemagglutinin and G glycoproteins secreted from confluent monolayers of MDCK cells transformed with vectors containing the corresponding truncated cDNAs was examined. It was found that, even though the transformed cells continued to secrete a major endogenous glycoprotein exclusively from the apical surface, the modified viral glycoproteins were secreted in a nonpolarized fashion from both sides of the monolayers. These observations suggest that important information for the sorting of the viral glycoprotein is contained within their membrane anchoring or cytoplasmic segments or that, if sorting signals are luminally located, these signals must be present in a conformation that is not attainable when the polypeptides are not attached to the membrane.

Variants of vaccinia virus hemagglutinin altered in intracellular transport

Two classes of revertants were isolated from a vaccinia virus mutant whose hemagglutinins (HAs) accumulate on nuclear envelopes and rough endoplasmic reticulums. The HAs of one of the revertants had the same phenotype as the wild type, i.e., rapid and efficient movement to the cell surface. The HAs of the second class had biphasic transport: rapid export to the cell surface as in the wild type and slow movement to the medial cisternae of the Golgi apparatus. Biochemical and nucleotide sequence analyses showed that the HAs of all the mutants examined that have defects in transport from the rough endoplasmic reticulum to the Golgi apparatus have altered cytoplasmic domains and that the HAs of the second class of revertants lack the whole cytoplasmic domain, while the HAs of the first class of revertants have a wild-type cytoplasmic domain.

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.

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.

Cell surface expression of membrane-anchored v-sis gene products: glycosylation is not required for cell surface transport

The v-sis gene is able to transform cells by production of a growth factor that is structurally related to platelet-derived growth factor. This growth factor has been detected in the conditioned media of v-sis transformed cells, and is able to stimulate the autophosphorylation of the platelet-derived growth factor receptor. We have used the v-sis gene product to analyze the role of protein-encoded signals in cell surface transport. We constructed several gene fusions that encode transmembrane forms of the v-sis gene product. These membrane-anchored forms of the v-sis gene product are properly folded into a native structure, as indicated by their dimerization, glycosylation, and NH2-terminal proteolytic processing. Indirect immunofluorescence demonstrated that several of these membrane-anchored gene products are transported to the cell surface. Removal of the N-linked glycosylation site from the v-sis gene product did not prevent cell surface transport. Several of these mutant genes are able to induce focus formation in NIH3T3 cells, providing further evidence that the membrane-anchored proteins are properly folded. These results demonstrate that N-linked glycosylation is not required for the cell surface transport of a protein that is in a native, biologically active conformation. These results provide a correlation between cell surface expression of the membrane-anchored v-sis gene products and transformation.

Variants of vaccinia virus hemagglutinin altered in intracellular transport

Two classes of revertants were isolated from a vaccinia virus mutant whose hemagglutinins (HAs) accumulate on nuclear envelopes and rough endoplasmic reticulums. The HAs of one of the revertants had the same phenotype as the wild type, i.e., rapid and efficient movement to the cell surface. The HAs of the second class had biphasic transport: rapid export to the cell surface as in the wild type and slow movement to the medial cisternae of the Golgi apparatus. Biochemical and nucleotide sequence analyses showed that the HAs of all the mutants examined that have defects in transport from the rough endoplasmic reticulum to the Golgi apparatus have altered cytoplasmic domains and that the HAs of the second class of revertants lack the whole cytoplasmic domain, while the HAs of the first class of revertants have a wild-type cytoplasmic domain.

Analysis of progressive deletions of the transmembrane and cytoplasmic domains of influenza hemagglutinin

Site-directed oligonucleotide mutagenesis has been used to introduce chain termination codons into the cloned DNA sequences encoding the carboxy-terminal transmembrane (27 amino acids) and cytoplasmic (10 amino acids) domains of influenza virus hemagglutinin (HA). Four mutant genes were constructed which express truncated forms of HA that lack the cytoplasmic domain and terminate at amino acids 9, 14, 17, or 27 of the wild-type hydrophobic domain. Analysis of the biosynthesis and intracellular transport of these mutants shows that the cytoplasmic tail is not needed for the efficient transport of HA to the cell surface; the stop-transfer sequences are located in the hydrophobic domain; 17 hydrophobic amino acids are sufficient to anchor HA stably in the membrane; and mutant proteins with truncated hydrophobic domains show drastic alterations in transport, membrane association, and stability.

Cytoplasmic domains of cellular and viral integral membrane proteins substitute for the cytoplasmic domain of the vesicular stomatitis virus glycoprotein in transport to the plasma membrane

Oligonucleotide-directed mutagenesis was used to construct chimeric cDNAs that encode the extracellular and transmembrane domains of the vesicular stomatitis virus glycoprotein (G) linked to the cytoplasmic domain of either the immunoglobulin mu membrane heavy chain, the hemagglutinin glycoprotein of influenza virus, or the small glycoprotein (p23) of infectious bronchitis virus. Biochemical analyses and immunofluorescence microscopy demonstrated that these hybrid genes were correctly expressed in eukaryotic cells and that the hybrid proteins were transported to the plasma membrane. The rate of transport to the Golgi complex of G protein with an immunoglobulin mu membrane cytoplasmic domain was approximately sixfold slower than G protein with its normal cytoplasmic domain. However, this rate was virtually identical to the rate of transport of micron heavy chain molecules measured in the B cell line WEHI 231. The rate of transport of G protein with a hemagglutinin cytoplasmic domain was threefold slower than wild type G protein and G protein with a p23 cytoplasmic domain, which were transported at similar rates. The combined results underscore the importance of the amino acid sequence in the cytoplasmic domain for efficient transport of G protein to the cell surface. Also, normal cytoplasmic domains from other transmembrane glycoproteins can substitute for the G protein cytoplasmic domain in transport of G protein to the plasma membrane. The method of constructing precise hybrid proteins described here will be useful in defining functions of specific domains of viral and cellular integral membrane proteins.

Mutants of the membrane-binding region of Semliki Forest virus E2 protein. II. Topology and membrane binding

The p62/E2 protein of Semliki Forest virus (SFV) is a typical transmembrane glycoprotein, with an amino-terminal lumenal domain, a transmembrane (hydrophobic) domain, and a carboxy-terminal cytoplasmic domain (or tail). Our hypothesis has been that the membrane-binding polypeptide region (membrane anchor) of this protein consists of both the transmembrane domain and the adjacent positively charged peptide, Arg-Ser-Lys, which is part of the cytoplasmic domain. We have investigated three anchor mutants of the p62 protein with respect to both their disposition and their stability in cell membranes. The construction of the three mutants has been described (Cutler, D.F., and H. Garoff, J. Cell Biol., 102:889-901). They are as follows: A1, changing the basic charge cluster from Arg-Ser-Lys(+2) to Gly-Ser-Glu(-1); A2, replacing an Ala in the middle of the hydrophobic stretch with a Glu; A3, changing the charge cluster from Arg-Ser-Lys(+2) to Gly-Ser-Met(0). All three mutants retain the transmembrane configuration of the wild-type p62. In a cell homogenate they have a cytoplasmic domain that is accessible to protease. In living cells an anti-peptide antibody specific for the cytoplasmic tail of p62 reacts with the tails of both wild-type and mutant p62s following its introduction into the cytoplasm. All three mutant proteins have Triton X-114 binding properties similar to the wild-type p62. However, when the membranes of cells expressing the three mutants or the wild-type p62 protein are washed with sodium carbonate, pH 11.5, three to four times as much mutant protein as wild-type p62 is released from the membranes. Thus the stability in cell membranes of the three mutant p62 proteins is significantly reduced.

Mutants of the membrane-binding region of Semliki Forest virus E2 protein. I. Cell surface transport and fusogenic activity

Three mutations of the membrane-binding region of the Semliki Forest virus (SFV) p62 polypeptide (the precursor for virion E3 and E2) have been made by oligonucleotide-directed mutagenesis of a cDNA clone encoding the SFV structural proteins. One of the mutations (A2) substitutes a Glu for an Ala in the middle of the hydrophobic stretch which spans the bilayer. A1 and A3 alter the two basic charged amino acids in the cytoplasmic domain next to the hydrophobic region. The wild-type charge cluster of Arg-Ser-Lys (+2) has been changed to Gly-Ser-Met (0;A3) or to Gly-Ser-Glu (-1;A1). The mutant p62 proteins have been analyzed both in the presence and the absence of E1, the other half of the heterodimer spike complex of SFV. The mutant proteins expressed in COS-7 cells are glycosylated and are of the expected sizes. When co-expressed with E1, all three mutants are cleaved to yield the E2 protein and transported to the surface of COS-7 cells. When expressed in the absence of E1, the mutant p62 proteins remain uncleaved but still reach the cell surface. Once at the cell surface, all three mutants, when co-expressed with E1, can promote low pH-triggered cell-cell fusion. These results show that the three mutant p62/E2 proteins are still membrane associated in a functionally unaltered way.