A glutamine residue in the membrane-associating domain of the bovine papillomavirus type 1 E5 oncoprotein mediates its binding to a transmembrane component of the vacuolar H(+)-ATPase

New approach for genomic characterisation of equine sarcoid-derived BPV-1/-2 using nanopore-based sequencing

BACKGROUND

Bovine papillomavirus (BPV) types 1 and 2 play a central role in the etiology of the most common neoplasm in horses, the equine sarcoid. The unknown mechanism behind the unique variety in clinical presentation on the one hand and the host dependent clinical outcome of BPV-1 infection on the other hand indicate the involvement of additional factors. Earlier studies have reported the potential functional significance of intratypic sequence variants, along with the existence of sarcoid-sourced BPV variants. Therefore, intratypic sequence variation seems to be an important emerging viral factor. This study aimed to give a broad insight in sarcoid-sourced BPV variation and explore its potential association with disease presentation.

METHODS

In order to do this, a nanopore sequencing approach was successfully optimized for screening a wide spectrum of clinical samples. Specimens of each tumour were initially screened for BPV-1/-2 by quantitative real-time PCR. A custom-designed primer set was used on BPV-positive samples to amplify the complete viral genome in two multiplex PCR reactions, resulting in a set of overlapping amplicons. For phylogenetic analysis, separate alignments were made of all available complete genome sequences for BPV-1/-2. The resulting alignments were used to infer Bayesian phylogenetic trees.

RESULTS

We found substantial genetic variation among sarcoid-derived BPV-1, although this variation could not be linked to disease severity. Several of the BPV-1 genomes had multiple major deletions. Remarkably, the majority of them cluster within the region coding for late viral genes. Together with the extensiveness (up to 603 nucleotides) of the described deletions, this suggests an altered function of L1/L2 in disease pathogenesis.

CONCLUSIONS

By generating a significant amount of complete-length BPV genomes, we succeeded to introduce next-generation sequencing into veterinary research focusing on the equine sarcoid, thus facilitating the first report of both nanopore-based sequencing of complete sarcoid-sourced BPV-1/-2 and the simultaneous nanopore sequencing of multiple complete genomes originating from a single clinical sample.

The viral protein U (Vpu)-interacting host protein ATP6V0C down-regulates cell-surface expression of tetherin and thereby contributes to HIV-1 release

Host proteins with antiviral activity have evolved as first-line defenses to suppress viral replication. The HIV-1 accessory protein viral protein U (Vpu) enhances release of the virus from host cells by down-regulating the cell-surface expression of the host restriction factor tetherin. However, the exact mechanism of Vpu-mediated suppression of antiviral host responses is unclear. To further understand the role of host proteins in Vpu's function, here we carried out yeast two-hybrid screening and identified the V0 subunit C of vacuolar ATPase (ATP6V0C) as a Vpu-binding protein. To examine the role of ATP6V0C in Vpu-mediated tetherin degradation and HIV-1 release, we knocked down ATP6V0C expression in HeLa cells and observed that ATP6V0C depletion impairs Vpu-mediated tetherin degradation, resulting in defective HIV-1 release. We also observed that ATP6V0C overexpression stabilizes tetherin expression. This stabilization effect was specific to ATP6V0C, as overexpression of another subunit of the vacuolar ATPase, ATP6V0C″, had no effect on tetherin expression. ATP6V0C overexpression did not stabilize CD4, another target of Vpu-mediated degradation. Immunofluorescence localization experiments revealed that the ATP6V0C-stabilized tetherin is sequestered in a CD63- and lysosome-associated membrane protein 1 (LAMP1)-positive intracellular compartment. These results indicate that the Vpu-interacting protein ATP6V0C plays a role in down-regulating cell-surface expression of tetherin and thereby contributes to HIV-1 assembly and release.

Hydrophobic Mismatch Drives the Interaction of E5 with the Transmembrane Segment of PDGF Receptor

The oncogenic E5 protein from bovine papillomavirus is a short (44 amino acids long) integral membrane protein that forms homodimers. It activates platelet-derived growth factor receptor (PDGFR) β in a ligand-independent manner by transmembrane helix-helix interactions. The nature of this recognition event remains elusive, as numerous mutations are tolerated in the E5 transmembrane segment, with the exception of one hydrogen-bonding residue. Here, we examined the conformation, stability, and alignment of the E5 protein in fluid lipid membranes of substantially varying bilayer thickness, in both the absence and presence of the PDGFR transmembrane segment. Quantitative synchrotron radiation circular dichroism analysis revealed a very long transmembrane helix for E5 of ∼26 amino acids. Oriented circular dichroism and solid-state ¹⁵N-NMR showed that the alignment and stability of this unusually long segment depend critically on the membrane thickness. When reconstituted alone in exceptionally thick DNPC lipid bilayers, the E5 helix was found to be inserted almost upright. In moderately thick bilayers (DErPC and DEiPC), it started to tilt and became slightly deformed, and finally it became aggregated in conventional DOPC, POPC, and DMPC membranes due to hydrophobic mismatch. On the other hand, when E5 was co-reconstituted with the transmembrane segment of PDGFR, it was able to tolerate even the most pronounced mismatch and was stabilized by binding to the receptor, which has the same hydrophobic length. As E5 is known to activate PDGFR within the thin membranes of the Golgi compartment, we suggest that the intrinsic hydrophobic mismatch of these two interaction partners drives them together. They seem to recognize each other by forming a closely packed bundle of mutually aligned transmembrane helices, which is further stabilized by a specific pair of hydrogen-bonding residues.

Viral channel proteins in intracellular protein-protein communication: Vpu of HIV-1, E5 of HPV16 and p7 of HCV

Viral channel forming proteins are known for their capability to make the lipid membrane of the host cell and its subcellular compartments permeable to ions and small compounds. There is increasing evidence that some of the representatives of this class of proteins are also strongly interacting with host proteins and the effectiveness of this interaction seems to be high. Interaction of viral channel proteins with host factors has been proposed by bioinformatics approaches and has also been identified experimentally. An overview of the interactions with host proteins is given for Vpu from HIV-1, E5 from HPV-16 and p7 from HCV. This article is part of a Special Issue entitled: Viral Membrane Proteins - Channels for Cellular Networking.

Compensatory mutants of the bovine papillomavirus E5 protein and the platelet-derived growth factor β receptor reveal a complex direct transmembrane interaction

The 44-amino-acid E5 protein of bovine papillomavirus is a dimeric transmembrane protein that exists in a stable complex with the platelet-derived growth factor (PDGF) β receptor, causing receptor activation and cell transformation. The transmembrane domain of the PDGF β receptor is required for complex formation, but it is not known if the two proteins contact one another directly. Here, we studied a PDGF β receptor mutant containing a leucine-to-isoleucine substitution in its transmembrane domain, which prevents complex formation with the wild-type E5 protein in mouse BaF3 cells and inhibits receptor activation by the E5 protein. We selected E5 mutants containing either a small deletion or multiple substitution mutations that restored binding to the mutant PDGF β receptor, resulting in receptor activation and growth factor independence. These E5 mutants displayed lower activity with PDGF β receptor mutants containing other transmembrane substitutions in the vicinity of the original mutation, and one of them cooperated with a receptor mutant containing a distal mutation in the juxtamembrane domain. These results provide strong genetic evidence that the transmembrane domains of the E5 protein and the PDGF β receptor contact one another directly. They also demonstrate that different mutations in the E5 protein allow it to tolerate the same mutation in the PDGF β receptor transmembrane domain and that a mutation in the E5 protein can allow it to tolerate different mutations in the PDGF β receptor. Thus, the rules governing direct interactions between transmembrane helices are complex and not restricted to local interactions.

Viral transformation of epithelial cells

Approximately 18% of human cancers have a viral etiology and the majority of these involve transformation of epithelial cells. Viral proteins transform epithelia by inducing alterations in the normal cell growth and differentiation pathways through the targeting of host proteins. Among the DNA viruses responsible for causing carcinomas are the human papillomaviruses as well as several members of the herpes and polyomavirus families. A number of techniques have been developed to study the mechanisms by which viruses immortalize epithelial cells and alter differentiation properties. These methods include the generation of immortalized lines by transfection or infection as well as the use of organotypic raft cultures, suspension in methylcellulose, and treatment with high calcium levels to examine how differentiation is altered.

Structural role of the conserved cysteines in the dimerization of the viral transmembrane oncoprotein E5

The E5 oncoprotein is the major transforming protein of bovine papillomavirus type 1. This 44-residue transmembrane protein can interact with the platelet-derived growth factor receptor β, leading to ligand-independent activation and cell transformation. For productive interaction, E5 needs to dimerize via a C-terminal pair of cysteines, though a recent study suggested that its truncated transmembrane segment can dimerize on its own. To analyze the structure of the full protein in a membrane environment and elucidate the role of the Cys-Ser-Cys motif, we produced recombinantly the wild-type protein and four cysteine mutants. Comparison by circular dichroism in detergent micelles and lipid vesicular dispersion and by NMR in trifluoroethanol demonstrates that the absence of one or both cysteines does not influence the highly α-helical secondary structure, nor does it impair the ability of E5 to dimerize, observations that are further supported by sodium dodecylsulfate polyacrylamide gel electrophoresis. We also observed assemblies of higher order. Oriented circular dichroism in lipid bilayers shows that E5 is aligned as a transmembrane helix with a slight tilt angle, and that this membrane alignment is also independent of any cysteines. We conclude that the Cys-containing motif represents a disordered region of the protein that serves as an extra covalent connection for stabilization.

Bap31 is a novel target of the human papillomavirus E5 protein

The E5 proteins of human papillomaviruses (HPVs) are small hydrophobic proteins that are expressed in the early and late stages of the viral life cycle; however, their role in HPV pathogenesis is not clearly understood. In this study, a split-ubiquitin yeast (Saccharomyces cerevisiae) two-hybrid system was used to identify B-cell-associated protein 31 (Bap31) as a binding partner of HPV E5 proteins. The association of these proteins was confirmed by coimmunoprecipitation of complexes of Bap31 with either HPV type 16 (HPV16) or HPV31 E5. In addition, Bap31 and E5 were found to colocalize in perinuclear patterns consistent with localization to the endoplasmic reticulum. Mutational analysis of E5 identified amino acids in the extreme C terminus as important for stabilizing the interaction with Bap31. Deletion of these C-terminal amino acids of E5 in the context of complete HPV31 genomes resulted in impaired proliferative capacity of HPV-positive keratinocytes following differentiation. When small interfering RNAs were used to reduce the levels of Bap31, the proliferative ability of HPV-positive keratinocytes upon differentiation was also reduced, implicating Bap31 as a regulator of this process. These studies identify a novel binding partner of the high-risk HPV E5 proteins and provide insight into how the E5 proteins may modulate the life cycle in differentiating cells.

E5 protein of human papillomavirus 16 downregulates HLA class I and interacts with the heavy chain via its first hydrophobic domain

Human papillomavirus type 16 E5 protein (HPV-16 E5) is expressed early in papillomavirus infection and is localised primarily in the cell Golgi apparatus (GA) and endoplasmic reticulum. E5 prevents transport of the major histocompatibility class I (MHC I; HLA class I in humans) to the cell surface and retains the complex in the GA. We report that these effects are due, at least in part, to the interaction between E5 and HLA I heavy chain (HC). We also demonstrate that the down-regulation of surface HLA I and interaction with HC are mediated by the first hydrophobic domain of E5. Although E5 downregulates classical HLA selectively as it does not downregulate non-classical HLA, the interaction with the HC of classical HLA I is not specific for a particular haplotype of HLA I. This suggests that E5 can interfere with antigen presentation by most, if not all, classical HLA I haplotypes, with potentially serious consequences as the ability of infected cells to present antigenic peptides to effector T cells would be compromised.

The E5 protein of BPV-4 interacts with the heavy chain of MHC class I and irreversibly retains the MHC complex in the Golgi apparatus

BPV-4 E5 inhibits transcription of the bovine MHC class I heavy chain (HC) gene, increases degradation of HC and downregulates surface expression of MHC class I by retaining the complex in the Golgi apparatus (GA). Here we report that transcription inhibition can be alleviated by interferon treatment and the degradation of HC can be reversed by treatment with inhibitors of proteasomes and lysosomes. However, the inhibition of transport of MHC class I to the cell surface is irreversible. We show that E5 is capable of physically interacting with HC. Together with the inhibition of the vacuolar ATPase (due to the interaction between E5 and 16k subunit c), the interaction between E5 and HC is likely to be responsible for retention of MHC class I in the GA. C-terminus deletion mutants of E5 are incapable of either downregulating surface MHC class I or interacting with HC, establishing that the C-terminus domain of E5 is important in the inhibition of MHC class I.

The role of pH dynamics and the Na+/H+ antiporter in the etiopathogenesis and treatment of cancer. Two faces of the same coin--one single nature

Looked at from the genetic point-of-view cancer represents a daunting and, frankly, confusing multiplicity of diseases (at least 100) that require an equally large variety of therapeutic strategies and substances designed to treat the particular tumor. However, when analyzed phenotypically cancer is a relatively uniform disease of very conserved 'hallmark' behaviors across the entire spectrum of tissue and genetic differences [D. Hanahan, R.A. Weinberg, Hallmarks of cancer, Cell 100 (2000) 57-70]. This suggests that cancers do, indeed, share common biochemical and physiological characteristics that are independent of the varied genetic backgrounds, and that there may be a common mechanism underlying both the neoplastic transformation/progression side and the antineoplastic/therapy side of oncology. The challenge of modern oncology is to integrate all the diverse experimental data to create a physiological/metabolic/energetic paradigm that can unite our thinking in order to understand how both neoplastic progression and therapies function. This reductionist view gives the hope that, as in chemistry and physics, it will possible to identify common underlying driving forces that define a tumor and will permit, for the first time, the actual calculated manipulation of their state. That is, a rational therapeutic design. In the present review, we present evidence, obtained from a great number of studies, for a fundamental, underlying mechanism involved in the initiation and evolution of the neoplastic process. There is an ever growing body of evidence that all the important neoplastic phenotypes are driven by an alkalization of the transformed cell, a process which seems specific for transformed cells since the same alkalinization has no effect in cells that have not been transformed. Seen in that light, different fields of cancer research, from etiopathogenesis, cancer cell metabolism and neovascularization, to multiple drug resistance (MDR), selective apoptosis, modern cancer chemotherapy and the spontaneous regression of cancer (SRC) all appear to have in common a pivotal characteristic, the aberrant regulation of hydrogen ion dynamics [S. Harguindey, J.L. Pedraz, R. García Cañero, J. Pérez de Diego, E.J. Cragoe Jr., Hydrogen ion-dependent oncogenesis and parallel new avenues to cancer prevention and treatment using a H+-mediated unifying approach: pH-related and pH-unrelated mechanisms, Crit. Rev. Oncog. 6 (1) (1995) 1-33]. Cancer cells have an acid-base disturbance that is completely different than observed in normal tissues and that increases in correspondence with increasing neoplastic state: an interstitial acid microenvironment linked to an intracellular alkalosis.

Endoplasmic reticulum-localized human papillomavirus type 16 E5 protein alters endosomal pH but not trans-Golgi pH

The human papillomavirus type 16 (HPV-16) E5 protein is a small, hydrophobic polypeptide that is expressed in virus-infected keratinocytes and alters receptor signaling pathways, apoptotic responses, and endosomal pH. Despite its ability to inhibit endosomal acidification, the HPV-16 E5 protein is found predominantly in the endoplasmic reticulum (ER), suggesting that its effect may be indirect and perhaps global. To determine whether E5 alters the pHs of additional intracellular compartments, we transduced human keratinocytes with a codon-optimized E5 vector and then quantified endosomal and trans-Golgi pHs using sensitive, compartment-specific, ratiometric pHluorin constructs. E5 protein increased endosomal pH from 5.9 to 6.9 but did not affect the normal trans-Golgi pH of 6.3. Confirming the lack of alteration in trans-Golgi pH, we observed no alterations in the acidification-dependent processing of the proH3 protein. C-terminal deletions of E5, which retained normal expression and localization in the ER, were defective for endosomal alkalization. Thus, E5 does not uniformly alkalinize intracellular compartments, and its C-terminal 10 amino acids appear to mediate interactions with critical ER targets that modulate proton pump function and/or localization.

Sequence variants of bovine papillomavirus E5 detected in equine sarcoids

The equine sarcoid, one of the most common dermatological lesions in equids, is a benign, locally invasive dermal fibroblastic lesion. Previous studies have suggested an association with two bovine papilloma virus (BPV) types, BPV-1 and BPV-2. In the present study, we examined sarcoids from horses from two geographical areas, Switzerland and the UK, for the major transforming gene of BPV, E5. We detected BPV DNA for the E5 open reading frame and viral E5 RNA transcripts in most sarcoids. Sequence analysis of the E5 open reading frame of sarcoid-associated BPV detected several unique DNA sequence variants, three of which resulted in sarcoid specific amino acid sequence variations. It is unclear if these sequence variants contribute to the unique clinical presentation of the sarcoid. However, our work provides further evidence of the association between BPV and sarcoid development and the direct involvement of the virus in the pathogenesis of sarcoids.

Modelling of the human papillomavirus type 16 E5 protein

The product of the E5 oncogene in human papillomaviruses (HPVs) participates in cellular transformation. The sequences of E5 from high-risk HPV types are closely related, and the ability to transform is thought to be associated with their structure. Structural determination by standard biophysical methods has proved impossible due to the extreme hydrophobicity of the gene product. We have achieved limited solubility by dividing the sequence into three, structurally distinct domains. Synthetic peptides corresponding to these domains have been examined using circular dichroism (CD) spectroscopy, a method that can detect secondary structure elements in highly dilute protein solutions. Using data on the secondary structure content of these domains under different conditions and in systematic combination to detect constructive domain interactions, a model of HPV E5 structure and position in the membrane is proposed that is consistent with what is known of the larger family of leucine-rich repeat (LRR) proteins to which it belongs.

Vacuolar H(+)-ATPase: functional mechanisms and potential as a target for cancer chemotherapy

Tumor cells in vivo often exist in a hypoxic microenvironment with a lower extracellular pH than that surrounding normal cells. Ability to upregulate proton extrusion may be important for tumor cell survival. Such microenvironmental factors may be involved in the development of resistant subpopulations of tumor cells. In solid tumors, both intracellular and extracellular pH differ between drug-sensitive and -resistant cells, and pH appears critical to the therapeutic effectiveness of anticancer agents. Four major types of pH regulators have been identified in tumor cells: the sodium-proton antiporter, the bicarbonate transporter, the proton-lactate symporter and proton pumps. Understanding mechanisms regulating tumor acidity opens up novel opportunities for cancer chemotherapy. In this minireview, we describe the structure and function of certain proton pumps overexpressed in many tumors--vacuolar H(+)-ATPases--and consider their potential as targets for cancer chemotherapy.

Mechanisms of cell transformation by papillomavirus E5 proteins

The papillomavirus E5 proteins are short, hydrophobic transforming proteins. The transmembrane E5 protein encoded by bovine papillomavirus transforms cells by activating the platelet-derived growth factor beta receptor tyrosine kinase in a ligand-independent fashion. The bovine papillomavirus E5 protein forms a stable complex with the receptor, thereby inducing receptor dimerization and activation, trans-phosphorylation, and recruitment of cellular signaling proteins to the receptor. The E5 proteins of the human papillomaviruses also appear to affect the activity of growth factor receptors and their signaling pathways. The interaction of papillomavirus E5 proteins with a subunit of the vacuolar ATPase may also contribute to transformation. Further analysis of these unique mechanisms of viral transformation will yield new insight into the regulation of growth factor receptor activity and cellular signal transduction pathways.

E5 transforming proteins of papillomaviruses do not disturb the activity of the vacuolar H(+)-ATPase

Papillomaviruses contain a gene, E5, that encodes a short hydrophobic polypeptide that has transforming activity. E5 proteins bind to the 16 kDa subunit c (proteolipid) of the eukaryotic vacuolar H(+)-ATPase (V-ATPase) and this binding is thought to disturb the V-ATPase and to be part of transformation. This link has been examined in the yeast Saccharomyces cerevisiae. The E5 proteins from human papillomavirus (HPV) type 16, bovine papillomavirus (BPV) type 1, BPV-4 E5 and various mutants of E5 and the p12' polypeptide from human T-lymphotropic virus (HTLV) type I all bound to the S. cerevisiae subunit c (Vma3p) and could be found in vacuolar membranes. However, none affected the activity of the V-ATPase. In contrast, a dominant-negative mutant of Vma3p (E137G) inactivated the enzyme and gave the characteristic VMA phenotype. A hybrid V-ATPase containing a subunit c from Norway lobster also showed no disruption. Sedimentation showed that HPV-16 E5 was not part of the active V-ATPase. It is concluded that the binding of E5 and E5-related proteins to subunit c does not affect V-ATPase activity or function and it is proposed that the binding may be due to a chaperone function of subunit c.

Expression of a transforming gene (E5) of bovine papillomavirus in sarcoids obtained from horses

OBJECTIVE

To determine expression of a transforming gene (E5) of bovine papillomavirus in sarcoids, other tumors, and normal skin samples collected from horses with and without sarcoids.

SAMPLE POPULATION

23 sarcoids and 6 samples of normal skin obtained from 16 horses with sarcoids, 2 samples of normal skin and 2 papillomas obtained from horses without sarcoids, and 1 papilloma obtained from a cow.

PROCEDURE

Protein was extracted from tissue samples collected from horses and incubated with agarose beads covalently coupled to Staphylococcus aureus protein A and an anti-E5 polyclonal antibody. Following incubation, proteins were eluted from the beads and electrophoresed on a 14% polyacrylamide gel and transferred to a polyvinylidene difluoride membrane. The E5 protein was detected by use of western blot analysis, using a chemiluminescence detection system.

RESULTS

All 23 sarcoids had positive results for expression of E5 protein. Quantity of viral protein appeared to vary among sarcoids. All other tissues examined had negative results for E5 protein. Highest expression for E5 protein was observed in biologically aggressive fibroblastic variants of sarcoids, compared with expression in quiescent tumors.

CONCLUSIONS AND CLINICAL RELEVANCE

This study documented that activation and expression of the E5 gene is evident in sarcoids obtained from horses. These data support the conclusion that infection with bovine papillomavirus is important in the initiation or progression of sarcoids in horses. Treatment strategies designed to increase immune recognition of virally infected cells are warranted.

The platelet-derived growth factor beta receptor as a target of the bovine papillomavirus E5 protein

The 44-amino acid E5 protein of bovine papillomavirus is a homo-dimeric, transmembrane protein that transforms cells by activating the platelet-derived growth factor ss receptor in a ligand-independent fashion. The E5 protein induces receptor activation by forming a stable complex with the receptor, thereby inducing receptor dimerization, trans-phosphorylation of tyrosine residues in the cytoplasmic domain of the receptor, and recruitment of cellular SH2 domain-containing proteins into a signal transduction complex. Direct interactions between specific transmembrane and juxtamembrane amino acids in the E5 protein and the PDGF ss receptor appear to drive complex formation and dimerization of the receptor. Further analysis of this unique mechanism of viral transformation promises to yield new insight into the regulation of growth factor receptor activity and cellular signal transduction pathways.

Binding of human papillomavirus 16 E5 to the 16 kDa subunit c (proteolipid) of the vacuolar H+-ATPase can be dissociated from the E5-mediated epidermal growth factor receptor overactivation

Human papillomavirus type 16 E5 protein (HPV16 E5) upregulates ligand-mediated activation of the epidermal growth factor receptor (EGFR) in transfected human keratinocytes. HPV16 E5 binds to the 16 kDa proteolipid (subunit c) of the vacuolar H+-ATPase (16K), responsible for endosomal acidification, and this binding has been suggested to be responsible for increased recycling of the EGFRs. Using mutant deletions we show here that amino acids 54-78, but not 79-83 are necessary for binding to the 16K proteolipid. EGF treatment of cells expressing wild type or mutants of the E5 protein show that deletion of the last carboxy terminal 5 amino acids results in loss of E5-mediated EGFR overactivation. Thus, our results show that the binding capacity of HPV16 E5 to 16K can be dissociated from the effect of the viral protein on EGFR activation.

A mutagenic analysis of the E5 protein of human papillomavirus type 16 reveals that E5 binding to the vacuolar H+-ATPase is not sufficient for biological activity, using mammalian and yeast expression systems

The E5 gene of human papillomavirus type 16 encodes a highly hydrophobic membrane protein previously shown to inhibit endosomal acidification, presumably by binding to the 16-kDa pore-forming subunit of the vacuolar H(+)-ATPase (v-ATPase). The role of this interaction in the disruption of v-ATPase activity was explored through extensive mutagenesis of E5 to identify residues that mediate binding to the 16-kDa subunit. Coimmunoprecipitations revealed that the hydrophobic span between residues 41 and 54 is primarily responsible for this interaction and can be replaced with random hydrophobic amino acids. Studies using mutated 16-kDa proteins indicated that the fourth transmembrane domain of the pore subunit mediates binding to E5. Analysis of the E5 mutants in a yeast expression system revealed that several mutants that retained the capacity to bind to the 16-kDa subunit in COS-1 cells failed to disrupt vacuolar acidification. These data argue that E5 binding to the pore subunit is not sufficient for the associated activity of disruption of v-ATPase function.

Golgi alkalinization by the papillomavirus E5 oncoprotein

The E5 oncoprotein of bovine papillomavirus type I is a small, hydrophobic polypeptide localized predominantly in the Golgi complex. E5-mediated transformation is often associated with activation of the PDGF receptor (PDGF-R). However, some E5 mutants fail to induce PDGF-R phosphorylation yet retain transforming activity, suggesting an additional mechanism of action. Since E5 also interacts with the 16-kD pore-forming subunit of the vacuolar H(+)-ATPase (V-ATPase), the oncoprotein could conceivably interfere with the pH homeostasis of the Golgi complex. A pH-sensitive, fluorescent bacterial toxin was used to label this organelle and Golgi pH (pH(G)) was measured by ratio imaging. Whereas pH(G) of untreated cells was acidic (6.5), no acidification was detected in E5-transfected cells (pH approximately 7.0). The Golgi buffering power and the rate of H(+) leakage were found to be comparable in control and transfected cells. Instead, the E5-induced pH differential was attributed to impairment of V-ATPase activity, even though the amount of ATPase present in the Golgi complex was unaltered. Mutations that abolished binding of E5 to the 16-kD subunit or that targeted the oncoprotein to the endoplasmic reticulum abrogated Golgi alkalinization and cellular transformation. Moreover, transformation-competent E5 mutants that were defective for PDGF-R activation alkalinized the Golgi lumen. Neither transformation by sis nor src, two oncoproteins in the PDGF-R signaling pathway, affected pH(G). We conclude that alkalinization of the Golgi complex represents a new biological activity of the E5 oncoprotein that correlates with cellular transformation.

The transmembrane domain of the E5 oncoprotein contains functionally discrete helical faces

The E5 protein of bovine papillomavirus is a 44-amino acid, Golgi-resident, type II transmembrane protein that efficiently transforms immortalized mouse fibroblasts. The transmembrane (TM) domain of E5 is not only critical for biological activity, it also regulates interactions with cellular targets including the platelet derived growth factor receptor (PDGF-R) and the 16-kDa subunit of the vacuolar proton ATPase (V-ATPase). In order to define the specific TM amino acids essential for E5 biological and biochemical activity, we performed scanning alanine mutagenesis on 25 of the 30 potential TM residues and genetically mapped discrete alpha-helical domains which separately regulated the ability of E5 to bind PDGF-R, activate PDGF-R, and to form oligomers. Alanine substitutions at positions 17, 21, and 24 (which lie on the same helical face) greatly inhibited E5 association with the PDGF-R, suggesting that this region comprises the receptor binding site. PDGF-R activation also mapped to a specific but broader domain in E5; mutant proteins with alanines on one helical face (positions 8, 9, 11, 16, 19, 22, and 23) continued to induce PDGF-R tyrosine phosphorylation, whereas mutant proteins with alanines on the opposite helical face (positions 7, 10, 13, 17, 18, 21, 24, and 25) did not, indicating that the latter helical face was critical for mediating receptor transphosphorylation. Interestingly, these "activation-defective" mutants segregated into two classes: 1) those that were unable to form dimers but that could still form higher order oligomers and transform cells, and 2) those that were defective for PDGF-R binding and were transformation-incompetent. These findings suggest that the ability of E5 to dimerize and to bind PDGF-R is important for receptor activation. However, since several transformation-competent E5 mutants were defective for binding and/or activating PDGF-R, it is apparent that E5 must have additional activities to mediate cell transformation. Finally, alanine substitutions also defined two separate helical faces critical for E5/E5 interactions (homodimer formation). Thus, our data identify distinct E5 helical faces that regulate homologous and heterologous intramembrane interactions and define two new classes of biologically active TM mutants.

A mutational analysis of the transforming functions of the E8 protein of bovine papillomavirus type 4

The E8 protein of BPV-4 contributes to transformation of primary bovine cells (PalFs) by inducing anchorage-independent growth and by down-regulating gap junction intercellular communication, likely due to its binding to 16K ductin. We show here that, in addition, E8 confers on PalF cells the ability to grow in low serum and to escape from contact inhibition (focus formation). E8 also transactivates an exogenous human cyclin A gene promoter, suggesting that overexpression of cyclin A is responsible for the transformed phenotype. Mutant forms of E8 were generated to establish whether the transforming functions of the protein could be segregated. Mutations were introduced both in the hydrophobic domain and in the hydrophilic C-terminal "tail", and chimeras with BPV-1 E5 were constructed. Cells expressing either wild-type E8 or mutant forms were analyzed for their ability to grow in low serum and in suspension and to form foci. Wild-type E8 and its mutants were also analyzed for their ability to transactivate the cyclin A promoter. We show here that the transforming functions of E8 can be segregated and that both the hydrophilic C-terminal tail and the residue at position 17 in the hydrophobic domain are crucial for E8 functions and for the transactivation of the cyclin A promoter. These results support the hypothesis that the different aspects of cellular transformation brought about by E8 might be due to interaction with different cellular targets. They suggest that E8 might function differently from BPV-1 E5 and demonstrate that the separate domains of E5 and E8 are not functionally interchangeable.

Structure, function and regulation of the vacuolar (H+)-ATPase

The vacuolar (H+)-ATPases (or V-ATPases) function in the acidification of intracellular compartments in eukaryotic cells. The V-ATPases are multisubunit complexes composed of two functional domains. The peripheral V1 domain, a 500-kDa complex responsible for ATP hydrolysis, contains at least eight different subunits of molecular weight 70-13 (subunits A-H). The integral V0 domain, a 250-kDa complex, functions in proton translocation and contains at least five different subunits of molecular weight 100-17 (subunits a-d). Biochemical and genetic analysis has been used to identify subunits and residues involved in nucleotide binding and hydrolysis, proton translocation, and coupling of these activities. Several mechanisms have been implicated in the regulation of vacuolar acidification in vivo, including control of pump density, regulation of assembly of V1 and V0 domains, disulfide bond formation, activator or inhibitor proteins, and regulation of counterion conductance. Recent information concerning targeting and regulation of V-ATPases has also been obtained.

Transmembrane domain sequence requirements for activation of the p185c-neu receptor tyrosine kinase

The receptor tyrosine kinase p185c-neu can be constitutively activated by the transmembrane domain mutation Val664-->Glu, found in the oncogenic mutant p185neu. This mutation is predicted to allow intermolecular hydrogen bonding and receptor dimerization. Understanding the activation of p185c-neu has assumed greater relevance with the recent observation that achondroplasia, the most common genetic form of human dwarfism, is caused by a similar transmembrane domain mutation that activates fibroblast growth factor receptor (FGFR) 3. We have isolated novel transforming derivatives of p185c-neu using a large pool of degenerate oligonucleotides encoding variants of the transmembrane domain. Several of the transforming isolates identified were unusual in that they lacked a Glu at residue 664, and others were unique in that they contained multiple Glu residues within the transmembrane domain. The Glu residues in the transforming isolates often exhibited a spacing of seven residues or occurred in positions likely to represent the helical interface. However, the distinction between the sequences of the transforming clones and the nontransforming clones did not suggest clear rules for predicting which specific sequences would result in receptor activation and transformation. To investigate these requirements further, entirely novel transmembrane sequences were constructed based on tandem repeats of simple heptad sequences. Activation was achieved by transmembrane sequences such as [VVVEVVA]n or [VVVEVVV]n, whereas activation was not achieved by a transmembrane domain consisting only of Val residues. In the context of these transmembrane domains, Glu or Gln were equally activating, while Lys, Ser, and Asp were not. Using transmembrane domains with two Glu residues, the spacing between these was systematically varied from two to eight residues, with only the heptad spacing resulting in receptor activation. These results are discussed in the context of activating mutations in the transmembrane domain of FGFR3 that are responsible for the human developmental syndromes achondroplasia and acanthosis nigricans with Crouzon Syndrome.

Mutational analysis of human papillomavirus type 11 E5a oncoprotein

In this study, we investigated the structural basis of human papillomavirus type 11 (HPV-11) E5a transforming activity at the amino acid level. The effects of insertion, deletion , and substitution mutations on teh E5a transforming activity were determined by the assay of anchorage-independent growth. In the conserved Cys-X-Cys structure, substitution of Ser for Cys-73 resulted in indistinguishable transforming activity, whereas substitution of Ser for Cys-75 or Ser for both Cys-73 and Cys-75 retained 50 and 42% transformation, respectively. This suggests that Cys at position 75 may be important for transformation. Charge and structural changes at teh COOH termini of several mutants impaired transformation significantly, but those at the middle region did so only mildly. In addition, the 16,000-molecular-weight pore-forming protein (16K protein) is known to associate with BPV-1, HPV-6, and HPV-16 E5 proteins. In this study, we investigated the correlation between E5a-16K binding affinity and the transforming activity of E5a by the use of 11 E5a mutants. Results show that E5a and these 11 E5a mutants could bind to the 16K protein when these proteins were coexpressed in COS cells, suggesting that simple binding of the 16K protein by E5a may not be sufficient for cell transformation.

E5 oncoprotein transmembrane mutants dissociate fibroblast transforming activity from 16-kilodalton protein binding and platelet-derived growth factor receptor binding and phosphorylation

The E5 oncoprotein of bovine papillomavirus type 1 is a 44-amino-acid, hydrophobic polypeptide which localizes predominantly in Golgi membranes and appears to transform cells through the activation of tyrosine kinase growth factor receptors. In fibroblasts, E5 interacts with both the 16-kilodalton vacuolar ATPase subunit and the platelet-derived growth factor receptor (PDGF-R) via its hydrophobic transmembrane domain and induces autophosphorylation of the receptor. To further analyze the correlation between E5 biological activity and its ability to bind these cellular proteins, a series of nine E5 transmembrane mutants was evaluated. In 32D mouse hematopoietic cells, there was an incomplete correlation between the abilities of the E5 mutant proteins to associate the PDGF-R and to transform cells. However, all transforming E5 mutant proteins induced PDGF-R tyrosine phosphorylation. In NIH 3T3 and C127 mouse fibroblasts, both transforming and nontransforming E5 mutant proteins were defective for PDGF-R binding. In addition, while most of the transforming E5 proteins induced PDGF-R phosphorylation, one hypertransforming mutant (serine 17) neither bound nor induced receptor autophosphorylation. These findings support the hypothesis that the transformation of fibroblasts by E5 transmembrane mutants can involve alternative cellular targets or potentially independent activities of the E5 protein. In addition, these results underscore the critical role of the transmembrane domain in mediating E5 biological activities.

Mutational analysis of the beta-type platelet-derived growth factor receptor defines the site of interaction with the bovine papillomavirus type 1 E5 transforming protein

The E5 polypeptide of bovine papillomavirus type 1 is a small membrane-bound protein which induces the transformation of immortalized fibroblasts, apparently via the formation of a ternary complex with the platelet-derived growth factor receptor (PDGFR) and the 16-kDa V-ATPase protein. This interaction seems to be mediated, at least in part, by their respective transmembrane domains. E5 also cooperates with transfected beta PDGFR to induce interleukin-3 (IL-3)-independent growth of a mouse myeloid precursor cell line (32D) which normally lacks expression of most known tyrosine kinase growth factor receptors. Cell proliferation induced by beta PDGFR and E5 is also highly specific, since the highly conserved alpha PDGFR and other related receptors did not physically or functionally interact with E5 in these cells. In the current study, analysis of chimeric alpha and beta PDGFRs confirmed that a short region encompassing the beta PDGFR transmembrane domain was sufficient for complex formation with E5, receptor autophosphorylation, and sustained proliferation of 32D cells in the absence of IL-3. Furthermore, a deletion mutant lacking the entire extracellular domain efficiently bound E5 and induced IL-3-independent growth. These data provide direct evidence that the interaction between E5 and the beta PDGFR involves amino acids 531 to 556 of the receptor transmembrane region and that this specific interaction is critical for activation of the PDGFR signaling complex.

Mutational analysis of the interaction between the bovine papillomavirus E5 transforming protein and the endogenous beta receptor for platelet-derived growth factor in mouse C127 cells

The bovine papillomavirus E5 protein is a 44-amino-acid membrane-associated protein that forms a stable complex with the endogenous platelet-derived growth factor (PDGF) beta receptor in rodent and bovine fibroblasts, resulting in sustained receptor activation and cell transformation. We report here that high-level expression of the E5 protein caused a reduction in the level of the mature form of the PDGF beta receptor in acutely and stably transformed mouse C127 cells. To explore in more detail the interaction of the E5 protein and the PDGF beta receptor, we tested the abilities of various E5 point mutants to bind the PDGF receptor, to induce PDGF receptor down-regulation and tyrosine phosphorylation, and to transform cells. A transformation-competent mutant, like the wild-type E5 protein, bound the receptor and induced receptor tyrosine phosphorylation and down-regulation. Transformation-defective E5 proteins either failed to interact with the endogenous PDGF beta receptor in mouse fibroblasts or underwent an aberrant interaction with the receptor. Mutation of glutamine at position 17, aspartic acid at position 33, or both carboxyl-terminal cysteine residues required for E5 homodimerization interfered with stable complex formation with the PDGF receptor, tyrosine phosphorylation and down-regulation of the receptor, and cell transformation. Point mutations at several other carboxyl-terminal positions generated transformation-defective E5 proteins that formed a complex with the PDGF receptor and induced receptor tyrosine phosphorylation but did not induce PDGF receptor down-regulation. Either PDGF receptor activation is not sufficient for transformation of C127 cells or the receptors that are tyrosine phosphorylated in response to these mutant E5 proteins are not fully activated and therefore are not able to deliver a mitogenic signal.

Vacuolar H(+)-ATPase mutants transform cells and define a binding site for the papillomavirus E5 oncoprotein

The 16K subunit of the vacuolar H(+)-ATPase binds specifically to the bovine (BPV) and human (HPV) papillomavirus E5 oncoproteins, and it has been suggested that this interaction may contribute to cell transformation (Goldstein, D. J., and Schlegel, R. (1990) EMBO J. 9, 137-146; Goldstein, D. J., Finbow, M. E., Andresson, T., McLean, P., Smith, K., Bubb, V. J., and Schlegel, R. (1991) Nature 352, 347-349; Conrad, M., Bubb, V. J., and Schlegel, R. (1993) J. Virol. 67, 6170-6178; Goldstein, D. J., Toyama, R., Schlegel, R., and Dhar, R. (1992) Virology 190, 889-893). We generated mutations within the 16K protein to define binding domains for BPV-1 E5 as well as to characterize the role of 16K in cell transformation. 16K consists predominantly of 4 transmembrane (TM) domains. We showed that mutations within the TM4 domain severely inhibited E5 binding. More specifically, conversion of glutamic acid 143 to arginine within TM4 severely reduced 16K/E5 binding, suggesting that charged interactions facilitated efficient binding. This hypothesis was confirmed by demonstrating that binding to the defective 16K arginine mutant could be restored by complementary charge mutations in E5; conversion of E5 glutamine 17 to glutamic acid or aspartic acid enhanced interactions with the 16K arginine mutant. Surprisingly, mutants in TM4 not only bound poorly to wild-type E5 but were converted into an oncoprotein and induced anchorage-independent growth of NIH 3T3 cells. These data define glutamic acid 143 in the 16K TM4 domain and glutamine 17 within E5 as important contributors to E5/16K binding and suggest a role for the 16K protein in the regulation of cell proliferation.

The genomes of the animal papillomaviruses European elk papillomavirus, deer papillomavirus, and reindeer papillomavirus contain a novel transforming gene (E9) near the early polyadenylation site

We report that the genomes of reindeer papillomavirus (RPV), European elk papillomavirus (EEPV), and deer papillomavirus (DPV) contain a short conserved translational open reading frame (ORF), E9, which is located between the E5 ORF and the early polyadenylation site. In RPV, DPV, and EEPV, E9 ORFs have the potential to encode extremely hydrophobic polypeptides of approximately 40 amino acids. In mouse C127 cells transformed by EEPV and RPV, there exists a unique, abundant mRNA species of approximately 700 nucleotides which has the capacity to encode an E9 polypeptide. This mRNA is transcribed from a previously unrecognized promoter at position 4030 in the EEPV genome. The EEPV E9 ORF exhibits weak transforming activity in C127 cells and primary rat embryo fibroblasts. We also show that EEPV E5 is the major oncogene in the EEPV genome when assayed in C127 cells, although it is less efficient in transformation than the E5 genes of bovine papillomavirus type 1, DPV, and RPV.

Mutation of the bovine papillomavirus E5 oncoprotein at amino acid 17 generates both high- and low-transforming variants

The E5 transforming protein of bovine papillomavirus type 1 is a 44-amino-acid, hydrophobic protein which localizes predominantly to Golgi membranes. The E5 transmembrane domain contains a highly conserved glutamine residue at position 17 which, from previous limited mutagenic analysis, appeared essential for transforming activity. In order to determine the specific amino acid requirements at this position, we constructed a series of substitution mutants, representing all classes of amino acids, employing a vector which expressed E5 independently of other bovine papillomavirus gene products. All of the expressed E5 mutant proteins were stable, dimerized normally, and localized to the Golgi. Our results obtained with C127 mouse cells demonstrated that acidic amino acids (and serine) increased E5 transforming activity, whereas basic amino acids greatly inhibited E5 activity. Nonpolar amino acid substitutions were also defective. Interestingly, the relative transforming activities of these E5 mutant proteins changed dramatically when assayed with NIH 3T3 cells, suggesting that an auxiliary cellular protein(s) may modulate E5 transformation or that there are additional or different mechanisms of E5 transformation which are utilized in these two cell lines.

The bovine papillomavirus type 1 E5 transforming protein specifically binds and activates the beta-type receptor for the platelet-derived growth factor but not other related tyrosine kinase-containing receptors to induce cellular transformation

The 44-amino-acid E5 protein of bovine papillomavirus type 1 is a highly hydrophobic protein which appears to transform cells through the activation of growth factor receptors. To investigate the specificity of E5-growth factor receptor interactions required for mitogenic signaling, we utilized a nontumorigenic, murine myeloid cell line (32D) which is strictly dependent on interleukin-3 (IL-3) for sustained proliferation in culture. This IL-3 dependence can be functionally substituted by the expression of a variety of surrogate growth factor receptors and the addition of the corresponding ligand. Several receptor cDNAs for the alpha- and beta-type platelet-derived growth factor receptors [alpha PDGFR and beta PDGFR], the epidermal growth factor receptor, and the colony-stimulating factor 1 receptor) were transfected into 32D cells constitutively expressing the E5 protein to test for IL-3-independent growth. Only beta PDGFR was capable of abrogating the IL-3 dependence of 32D cells. The proliferative signal induced by the coexpression of beta PDGFR and E5 was accompanied by stable complex formation between these proteins, constitutive tyrosine phosphorylation of the receptor, and tumorigenicity in nude mice. The lack of cooperative interaction between E5 and the epidermal growth factor receptor, the colony-stimulating factor 1 receptor, and the highly related alpha PDGFR was paralleled by the inability of E5 to bind to these receptors and failure to increase receptor tyrosine phosphorylation. Thus, these data indicate that the ability of E5 to induce sustained proliferation and transformation of 32D cells is a direct consequence of specific interaction between the E5 protein and the beta PDGFR signaling complex and the subsequent stimulation of receptor tyrosine phosphorylation.

Specific interaction between the bovine papillomavirus E5 transforming protein and the beta receptor for platelet-derived growth factor in stably transformed and acutely transfected cells

The E5 protein of bovine papillomavirus is a 44-amino-acid membrane protein which induces morphologic and tumorigenic transformation of fibroblasts. We previously showed that the E5 protein activates and forms a complex with the endogenous beta receptor for platelet-derived growth factor (PDGF) in transformed rodent fibroblasts and that the PDGF beta receptor can mediate tumorigenic transformation by the E5 protein in a heterologous cell system. Other workers have identified the receptor for epidermal growth factor (EGF) as a potential target of the E5 protein in NIH 3T3 cells. Here, we investigate the specificity of the interaction of the E5 protein with various growth factor receptors, with particular emphasis on the PDGF beta receptor and the EGF receptor. Under conditions where both the PDGF beta receptor and the EGF receptor are stably expressed in E5-transformed mouse and bovine fibroblasts and in E5-transformed epithelial cells, the E5 protein specifically forms a complex with and activates the PDGF receptor and not the EGF receptor. Under conditions of transient overexpression in COS cells, the E5 protein has the potential to associate with several growth factor receptors, including the EGF receptor. However, upon coexpression of PDGF beta receptors and EGF receptors in COS cells, the E5 protein preferentially forms a complex with the PDGF receptor. Therefore, we conclude that the PDGF beta receptor is the primary target for the E5 protein in a variety of cell types, including bovine fibroblasts.

Cellular transformation by a transmembrane peptide: structural requirements for the bovine papillomavirus E5 oncoprotein

The E5 oncoprotein of bovine papillomavirus, only 44 amino acids long, occurs as a disulfide-bonded transmembrane dimer. This remarkable oncoprotein stimulates signal transduction through activation of the platelet-derived growth factor (PDGF) receptor, and E5 exhibits limited amino acid sequence similarity with PDGF. Results presented here suggest that a key feature of the hydrophobic transmembrane domain is an amino acid side chain that participates in interhelical hydrogen bond formation. These data are reminiscent of the activated neu oncogene, in which a point mutation in the transmembrane domain leads to ligand-independent dimerization and activation of a receptor tyrosine kinase. Significantly, the transmembrane domain of E5 can be largely replaced by the transmembrane domain from the activated neu receptor tyrosine kinase. Extensive mutagenesis defines the minimal structural features required for transformation by the E5 oncoprotein as, first, the ability to dimerize and, second, presentation of a negatively charged residue at the extracellular side of the membrane. The biological activity of E5 mutants that lack most amino acid residues similar to PDGF suggests that E5 and PDGF activate the PDGF receptor by distinct mechanisms.

Specificity and promiscuity in membrane helix interactions

The membrane-spanning portions of many integral membrane proteins consist of one or a number of transmembrane α-helices, which are expected to be independently stable on thermodynamic grounds. Side-by-side interactions between these transmembrane α-helices are important in the folding and assembly of such integral membrane proteins and their complexes. In considering the contribution of these helix–helix interactions to membrane protein folding and oligomerization, a distinction between the energetics and specificity should be recognized. A number of contributions to the energetics of transmembrane helix association within the lipid bilayer will be relatively non-specific, including those resulting from charge–charge interactions and lipid–packing effects. Specificity (and part of the energy) in transmembrane α-helix association, however, appears to rely mainly upon a detailed stereochemical fit between sets of dynamically accessible states of particular helices. In some cases, these interactions are mediated in part by prosthetic groups.

The human T-cell leukemia/lymphotropic virus type I p12I protein cooperates with the E5 oncoprotein of bovine papillomavirus in cell transformation and binds the 16-kilodalton subunit of the vacuolar H+ ATPase

The human T-cell leukemia/lymphotropic virus type I (HTLV-I) induces T-cell leukemia and transforms human T cells in vitro. A recently identified protein with a molecular weight of 12,000 (12K) (p12I), encoded by single- and double-spliced mRNAs transcribed from the 3' end of the HTLV-I genome, has been shown to localize in the perinuclear compartment and in the cellular endomembranes. The p12I protein exhibits significant amino acid sequence similarity to the E5 oncoprotein of bovine papillomavirus type 1 (BPV-1). Both proteins are very hydrophobic, contain a glutamine residue in the middle of a potential transmembrane region(s), and are localized in similar cellular compartments. Because of these observations, we investigated whether the p12I resemblance to E5 correlated with a similarity in their biological behavior. We expressed the p12I protein to evaluate its ability to functionally cooperate with the BPV-1 E5 oncoprotein and to bind to a cellular target of the E5 protein, the 16K component of the vacuolar H+ ATPase. Cotransfection of the mouse C127 cell line with the p12I and E5 cDNAs showed that although p12I alone could not induce focus formation, it strongly potentiated the transforming activity of E5. In addition, the p12I protein bound to the 16K protein as efficiently as the E5 protein. These findings might provide new insight for potential mechanisms of HTLV-I transformation and suggest that p12I and E5 represent an example of convergent evolution between RNA and DNA viruses.

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.

The human papillomavirus type 6 and 16 E5 proteins are membrane-associated proteins which associate with the 16-kilodalton pore-forming protein

The human papillomavirus (HPV) E5 proteins are predicted from DNA sequence analysis to be small hydrophobic molecules, and the HPV type 6 (HPV-6) and HPV-11 E5 proteins share several structural similarities with the bovine papillomavirus type 1 (BPV-1) E5 protein. Also similar to the BPV-1 E5 protein, the HPV-6 and HPV-16 E5 proteins exhibit transforming activity when assayed on NIH 3T3 and C127 cells. In this study, we expressed epitope-tagged E5 proteins from both the "low-risk" HPV-6 and the "high-risk" HPV-16 in order to permit their immunologic identification and biochemical characterization. While the HPV-6 and HPV-16 E5 proteins fail to form disulfide-linked dimers and oligomers, they did resemble the BPV-1 E5 protein in their intracellular localization to the Golgi apparatus, endoplasmic reticulum, and nuclear membranes. In addition, the HPV E5 proteins also bound to the 16-kDa pore-forming protein component of the vacuolar ATPase, a known characteristic of the BPV-1 E5 protein. These studies reveal a common intramembrane localization and potential cellular protein target for both the BPV and HPV E5 proteins.

Transformation-specific interaction of the bovine papillomavirus E5 oncoprotein with the platelet-derived growth factor receptor transmembrane domain and the epidermal growth factor receptor cytoplasmic domain

The bovine papillomavirus E5 transforming protein appears to activate both the epidermal growth factor receptor (EGF-R) and the platelet-derived growth factor receptor (PDGF-R) by a ligand-independent mechanism. To further investigate the ability of E5 to activate receptors of different classes and to determine whether this stimulation occurs through the extracellular domain required for ligand activation, we constructed chimeric genes encoding PDGF-R and EGF-R by interchanging the extracellular, membrane, and cytoplasmic coding domains. Chimeras were transfected into NIH 3T3 and CHO(LR73) cells. All chimeras expressed stable protein which, upon addition of the appropriate ligand, could be activated as assayed by tyrosine autophosphorylation and biological transformation. Cotransfection of E5 with the wild-type and chimeric receptors resulted in the ligand-independent activation of receptors, provided that a receptor contained either the transmembrane domain of the PDGF-R or the cytoplasmic domain of the EGF-R. Chimeric receptors that contained both of these domains exhibited the highest level of E5-induced biochemical and biological stimulation. These results imply that E5 activates the PDGF-R and EGR-R by two distinct mechanisms, neither of which specifically involves the extracellular domain of the receptor. Consistent with the biochemical and biological activation data, coimmunoprecipitation studies demonstrated that E5 formed a complex with any chimera that contained a PDGF-R transmembrane domain or an EGF-R cytoplasmic domain, with those chimeras containing both domains demonstrating the greatest efficiency of complex formation. These results suggest that although different domains of the PDGF-R and EGF-R are required for E5 activation, both receptors are activated directly by formation of an E5-containing complex.

Human immunodeficiency virus type 1 Vpu protein is an oligomeric type I integral membrane protein

The human immunodeficiency virus type 1 Vpu protein is a 16-kDa phosphoprotein which enhances the efficiency of virion production and induces rapid degradation of CD4, the cellular receptor for human immunodeficiency virus. The topology of membrane-inserted Vpu was investigated by using in vitro-synthesized Vpu cotranslationally inserted into canine microsomal membranes. Proteolytic digestion and immunoprecipitation studies revealed that Vpu was a type I integral membrane protein, with the hydrophilic domain projecting from the cytoplasmic membrane face. In addition, several high-molecular-weight proteins containing Vpu were identified by chemical cross-linking. Such complexes also formed when wild-type Vpu and a Tat-Vpu fusion protein were coexpressed. Subsequent analysis by one- and two-dimensional electrophoresis revealed that these high-molecular-weight complexes consisted of homo-oligomers of Vpu. These findings indicate that Vpu is a type I integral membrane protein capable of multimerization.

Analysis of the gene encoding a 16-kDa proteolipid subunit of the vacuolar H(+)-ATPase from Manduca sexta midgut and tubules

Vacuolar ATPases (V-ATPases), originally characterised as components of endomembranes, have also been implicated in epithelial ion transport, both in vertebrates and in insects. The ATPase is particularly noteworthy in lepidopteran larvae, where it generates large transepithelial potential differences and short-circuit currents across the midgut epithelium. A cDNA library from Manduca sexta larval midguts and Malpighian tubules was screened with a Drosophila melanogaster cDNA encoding the 16-kDa proteolipid subunit of the V-ATPase, and a 1.4-kb cDNA sequenced in its entirety. The sequence contains a long open reading frame, encoding a putative peptide of 156 amino acids (aa) and with an M(r) of 15,967, in close agreement with values previously suggested by sodium dodecyl sulfate-polyacrylamide gels of M. sexta midgut proteins. Correspondence of the deduced aa sequence with those of other species, particularly D. melanogaster, was extremely close. Northern blots of M. sexta midgut mRNA at high stringency revealed two transcripts of 1.4 and 1.9 kb, whereas genomic Southern blots suggest that there is only a single copy of the gene in M. sexta. The possibility that members of the 16-kDa gene family might serve multiple roles in transport and membrane communication is discussed.

The BPV-1 E5 oncoprotein expressed in Schizosaccharomyces pombe exhibits normal biochemical properties and binds to the endogenous 16-kDa component of the vacuolar proton-ATPase

The 44-amino-acid E5 oncoprotein of bovine papillomavirus type 1 transforms immortalized murine fibroblast cell lines. This highly hydrophobic protein forms homodimers, localizes to intracellular membrane compartments (including the Golgi apparatus), and forms a complex with the 16-kDa membrane-embedded constituent (16k) of the vacuolar proton-ATPase. To develop a system for the genetic and biochemical analysis of the E5/16k interaction, the E5 gene was cloned into a new vector which was designed for expression in the fission yeast Schizosaccharomyces pombe. The E5 protein synthesized in this system dimerized normally and bound to endogenous and overexpressed S. pombe 16k protein. Comparison of the S. pombe and mammalian 16k proteins showed strong conservation in carboxyl-terminal amino acids but greater variation in the amino-terminal sequences, suggesting that E5 was interacting with the 16k carboxyl domains. Finally, a new protein epitope tag is described which permitted for the first time the coprecipitation of E5 with antibodies directed against the 16k protein.