PREPARATION OF HUMAN PAPILLOMAVIRUS E1 HAVING HELICASE ACTIVITY AND METHOD THEREFOR

Abstract: The present invention relates to a method for isolating cloned papillomavirus E1 protein from a eukaryotic expression system having demonstrable and reproducible viral helicase activity and preparation containing essentially pure E1 protein. The invention further relates to the use of this E1 protein preparation in a screening assay for identifying antiviral agents. More particularly a high throughput assay to screen for agents capable of inhibiting HPV DNA replication. The assay is based on measuring the effect of antiviral agents on the activity of the E1 protein and more specifically on its helicase activity.

Description: PREPARATION OF HUMAN PAPILLOMAVIRUS El HAVING HELICASE ACTIVITY AND METHOD THEREFOR FIELD OF THE INVENTION The present invention relates to a method for isolating and purifying cloned papillomavirus (PV) El protein from a eukaryotic expression system, having demonstrable and reproducible viral helicase activity and devoid of contaminating activities . The invention further relates to the use of this novel protein extraction method to isolate substantially purified, preferably essentially pure, El protein having helicase activity to establish a screening assay for antiviral agents . More particularly the invention relates to El protein to establish a high throughput assay to screen for agents capable of inhibiting PV DNA replication. The assay is based on measuring the inhibition of the antiviral agents on the activity of the El protein and more specifically on its helicase activity. BACKGROUND OF THE INVENTION Papillomaviruses (PV) are non-enveloped DNA viruses that induce hyperproliferative lesions of the epithelia. The papillomaviruses are widespread in nature and have been recognized in higher vertebrates. Viruses have been characterized, amongst others, from humans, cattle, rabbits, horses, and dogs. The first papillomavirus was described in 1933 as cottontail rabbit papillomavirus (CRPV) . Since then, the cottontail rabbit as well as bovine papillomavirus type 1 (BPV-1) have served as experimental prototypes for studies on papillomaviruses . Most animal papillomaviruses are associated with purely epithelial proliferative lesions, and most lesions in animals are cutaneous . In the human there are more than 75 types of papillomavirus (HPV) that have been identified and they have been catalogued by site of infection: cutaneous epithelium and mucosal epithelium (oral and genital mucosa) . The cutaneous-related diseases include flat warts, plantar warts, etc. The mucosal-related diseases include laryngeal papillomas and anogenital diseases comprising cervical carcinomas (Fields, 1996, Virology, 3rd ed. Lippincott - Raven Pub., Philadelphia, N.Y.). There are more than 25 HPV types that are implicated in anogenital diseases, these are grouped into "low risk" and "high risk" types. The low risk types include HPV type 6, type 11 and type 13 and induce mostly benign lesions such as condyloma acuminata (genital warts) and low grade squamous intraepithelial lesions (SIL) . In the United States there are 5 million people with genital warts of which 90% is attributed to HPV-6 and HPV-11. About 90% of SIL is also caused by low risk types 6 and 11. The other 10% of SIL is caused by high risk HPVs. The high risk types are associated with high grade SIL and cervical cancer and include most frequently HPV types 16, 18, 31, 33, 35, 45, 52, and 58. The progression from low- grade SIL to high-grade SIL is much more frequent for lesions that contain high risk HPV-16 and 18 as compared to those that contain low risk HPV types. In addition, only four HPV types are detected frequently in cervical cancer (types 16, 18, 31 and 45) . About 500,000 new cases of invasive cancer of the cervix are diagnosed annually worldwide (Fields, 1996, supra) . Treatments for genital warts include physical removal such as cryotherapy, C0 2 laser, electrosurgery, or surgical excision. Cytotoxic agents may also be used such as trichloroacetic acid (TCA) , podophyllin or podofilox. Immunomodulatory agents are also available such as Interferon or Imiquimod. These treatments are not completely effective in eliminating all viral particles and there is either a high cost incurred or uncomfortable side effects related thereto. In fact, there are currently no effective antiviral treatments for HPV infection since with all current therapies recurrent warts are common (Beutner & Ferenczy, 1997, Amer. J. Med. , 102 (5A) , 28-37) . The ineffectiveness of the current methods to treat HPV infections has demonstrated the need to identify new means to control or eliminate such infections. In recent years, efforts have been directed towards finding antiviral compounds, and especially compounds capable of interfering with viral replication at the onset of infection (Hughes, 1993, Nucleic Acids Res. 21:5817-5823) . To that end, it has therefore become important to study the genetics of HPVs in order to identify potential chemotherapeutic targets to contain and possibly eliminate any diseases caused by HPV infections at the onset of infection. It is equally important to identify a measurable viral activity that demonstrates specificity and reliability to be used as an indicator in assessing the effectiveness of the potential chemotherapeutic agents against PVs. The life cycle of PV is closely coupled to keratinocyte differentiation. Infection is believed to occur at a site of tissue disruption in the basal epithelium. As the infected cells undergo progressive differentiation the cellular machinery is maintained allowing viral gene expression to increase, with eventual late gene expression and virion assembly in terminally differentiated keratinocytes and the release of viral particles (Fields, supra) . The coding strands for each of the papillomavirus contains approximately ten designated translational open reading frames (ORFs) that have been classified as either early ORFs or late ORFs based on their location in the genome. El to E8 are expressed early in the viral replication cycle, and two late genes (LI and L2) represent the major and minor capside proteins respectively. The El and E2 gene products function in viral DNA replication, whereas E5, E6 and E7 are expressed in connection with host cell proliferation. The LI and L2 are involved in virion structure. The functions of E3, E4 and E8 gene products is uncertain at present. Studies of HPV have shown that proteins El and E2 are both essential and sufficient for viral DNA replication in vitro (Kuo et al . , 1994, J. Biol. Chem. 30: 24058-24065). This requirement is similar to that of bovine papillomavirus type 1 (BPV-1) . Indeed, there is a high degree of similarity between El and E2 proteins and the ori-sequences of all papillomaviruses (PV) regardless of the viral species and type (Kuo et al . , 1994, supra) . Of note, El is the most highly conserved protein in PV and its enzymatic activity is presumed to be similar for all PV types (Jenkins, 1996, J. Gen. Virol., 77:1805-1809) . Evidence emanating from studies of BPV-1 have shown that El possesses ATPase and helicase activities that are required in the initiation of viral DNA replication (Seo et al . , 1993a, Proc. Natl. Acad. Sci. USA 90:702-706; Yang et al . , 1993, Proc. Natl. Acad. Sci. 90:5086-5090; and MacPherson et al . , 1994, 204:403-408) . The E2 protein is a transcriptional activator that binds to El protein and forms a complex that binds specifically to the ori sequence (see FIGURE 1) (Mohr et al . , 1990, Science 250:1694-1699). It is believed that E2 enhances binding of El to the BPV origin of replication (Seo et al . , 1993b, Proc. Natl. Acad. Sci., 90:2865-2869). In HPV, Lui et al . suggested that E2 stabilizes El binding to the ori (1995, J. Biol. Chem., 270 (45) :27283-27291) . The helicase activity of the El proteins of papillomavirus therefore constitute a good molecular target to design chemical entities capable of inhibiting viral replication. Such objective requires that the El protein be extracted and purified to an extent where its helicase activity can be measured reliably and reproducibly. Such isolation of El helicase has however remained elusive or at best unreliable, especially on a scale sufficient to establish an assay to screen for such inhibitors . Seo et al. ( 1993a, supra) disclose the extraction and purification of BPV-E1 from a baculovirus expression system with the step consisting of the use of PEG and IM NaCl in the nuclear extraction buffer. They obtained BPV-E1 preparation about 90% pure. However, we have not found it possible to obtain pure HPV-11 El by this procedure, and in any case the procedure is not suitable to the large scale required to purify El for high-throughput screening. The two BPV-1 genes encoding El and E2 proteins have been cloned into a Baculovirus expression system and the proteins substantially purified (US patent 5,464,936). US patent '936 discloses a purification process for El consisting of a nuclear extraction in a hypertonic buffer (containing 300 mM NaCl) followed by 3 sequential chromatographic separations. The disclosure, however, does not demonstrate the purity and specific activity of the resulting El helicase. The absence of affinity chromatography purification step leads to the presence of contaminating nucleases that prevent accurate measurement of the El helicase activity. In addition, even if such a process would in fact yield El helicase of sufficient purity to assess the helicase activity, it is believed that it would be inapplicable to a high-yield, large-scale process for HTS purposes. An extraction process wherein nuclei were suspended in lysis buffer containing 300 mM NaCl followed by further purification has been described (Bream et al . , 1993, J. Virol., 2655). However, the authors were unable to detect helicase activity from these crude preparations of El. Further attempts to isolate HPVs El protein cloned into different expression systems, having demonstrable and specific helicase activity, have failed (Jenkins et al . , 1996, supra) . Kuo et al . , 1994, supra discloses a purification procedure (using 420 mM salt during the nuclear extraction) but does not discuss the scale on which the procedure was carried out or the total yield of protein. It has been hypothesized that the conformation of the El protein and its hydrophobicity cause the protein to be "sticky" and to form aggregates thus making it difficult to extract and purify. In addition, difficulties in establishing enzymatic activities that are specific and free of cellular contaminants have generally been encountered. For example, viral helicase and/or ATPase activity may not be distinguishable from cellular helicase and/or ATPase contaminants present in the host cell used to express the El gene. In addition, very low levels of nucleases will destroy the substrate rendering any assessment impossible. One common denominator in the various purification processes outlined above lies in the presence of high concentrations of salt (hypertonic conditions) during the nuclear extraction step. Indeed, according to conventional wisdom, it is believed that nucleic acid-binding proteins may be solubilized in high concentrations of salt and thereby separated from nucleic acids. At present, the prior art has not revealed satisfactory processes for the purification of El. There thus remains a need to isolate a demonstrable and reproducible viral helicase activity that can be used as an indicator of the inhibitory effect of antiviral chemotherapeutic agents . More particularly, there remains a need to provide a method of preparing a PV El preparation displaying a high helicase activity. There also remains a need to obtain a preparation of human papillomavirus El protein which displays a helicase activity sufficient for the purposes of a screening assay, particularly, a high throughput screening assay. Since El structure/function is highly conserved amongst different papillomaviruses and amongst subtypes, it is assumed that the BPV and CRPV El proteins can be extracted and purified by the procedure of the invention. Therefore, there remains a need for a method for the isolation/purification of El protein from several species of papillomavirus, including, but not limited to bovine papillomavirus (BPV) , cottontail rabbit papillomavirus (CRPV) and human papillomavirus (HPV) . There also remains a need to isolate and purify the El protein from different subtypes of HPV, including but not limited to, HPV-6, 11, 16, 18, 31, 33, 35, 45, 52, and 58. Before the present invention, El protein preparations, including human El preparations, did not demonstrate reproducible helicase activity. The deficiency in the prior art created a road block in being able to screen a large collection of antiviral agents capable of inhibiting papilloma viral DNA replication. This deficiency is overcome by the present invention which is capable of providing the means to design a HTS for the screening for such agents. The Applicant has now found a reliable and reproducible purification process for the preparation of El having helicase activity. The resulting El preparation is free from degradation products and amenable to large scale production of El. The present description refers to a number of documents, 8 the content of which is herein incorporated by reference. SUMMARY OF THE INVENTION Therefore, in accordance with a first embodiment of the present invention, there is provided, a means for the isolation and purification of El protein having demonstrable, reliable and reproducible helicase activity. Thus, the invention mostly concerns the isolation and purification of El protein from papillomavirus or a functional derivative thereof, having detectable helicase activity above background levels. The El preparation according to the present invention is significantly free of contaminating cellular helicase, ATPase and nuclease activities. The El preparation according to the present invention displays reproducible viral helicase activity. In accordance with this first embodiment, there is provided a method for the isolation of the expressed El protein from the cloned El gene. There is therefore provided a method for extracting from a nuclear extract the papillomavirus El protein or a functional derivative thereof having viral helicase activity comprising the steps of: a) producing an El recombinant protein in a eukaryotic expression system and isolating a nuclei preparation thereof; b) extracting El protein from said nuclei preparation in a buffer comprising salt at a concentration lower than 300 mM. This novel method for extracting El protein having helicase activity from a eukaryotic cell nuclei preparation, comprises the use of salt concentrations lower than those taught in the prior art . There is further provided a method for isolating said El protein further comprising the step of: c) purifying El protein from said nuclear extract by affinity chromatography. The applicant was the first to design a method for the isolation of human papillomavirus El protein capable of demonstrating reproducible viral helicase activity, thus providing the essential element for the design of an assay for identifying potential antiviral agents capable of inhibiting El helicase activity and thereby preventing viral DNA replication. This method can also be applied for the isolation and purification of BPV and CRPV El helicases. In accordance with a further aspect of the present invention, there is provided a preparation of recombinant papillomavirus El protein from a eukaryotic expression system, said El having viral helicase activity, wherein the El protein is extracted from a nuclei preparation in the presence of salt at a concentration less than 300 mM, and optionally purified by affinity chromatography. In accordance with a further embodiment of the present invention, there is therefore provided the means to use the isolated El protein preparation in screening for the level of inhibition of candidate antiviral agents on El helicase activity. There is therefore provided a method for assaying the specific viral helicase activity of papillomavirus El protein, said method comprising the steps of: -incubating a mixture of said El protein preparation as defined above, and a suitable substrate for said viral helicase enzymatic activity; and - measuring the amount of specific helicase activity of said El protein. There is further provided a method for identifying agents capable of modulating said helicase activity, said 10 method comprising the steps of: a) assaying the activity of said El helicase in the absence of said agent by the method as defined above; b) assaying the activity of said helicase in the presence of said agent by the method as defined above, wherein said agent is added to said helicase and substrate mixture during said incubation; and c) comparing the result of step a) with the result of step b) . In accordance with a further embodiment, the isolated El protein has detectable and specific helicase activity, and, in the presence of E2 protein is capable of binding DNA to form a complex at the origin of replication, and contribute to viral DNA replication. Therefore, an alternative way to measure inhibition of El helicase activity is to measure the inhibition of viral DNA replication. There is therefore provided a method for assaying papillomavirus DNA replication, said method comprising the steps of : - incubating a candidate agent with a mixture of El protein preparation as defined above, with E2 protein and a suitable DNA origin of replication; and - measuring the amount of DNA unwinding. There is also provided a method for identifying an agent capable of modulating papillomavirus DNA replication, said method comprising the steps of: a) assaying said DNA replication activity in the absence of said agent by the method as defined above; b) assaying said DNA replication activity in the presence of said agent by the method as defined above, wherein said agent is added to said mixture during said incubation; and 11 c) comparing the result of step a) with the result of step b) . Other aspects of the present invention will become more apparent upon reading of the following non-restrictive description of preferred embodiments with reference to the accompanying drawing which is exemplary and should not be interpreted as limiting the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS Having thus generally described the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, and in which: FIGURE 1 shows a pictorial representation of the El and E2 interaction at the origin of replication of the papillomavirus. Briefly, the El protein is recruited at the origin of replication by the E2 protein and then forms a complex activating the helicase activity of the El to unwind DNA. The E1-E2 complex later recruits cellular replication proteins to eventually initiate DNA replication. FIGURE 2 shows the alignment of the amino acid sequences of the El helicases of several papillomaviruses and shows their % identity compared to the sequence of the El helicase of HPV-11; FIGURE 3A shows a Coomassie blue-stained gel of different conditions for the nuclear extraction of the El protein according to the present invention (the legend of which is presented in Example 2) ; FIGURE 3B shows a Western immunoblot of the gel of Figure 3A, stained with an anti-El K72 polyclonal antibody and developed with a chemiluminescent reagent; FIGURE 4A shows a schematic representation of the purification process according to the invention; FIGURE 4B shows a Coomassie blue-stained gel of the 12 different fractions recovered from the affinity-chromatography purification (the legend of which is presented in Example 4) ; Figure 5A shows the results of the El/E2/ori binding assay described in Example 8; FIGURE 5B shows three experiments with purified wild- type and mutants HPV- 11 El proteins. The top panel shows the results of a helicase gel-based assay by detecting unwinding activity of the enzyme; the middle panel shows the results of an ATPase assay; and the bottom figure shows the results of a helicase assay, as detected by SPA. These experiments are described in Example 9; FIGURE 6 shows a schematic representation of the high throughput screening assay for the El helicase as described in Example 11; FIGURE 7 shows the IC 50 curve for inhibition of the El helicase activity by the M13 plasmid as described in Example 12; FIGURE 8 shows the IC 50 curve of inhibition of the El helicase activity by ethidium bromide as described in Example 12 ; FIGURE 9A shows a Coomassie blue-stained gel of the loaded material and the different fractions recovered from the affinity-chromatography purification (the legend of which is presented in Example 14) ; FIGURE 9B shows a Western immunoblot of the gel of the different fractions recovered from the affinity-chromatography purification (the legend of which is presented in Example 15) ; FIGURE 10A shows a Coomassie blue-stained gel of different conditions for the nuclear extraction of the El protein (the legend of which is presented in Example 15) and also the no-salt extraction of HPV-6 El protein; 13 FIGURE 10B shows a Western immunoblot of the gel of Figure 9A. The blot was incubated with an anti-El polyclonal antibody and horseradish peroxidase (HRP) -conjugated second antibody. Bands were visualized using a chemiluminescent reagent; FIGURE 11 represents the amino acid sequence of HPV-11 El protein as isolated by the method of the invention. The amino acids in bold indicate the modifications observed compared to the published sequence of Figure 2; and FIGURE 12 represents the amino acid sequence of HPV-6a El protein as isolated by the method of the invention. The amino acid in bold indicates the modification observed compared to the published sequence of Figure 2. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Definitions Unless defined otherwise, the scientific and technological terms and nomenclature used herein have the same meaning as commonly understood by a person of ordinary skill to which this invention pertains . Generally, the procedures for cell culture, infection, molecular biology methods and the like are common methods used in the art. Such standard techniques can be found in reference manuals such as for example Sambrook et al . (1989, Molecular Cloning - A Laboratory Manual, Cold Spring Harbor Laboratories) and Ausubel et al . (1994, Current Protocols in Molecular Biology, Wiley, New York) . Nucleotide sequences are presented herein by single strand, in the 5' to 3 ' direction, from left to right, using the one letter nucleotide symbols as commonly used in the art and in accordance with the recommendations of the IUPAC-IUB Biochemical Nomenclature Commission (Biochemistry, 1972, 11:1726-1732) . 14 The present description refers to a number of routinely used recombinant DNA (rDNA) technology terms. Nevertheless, definitions of selected examples of such rDNA terms are provided for clarity and consistency. The term "recombinant DNA" or "recombinant plasmid" as known in the art refers to a DNA molecule resulting from the joining of DNA segments. This is often referred to as genetic engineering. The term "DNA segment", is used herein, to refer to a DNA molecule comprising a linear stretch or sequence of nucleotides. This sequence when read in accordance with the genetic code, can encode a linear stretch or sequence of amino acids which can be referred to as a polypeptide, protein, protein fragment and the like. The term "oligonucleotide" or "DNA" molecule or sequence refers to a molecule comprised of the deoxyribonucleotides adenine (A) , guanine (G) , thymine (T) and/or cytosine (C) . The term "oligonucleotide" or "DNA" can be found in linear DNA molecules or fragments, viruses, plasmids, vectors, chromosomes or synthetically derived DNA. As used herein, DNA sequences are described according to the normal convention of giving only the sequence in the 5' to 3' direction. As used herein, the term "gene" is well known in the art and relates to a nucleic acid sequence defining a single protein or polypeptide. A "structural gene" defines a DNA sequence which is transcribed into RNA and translated into a protein having a specific amino acid sequence thereby giving rise to a specific polypeptide or protein. The term "fusion protein" as defined herein refers two polypeptidic segments that are not joined together in nature. Non-limiting examples of such "fusion proteins" according to the present invention include the El protein fused to the 15 polypeptide of an "affinity label". In some embodiments it may be beneficial to introduce a cleavage site between the two polypeptide sequences which have been fused. Such protease cleavage sites between two heterologously fused protein are well known in the art. The terms "vectors" or "DNA construct" are commonly known in the art and refer to any genetic element, including, but not limited to, plasmid DNA, phage DNA, viral DNA and the like which can incorporate the oligonucleotide sequences, or sequences of the present invention and serve as DNA vehicle into which DNA of the present invention can be cloned. Numerous types of vectors exist and are well known in the art . The term "expression" defines the process by which a structural gene is transcribed into mRNA (transcription) , the mRNA is then being translated (translation) into one polypeptide (or protein) or more. The terminology "expression vector" defines a vector or vehicle as described above but designed to enable the expression of an inserted sequence following transformation into a host. The cloned gene (inserted sequence) is usually placed under the control of control element sequences such as promoter sequences. Such expression control sequences will vary depending on whether the vector is designed to express the operably linked gene in a prokaryotic or eukaryotic host or both (shuttle vectors) and can additionally contain transcriptional elements such as enhancer elements, termination sequences, tissue-specificity elements, and/or translational initiation and termination sites . By "eukaryotic expression system" is meant the combination of an appropriate expression vector and a eukaryotic cell line which can be used to express a protein of interest . In some systems the gene for the protein may be 16 inserted into the genome of a virus which can infect the cell type being used. Plasmid vectors containing the desired gene may also be used. In all cases, the vector will contain appropriate control elements (promoter) to express protein in the cell type of interest. Additional components, for example a vector or viral genome coding for T7 polymerase, may also be necessary in certain expression systems. Eukaryotic cell types typically used are yeast (e.g. Saccharomyceε cerevisiae, Pischia pastoris) transfected with a plasmid vector; insect cells (e.g. SF9, SF21) infected with baculovirus {Autog-rapha californica or Bombyx ori) (Luckow, Curr. Op. Biotech., 1993, 4:564-572; Griffiths and Page, 1994, Methods in Molec. Biol. 75:427-440; and Merrington et al . , 1997, Molec. Biotech. 8 (3) :283-297) ; mammalian cells infected with adenovirus, vaccinia virus, Sindbis virus, or semliki forest virus; and mammalian cells transfected with DNA vectors for transient or constitutive expression. Particularly preferred here is the insect cell/baculovirus system. A host cell or indicator cell has been "transfected" by exogenous or heterologous DNA (e.g. a DNA construct) when such DNA has been introduced inside the cell. The transfecting DNA may or may not be integrated (covalently linked) into chromosomal DNA making up the genome of the cell . In prokaryotes, yeast, and mammalian cells for example, the transfecting DNA may be maintained on an episomal element such as a plasmid. With respect to eukaryotic cells, a stably transfected cell is one in which the transfecting DNA has become integrated into a chromosome so that it is inherited by daughter cells through chromosome replication. This stability is demonstrated by the ability of the eukaryotic cell to establish cell lines or clones comprised of a population of daughter cells containing the transfecting DNA. Transfection methods are well known in the art (Sambrook et al . , 1989, 17 supra; Ausubel et al . , 1994, supra) . The term "affinity label" or "affinity tag" as used herein refers to a label which is specifically trapped by a complementary ligand. Examples of pairs of affinity marker/affinity ligand include but are not limited to: Maltose-Binding Protein (MBP) /maltose; Glutathione S Transferase (GST)/ glutathione; histidine (His)/ metal. The metal used as affinity ligand may be selected from the group consisting of: cobalt, zinc, copper, iron, and nickel (Wong et al. (1991), Separation and Purification Methods, 20(1), 49- 106). Preferably, the metal selected is nickel. The affinity ligand can be set up in columns to facilitate separation by affinity chromatography. The affinity label may be positioned on the N- or C- terminal end of the protein, but preferably on the N-terminus of the protein. For certainty, the nucleotide sequences and polypeptides useful to practice the invention includes "functional derivatives". The term "functional derivatives" is intended to include "fragments", "segments", "variants", "analogs" or "chemical derivatives" of the subject matter of the present invention. The functional derivatives of the present invention can be synthesized chemically or produced through recombinant DNA technology. All these methods are well known in the art. Thus, the term "variant" refers herein to a protein or nucleic acid molecule which is substantially similar in structure and biological activity to the protein or nucleic acid of the present invention. As used herein, "chemical derivatives" is meant to cover additional chemical moieties not normally part of the subject matter of the invention. Such moieties could affect the 18 physico-chemical characteristic of the derivative (i.e. solubility, absorption, half life and the like, decrease of toxicity) . Such moieties are exemplified in Remington's Pharmaceutical Sciences (1980) . Methods of coupling these chemical-physical moieties to a polypeptide are well known in the art . As exemplified herein below, the nucleotide sequences and polypeptides used in the present invention can be modified, for example by in vi tro mutagenesis, to dissect the catalytic and structure-function relationship thereof and permit a better design and identification of the resulting proteins. As used herein, the designation "functional derivative" denotes, in the context of a functional derivative of a sequence whether a nucleic acid or amino acid sequence, a molecule that retains a biological activity (either function or structural) that is substantially similar to that of the original sequence. This functional derivative or equivalent may be a natural derivatives or may be prepared synthetically. Such derivatives include amino acid sequences having substitutions, deletions, or additions of one or more amino acids, provided that the biological activity of the protein is conserved. The same applies to derivatives of nucleic acid sequences which can have substitutions, deletions, or additions of one or more nucleotides, provided that the biological activity of the sequence is generally maintained. When relating to a protein sequence, the substituting amino acid has chemico-physical properties which usually, but not necessarily, are similar to that of the substituted amino acid. The similar chemico-physical properties include, similarities in charge, bulkiness, hydrophobicity, hydrophilicity and the like. Some of the most commonly known conservative amino acid substitutions include, but are not limited to: 19 Leu or Val or lie; Gly or Ala; Asp or Glu; Asp or Asn or His; Glu or Gin; Lys or Arg; Phe or Trp or Tyr; Val or Ala; Cys or Ser; Thr or Ser; and Met or Leu. As used herein, the term "purified" refers to a molecule having been separated from other cellular or viral components. Thus, for example, a "purified protein" has been purified to a level not found in nature. The term "substantially purified" refers to a protein that is pure to about 60% or higher. The term " substantially pure" refers to a protein that is pure to about 80% or higher. The term "essentially pure" refers to a protein that is pure to about 90% or higher. Preferred embodiments In a particularly preferred embodiment, the method of purification of El protein comprises an incubation of a nuclei extract from eukaryotic expression system at a salt concentration lower than 300 mM, preferably from 0-100 mM, more preferably from 0-50 mM and most preferably in the absence of salt. Preferably, the salt refers to NaCl although other salts well known in the art (such as LiCl or KC1) may be used for nuclear extractions . In accordance with a further embodiment of the invention, there is provided a method as described above wherein the El protein is the El helicase from bovine papillomavirus (BPV) , cottontail rabbit papillomavirus (CRPV) or human papillomavirus (HPV) . In a preferred embodiment, the El protein is from HPV low risk or high risk types . 20 Preferably, when the El protein is a low risk type, it is selected from type 6, type 11 and type 13, and especially HPV type 11 and type 6. Alternatively, when the El protein is a high risk type, it is selected from the group consisting of types 16, 18, 31, 33, 35, 45, 52, or 58, preferably type 16. A further aspect of the present invention provides the method as described above wherein the eukaryotic expression system is selected from the group consisting of : baculovirus in insect cells; Vaccinia, Sindbis, and Semliki forest viruses, or Adenovirus in mammalian cells (such as COS or Vero cells) ; and plasmid in yeast expression systems, preferably a baculovirus in insect cells expression system. A further aspect of the present invention provides the method as described above wherein said El protein comprises an affinity label selected from the group consisting of: histidine tag, glutathione-S-transferase, and maltose-binding- protein and the complementary affinity ligand is selected from the group consisting of: antibody, nickel, maltose and glutathione columns . Preferably, the antibody column comprises monoclonal or polyclonal antibodies, more preferably monoclonal antibodies. Most preferably, the El protein is labeled with a histidine-tag and the His-labeled protein is separated on a nickel affinity ligand column. Preferably, the affinity label is positioned at one terminus of the El protein, more preferably at the N-terminus thereof . Still, a further aspect of the present invention provides a HPV El preparation prepared from low salt concentration, preferably extracted from a nuclei preparation in the presence of 0-100 mM NaCl, more preferably 0-50 mM 21 NaCl, most preferably in the absence of NaCl, and further purified with affinity chromatography. Preferably, the El preparation as described above is "substantially purified" at least about 60% purity and above, more preferably "substantially pure" at least about 80% purity and above, especially "essentially pure" at least about 90% purity. Preferably, the El preparation as described above is the El helicase from bovine papillomavirus (BPV) , cottontail rabbit papillomavirus (CRPV) or human papillomavirus (HPV) , preferably from HPV low risk or high risk type. Preferably, when the El protein is a low risk type, it is selected from type 6, type 11 and type 13, and especially HPV type 11 and type 6. Alternatively, when the El protein is a high risk type, it is selected from the group consisting of types 16, 18, 31, 33, 35, 45, 52, or 58, preferably, type 16. Methodology The recombinant DNA constructs in accordance with the present invention can be constructed using conventional molecular biology, microbiology, and recombinant DNA techniques well known to those of skilled in the art (i.e. Sambrook et al, 1989, supra) . With a suitable DNA construct transfected into a host cell, the present invention provides a method for the expression of a gene of interest. Alternatively, the DNA construct comprises a sequence coding for a affinity label, such as nucleotides coding for histidine (His) . Transfection of the DNA construct into a host cell provides a convenient means for expressing a fusion protein comprised of the polypeptide of interest and the affinity label, thus allowing the isolation of the expressed fusion product by an affinity ligand column complementary to the affinity label . 22 Construction and Expression We have used a particular version of the system from Gibco Lifesciences, in which the gene of interest is subcloned into a transfer vector which is then transformed into an E. coli strain containing a baculovirus genome. Specific sites on the vector then allow transposition which inserts the gene into the baculovirus genome (bacmid) . This recombinant bacmid can then be isolated and transfected into SF9 or SF21 insect cells, which then produce the protein of interest, as well as infectious virus which can be used in the future to produce the protein of interest. In other baculovirus systems, the gene of interest may be recombined into the baculovirus genome within the insect cell. This is done by transfecting insect cells with a vector containing the gene of interest and at the same time infecting them with baculovirus. In a certain percentage of the cases, the gene of interest is transferred to the viral genome by homologous recombination. Various methods well known in the art may be used to select for recombinant genomes carrying the gene of interest . Extraction and Purification The El protein of the invention can be purified using a specific protocol enabling it to be separated quickly and in a limited number of steps from the bulk of eukaryotic cellular and nuclear proteins and other viral contaminating components . Contrary to conventional wisdom suggesting that nucleic acid binding proteins are more soluble in high salt concentrations, it has been established by the Applicant that the El protein is quickly and efficiently separated from the bulk of nuclear proteins and DNA by a low salt extraction protocol . Without wishing to be bound by theory, it is hypothesized that, when suspended in a hypotonic salt 23 solution, the El protein leaches out selectively from the nucleus preparation. That is why the critical step of the invention comprises the low salt extraction of the El protein from nuclear extracts of HPV-infected baculovirus cell culture. One of the peculiar aspect of the extraction protocol relies in the incubation time in which the nuclear extraction is carried out in the low salt solution (30-40 min as opposed to 5-10 min for the cell lysis) . Indeed, in a preferred embodiment of the invention, the cell lysis buffer may also be hypotonic, however in this case it is important not to leave the lysed cells in the cell lysis buffer before the nuclei are centrifuged and separated to avoid El leaching in the lysis buffer prior to extraction. Although our experiments allowed us to extract El at a salt concentration up to 500 mM, it was shown that some contaminants are observed at that concentration. It is therefore preferred to use salt concentrations that are below 300 mM, and preferably salt concentrations equal or below isotonic salt concentrations (150 mM) such as 100 mM, more preferably 50 mM, and most preferably, the extraction is carried out in the absence of salt . Following the nuclear extraction, the El protein is preferably further purified via affinity chromatography. For such purposes the protein can be expressed as a fusion protein comprising an affinity label which is specifically trapped by a complementary affinity ligand optionally bound to chromatographic column media. The affinity label is preferably localized on the N-terminus of the protein Examples of pairs of affinity label/affinity ligand column include but are not limited to: Maltose-Binding Protein (MBP) /maltose column; Glutathione S Transferase (GST)/ 24 glutathione column; histidine (His)/ Ni column. In a preferred embodiment, the El is expressed as a His- El fusion protein and is purified through Ni column affinity chromatography according to methods well known in the art. Alternatively, the protein can also be trapped by polyclonal or monoclonal antibodies, in which case it does not need to be modified with a affinity label. For that purpose, an antiserum must be prepared. In general, techniques for preparing antibodies (including monoclonal antibodies and hybridomas) and for detecting antigens using antibodies are well known in the art (Campbell, 1984, In "Monoclonal Antibody Technology: Laboratory Techniques in Biochemistry and Molecular Biology" , Elsevier Science Publisher, Amsterdam, The Netherlands) and in Harlow et al . , 1988 (in: Antibody - A Laboratory Manual, CSH Laboratories) . In accordance with an additional aspect of the present invention, there is provided the means for detecting antiviral agents using assays for screening their level of inhibition against HPV. The effectiveness of the candidate agents can be assessed by their ability to inhibit the viral helicase activity. This can be accomplished directly, by measuring the level of inhibition on the viral helicase activity or indirectly, by assessing the disruption on the interaction between the El/E2/ori complex by measuring the inhibition on the viral DNA replication process . Methods for detecting such antiviral agents include, without limitations, the use of colorimetric, fluorescent or radioactive reagents. Such detection methods can be applied in several types of assays such as culture plate assays or gel-based assays including for example Enzyme-Linked Immunosorbent Assay (ELISA) , or Scintillation Proximity Assay 25 (SPA) or any other assay well known in the art. In one particular embodiment, there is provided an assay for screening and identifying candidate agents which modulate the helicase activity of El, and more particularly the El helicase activity of HPV type 11 and type 6. A preferred embodiment of such assay relies in a high throughput screening (HTS) assay for candidate agents capable of inhibiting El helicase activity and to identify such agents . Such high throughput screening assay is preferably selected from a fluorescence assay or a scintillation proximity assay, more preferably the latter. In this assay, the duplex DNA substrate consists of M13 single-stranded DNA (about 8000 bases) to which is annealed a 19 base oligodeoxynucleotide (see Figure 6) . This partial duplex is extended to 24 bases with the incorporation of [ 33 P] - labeled dATP by a reaction with the Klenow fragment of DNA polymerase I. Helicase activity results in the separation of this radiolabeled oligo from the M13 DNA. In the absence of a functional helicase, the double-stranded DNA substrate is stable for several hours at the assay temperature. To detect activity, a second 24-base deoxyoligonucleotide, complementary to the substrate oligo, is added to the reaction mixture in a second step. This oligo anneals to any free radiolabeled oligo, but cannot interact with oligo still annealed to M13 DNA. A biotin is covalently attached to the 5 '-terminus of the second oligo. In the third step, streptavidin-coated SPA beads (Amersham Life Science, code TRKQ7030) are added to the mixture. The biotinylated oligo and any associated radiolabeled oligo then bind to these beads. SPA beads are impregnated with a scintillant, which allows detection of radiolabel in close proximity to the beads . Thus radiolabeled 26 oligo annealed to the biotinylated oligo will be detected, whereas unreacted substrate still hybridized to M13 is not in close proximity to the beads and will not be detected. In the presence of an inhibitor, less substrate is unwound, so a lower signal is detected. Positive controls used for the validation of this assay may be, among others cold substrate (such as the M13 single-stranded DNA) or DNA intercalators (such as ethidium bromide) . The M13 DNA competes with the labeled M13 substrate and inhibits the signal detected. Ethidium bromide is a recognized DNA intercalator and stabilizes the M13-oligo substrate, thereby preventing helicase activity. EXAMPLES The present invention is illustrated in further detail by the following non-limiting examples. EXAMPLE 1: El EXPRESSION Construction of recombinant plasmid Recombinant baculovirus construct (Bac-to-Bac™ Baculovirus Expression Systems) (Gibco BRL) : The El gene from HPV type 11 was pcr-amplified using recombinant plasmid pCR3-El as DNA template according to Lu et al . (1993, J. Virology 67:7131-7138). The forward primer was 5'-CGC GGA TCC AGG ATG CAT CAC CAT CAC CAT CAC GCG GAC GAT TCA CGT ACA GAA AAT GAG-3 ' (SEQ ID NO. 1) and the reverse was GG CTG AAT TCA TAA AGT TCT AAC AAC T (SEQ ID NO. 2) . Purified per products were then restricted with EcoRI and BamHI and ligated with donor plasmid pFASTBACl™ (Gibco, BRL) which had been linearized with the same enzymes. HPV-11 El protein expression His-El-pFASTBAC was then transformed into E. coli strain DHlOBac™ for transposition following the manufacturer's 27 instructions (Gibco-BRL) . White colonies were selected and transposition confirmed by analytical per using primers flanking the bacmid (baculovirus circular DNA) insertion site. Mini-preparation of recombinant bacmids was carried out and the purified bacmid DNA transfected into SF9 cells. Baculovirus-containing supematants were collected 72 h post- transfection, and infected cells resuspended in 2X Leammli buffer for expression analysis by Western using anti-El K72 polyclonal antibody (see description of E2-dependent El-DNA binding assay, Example 8) . Recombinant baculovirus, confirmed to express His-El protein was reamplified and further used to infect SF21 insect cells for large scale production. EXAMPLE 2: HPV-11 His-El extraction using different concentrations of salt El extraction. SF21 insect cells infected with El-pFASTBAC recombinant baculovirus were harvested from 425 ml culture in SF-900 II SFM medium to give a cell pellet of 5 ml which has been frozen rapidly in dry ice. Frozen pellet was then thawed rapidly and cells resuspended in 5 ml of cell lysis buffer A (20 mM tris, pH 8.0, 1 mM DTT, 1 mM EDTA, 5 mM KC1, ImM MgCl 2 - antipain, leupeptin and pepstatin each at 1 μg/ml - 1 mM Pefabloc™) . Following 15 min incubation on ice, cells were broken with a Dounce homogenizer («5 min, pestle B) and then centrifuged at 2500 g, 20 min, 4°. Pelleted nuclei were resuspended to 7 ml with resuspension buffer (20 mM Tris, pH 8.0, 1 mM DTT, 1 mM EDTA, antipain, leupeptin and pepstatin each at 2 μg/ml, 2 mM Pefabloc™) and distributed in 0,5 ml aliquots in 14 tubes. 0,5 ml of 13 different 2X extraction buffers (at varying concentrations of salt and detergents) were then mixed separately to 13 aliquots of nuclei, by pipetting up and down to give the final conditions listed below. Samples were incubated at 4° with rocking for 30 min 28 and centrifuged in a microcentrifuge at maximal speed for 30 min. Supematants were finally recovered and 4 μl of each run in 10% SDS-PAGE. 1 gel was stained with Coomassie Blue (Figure 3A) and another one transferred for the membrane to be hybridized with anti-El K72 polyclonal antibody and detected with "western blot chemiluminescent reagent" (DuPont NEN, Boston, MA) and the emitted light was captured on autoradiography film (Figure 3B) . Figures 3A and 3B Legend: Lane 0: 10 mM Tris, pH 8,0; 0 , 5 mM DTT; 0,5 mM EDTA Lane 1: 20 mM tris, pH 8 , 0 ; 1 mM DTT; 0,5 mM EDTA Lane 2: #1 + 100 mM NaCl Lane 3: #1 + 450 mM NaCl Lane 4: #1 + 0,01% Triton X-100 Lane 5: #1 + 0,01% Triton + 100 mM NaCl Lane 6: #1 + 0,01% Triton + 450 mM NaCl Lane 7: #1 + 0,1% Triton Lane 8: #1 + 0,1% Triton + 100 mM NaCl Lane 9: #1 + 0,1% Triton + 450 mM NaCl Lane 10: #1 + 10% glycerol Lane 11: #1 + 10% glycerol + 100 mM NaCl Lane 12: #1 + 10% glycerol + 450 mM NaCl Lanes indicated 50, 100, and 200 μg were samples of El fragment from E. coli that were used as positive control for the K72 antibody immunoblot. Figures 3A and 3B show that the extraction of El from the nuclei preparation is not greatly improved by the use of detergent (lanes 4 to 12) . As salt concentrations increase, more contaminants leach out of the nuclei . In absence of salt, almost all of the El protein is already extracted, and 100 mM does not show more El extracted. At 450 mM salt, the gel shows more contaminants and some degradation of the El protein. 29 EXAMPLE 3: HPV-11 His-El extraction Cells infected with recombinant baculovirus were harvested and frozen rapidly in liquid nitrogen before being stored at -80°. For nuclear extraction, frozen cell pellets were thawed and resuspended in 1 volume (relative to the volume of cell pellet) of cell lysis buffer B containing protease-inhibitors (20 mM Tris pH 8, 5 mM β-mercaptoethanol , 5 mM KCl, 1 mM MgCl 2 , 1 mM Pefabloc™, 1 μg/ml pepstatin, 1 μg/ml leupeptin/ and 1 μg/ml antipain) and left on ice for 15 min. Cells were then broken on ice with a Dounce homogenizer («5 min, pestle B) followed by centrifugation at 2500g, 4° for 20 min. Supernatant (cytosol) was discarded and nuclei resuspended to 1.4 volume with extraction buffer A (20 mM Tris pH 8, 5 mM β-mercaptoethanol, 2 mM Pefabloc™, 2 μg/ml pepstatin, 2 μg/ml leupeptin, and 2 μg/ml antipain) . Finally, 1.4 volume of extraction buffer B (20 mM Tris pH 8, 5 mM β- mercaptoethanol , and 0.02% Triton X-100) was added and the nuclei incubated at 4° with rocking for 30 min before ultracentrifugation at 148,000g, 4° for 45 min. Glycerol was added to the supernatant to 10% final concentration and the extract was frozen rapidly on dry ice and stored at -80°. EXAMPLE 4: HPV-11 His-El purification Nuclear extracts were thawed rapidly and the NaCl concentration adjusted to 500 mM before the preparation was loaded on 5 ml Hi-Trap™ chelating column previously charged with NiS0 4 according to the Manufacturer's instructions (Pharmacia, Biotech) . The column was then pre-equilibrated in equilibration buffer (20 mM Tris pH 8, 5 mM β-mercaptoethanol, 500 mM NaCl, 10 mM imidazole, and 10% glycerol) and the flow- through collected for analysis. The column was washed first with 10 volumes of equilibration buffer and then with 10 volumes of washing buffer (equilibration buffer but with 50 mM 30 imidazole) before His-El (or mutant proteins) was eluted (1 mL fractions 1 to 10) with elution buffer (equilibration buffer but with 180 mM imidazole) . El proteins were then dialyzed in dialysis buffer (20 mM MES pH 7.0, 500 mM NaCl, 1 mM DTT, 0.05 mM EDTA, and 10 % glycerol) before being frozen on dry ice and stored at -80°. As an example of the yields obtained from this preparation, one 10 L preparation gave a 10 mL solution of purified El at 30 μg/mL (3 mg protein total) . Legend of Figure 4B: A: total load of the column; B: flow-through; C: equilibration with 10 mM imidazole; D: washing with 50 mM imidazole; Lanes 1 to 10 represent 1 mL fractions eluted with the elution buffer (180 mM imidazole) . Figure 4B shows a Coomassie blue-stained gel where fractions 2, 3, and 4 contain most of the essentially pure El protein. EXAMPLE 5: Mutation analysis of HPV-11 El helicase Mutant El proteins were made by disabling the helicase active site to validate that the helicase activity observed was due to the El protein and not to contaminants co-purified with El. Mutant plasmids encoding the K484A, K484H, K484I and, K484R mutations were constructed using the QuickChange™ site- directed mutagenesis kit from Stratagene using the protocol supplied by the manufacturer. The template for mutagenesis was the El DNA sequence carrying the K484E mutation. This mutant DNA template was used instead of wild type El because the K484E mutation 31 creates a restriction site. This allowed us to identify quickly clones which carried the K484A, -H, -I, and -R mutations by simply screening for loss of the restriction site. The K484E mutation differed from the wild type El DNA sequence in the following way: WT El 5'-CCTGACACTGGGAAGTCGTGCTTTTGC-3 ' (SEQ ID NO. 3) K484E 5 ' -CCTGACACTGGGGAGTCGTGCTTTTGC-3 ' (SEQ ID NO. 4) (GAGTC is a site cut by the Pie 1 enzyme) Pairs of complementary primers used for mutagenesis were: K484A MUT-TOP 5 ' -CCTGACACTGGGGCGTCGTGCTTTTGC-3 ' (SEQ ID NO 5) MUT-BOT 5 ' -GCAAAAGCACGACGCCCCAGTGTCAGG-3 ' (SEQ ID NO. 22) K484H MUT-TOP 5 ' -CCTGACACTGGGCACTCGTGCTTTTGC-3 ' (SEQ ID NO. 6) MUT-BOT 5 ' -GCAAAAGCACGAGTGCCCAGTGTCAGG-3 » (SEQ ID NO. 23) K484I MUT-TOP 5 ' -CCTGACACTGGGATCTCGTGCTTTTGC-3 ' (SEQ ID NO. 7) MUT-BOT 5 ' -GCAAAAGCACGAGATCCCAGTGTCAGG-3 ' (SEQ ID NO. 24) 32 K484R MUT-TOP 5 ' -CCTGACACTGGGCGGTCGTGCTTTTGC-3 ' (SEQ ID NO 8) MUT-BOT 5 ' -GCAAAAGCACGACCGCCCAGTGTCAGG-3 » (SEQ ID NO 25) The subcloning of these mutant alleles in baculovirus was amplified by per using the same primers as described above . All His-El-K484A, -H, -I, and -R constructs were cloned into the same pFASTBac™ vector, transformed into E. coli DHlOBac™ plasmids according to Example 1. The resulting bacmids were transfected in SF9 cells, and the recombinant viruses infected in SF21 cells also according to Example 1. EXAMPLE 6: HPV-11 E2 protein expression HPV- 11 E2 was obtained by expression in baculovirus- infected insect cells. A baculovirus encoding the gene for HPV-11 E2 was obtained from R. Rose (U. Rochester, N.Y.) and used to infect SF21 insect cells. Infected cells were resuspended in cell lysis buffer C (30 mM HEPES pH 7.6, 1 mM EDTA, 2 mM DTT, 1% NP-40, and protease inhibitors: 1 mM Pefabloc™, 1 mM PMSF, and 2.5 μg/mL each antipain, leupeptin, and pepstatin) . Lysis occurred on stirring the cells, and nuclei were recovered by centrifugation. Nuclei were resuspended in nuclear extraction buffer C (30 mM HEPES pH 7.6, 10% glycerol, 250 mM NaCl, 5 mM EDTA, 2 mM DTT, 0.5% NP- 40, and the same protease inhibitors as above) . The suspension was stirred for 45 min, then sonicated. E2 was recovered in the supernatant following centrifugation. EXAMPLE 7: Purification of HPV-11 E2 E2 was purified from the nuclear extract using a DNA affinity chromatography by a procedure based on that of Seo et 33 al . (PNAS .90.(93) 2865) . To prepare the affinity ligand column, duplex DNA containing three E2 binding sites was prepared by annealing two oligos (5 ' -biotin-AGT GAC CGA AAA CGG TCG GGA CCG AAA ACG GTG TAG ACC GAA AAC GGT GTA-3 ' (SEQ ID NO. 9) and 5'-CTA CAC CGT TTT CGG TCT ACA CCG TTT TCG GTC CCG ACC GTT TTC GGT CAC T-3' (SEQ ID NO. 10)) . The duplex was bound to streptavidin agarose by virtue of the biotin incorporated into the first oligo. Chromatography was carried out using elution buffer D (nuclear extraction buffer C without protease inhibitors) and elution buffer E (elution buffer D plus IM NaCl) . A typical column consisted of 10 mL of the above resin. The nuclear extract was centrifuged at 50,000 g for 20 min. to remove any precipitated material, then applied to the column, washed with elution buffer D until the absorbance of the eluent at 280 nm reached baseline, then eluted with a linear gradient of elution buffers D and E, 60 mL of each. Fractions containing pure E2 (by SDS-PAGE) were pooled and concentrated to approximately 150 μg/mL using a Millipore centrifugal filter device (Ultrafree-15™) , then stored at -80°. EXAMPLE 8: E2-dependent El DNA binding assay This assay was modeled on a similar assay for SV40 T Antigen described by McKay (J. Mol . Biol., 1981,145:471). A 400bp radiolabeled DNA probe, containing the HPV-11 origin of replication (Chiang et al . , 1992, Proc. Natl. Acad. Sci. USA 89:5799) was produced by per, using plasmid pBluescript™ SK encoding the origin (nucleotides 7886-61 of the HPV-11 genome in unique BAMHI site) as template and primers flanking the origin. Radiolabel was incorporated as [ 33 P]dCTP. Binding assay buffer consisted of: 20 mM Tris pH 7.6, 100 mM NaCl, 1 mM DTT, 1 mM EDTA. Other reagents used were protein A-SPA beads (type II, Amersham) and K72 rabbit polyclonal antiserum, raised against 34 a peptide corresponding to the C-terminal 14 amino acids of HPV-11 El. Following the protocol from Amersham, one bottle of beads was mixed with 25 mL of binding assay buffer. For the assay, a saturating amount of K72 antiserum was added to the beads and the mixture was incubated for 1 h, washed with one volume of binding assay buffer, and then resuspended in the same volume of fresh binding assay buffer. Binding reactions contained 8 ng of E2, approximately 100-200 ng of purified El, and 0.4 ng of radiolabeled probe in a total of 80 μL of binding assay buffer. After 1 h at room temperature, 25 μL of K72 antibody-SPA bead suspension was added to with the binding reaction and mixed. After an additional hour of incubation at room temperature, the reactions were centrifuged briefly to pellet the beads and the extent of complex formation was determined by scintillation counting on a Packard TopCount™. Typically, the signal for reactions containing El and E2 was 20-30 fold higher than the background observed when either El, E2, or both was omitted. Figure 5A shows the DNA binding activity of the E1/E2 complex of the wild type (wt) El helicase and the four mutants produced in Example 5. There was no significant difference in the El/E2/ori binding between any of the proteins indicating that the mutant proteins were folding in a normal fashion. EXAMPLE 9: HELICASE/ ATPase ASSAYS Helicase/ATPase assays The substrate for the analytical helicase assay consisted of a 24-base oligonucleotide (GTA AAA CGA CCA GTG CCA AGO (SEQ ID NO. 11) end-labeled using [ 33 P]ATP and polynucleotide kinase, annealed to M13mpl8. Combined helicase/ATPase reactions contained 800 or 1600 ng of El, 2 mM MgCl 2 , 1 mM ATP, and 1 μM helicase substrate (concentration in nucleotides) in a total volume of 80 μL of helicase assay 35 buffer (20mM MES, pH 7.0, ImM DTT, 0.05mM EDTA, 10% glycerol). Reactions were incubated for 2 h at 37° and then placed on ice. Helicase gel-based detection: 25 μL of each reaction was mixed with 5x helicase stop/loading solution (12.5% Ficoll 4000, 0.5% SDS, 50 mM EDTA, and 0.125% each bromophenol blue and xylene cyanol) ; 20 μL of the mixture was electrophoresed for 1 h at 125 V through a 20% polyacrylamide/lx TBE gel. Blank reactions containing no enzyme were run in parallel. The gel was dried and scanned on a Molecular Dynamics Phosphorlmager™. The substrate and reaction product separated by size, with the substrate remaining at the top of the gel and the unwound radiolabeled oligonucleotide migrating approximately half-way down. In some cases degradation products due to nuclease activity are apparent further down the gel . Figure 5B top panel shows the gel migration of the helicase substrate and product after incubation with the wild- type (wt) and the mutants El proteins. Lane 13 is a boiled sample, lane 12 is a blank, whereas lane 11 is a blank which has been incubated for 2 h at 37°. As apparent from Figure 5B, none of the mutants show any significant helicase activity compared to the wild-type El protein. The intensity of the unwound oligonucleotide band may be quantitated using the Phosphorlmager™, and the amount of activity may be expressed relative to 100% unwinding as described below for the SPA. ATPase assay An additional 15 μL of each reaction was used to detect ATPase activity by the procedure of Lanzetta et al . (Anal. 36 Biochem . , 1979 , 100 , 95 ) . Figure 5B middle panel shows that the ATPase activity follows the helicase activity as demonstrated by the gel assay. Helicase Scintillation Proximity Detection (SPA) An additional 30 μL was transferred to another 96-well plate containing 30μL of SPA stop hybridization buffer, which is identical to the "stop" buffer in Example 11, except that the biotinylated capture oligonucleotide is complementary to the substrate sequence above. Helicase activity was quantitated as follows: A separate reaction mixture, containing no enzyme was heated to 95° for 10 min. , and the resulting free substrate oligonucleotide (completely denatured) was detected as described for reaction mixtures. The level of signal generated in this experiment represents 100% unwinding. Similar samples, which were not heated, serve a blanks, representing background signal . Quantitation of unwinding is calculated relative to the boiled sample with background subtracted. Figure 5B bottom panel shows that the percent of unwinding is negligible with the mutant proteins as compared to the wild-type (wt) control. These results are in accordance with the ones obtained from the gel-based assay. 37 EXAMPLE 10: ENZYMATIC ACTIVITY His-El helicase specific activity Enzymatic Activities of HPV-11 El -Comparison to Literature TABLE 1 Helicase V (% unwinding/ Reference Activity μM protein/ min) HPV-11 El 2.5 this work HPV-6a El 2.3 this work SV-40 TAg 5.0 this work BPV El 2.9 Yang PNAS (1993) Table 1 compares the enzymatic activity of the helicase from HPV-11 and -6 as purified according to the present invention, to another helicase reported in the literature. This represents the first instance where the human papillomavirus helicase El is purified to an extent where its unwinding activity can be quantified. In all cases the enzyme concentration was greater than the substrate concentration and the substrate was partial duplex DNA. All experiments were done at 37° except for the BPV-El which was assayed at 32°. The SV-40 TAg was also assessed in the literature and V of 50 and 80 % unwinding/μM protein/min were obtained from these groups respectively (Goetz, JBC (1988); Stahl, EMBO (1986)). The extent of the difference with our results stem from the fact that our assay conditions were optimized for El activity and may not be optimal for TAg activity (lower pH, etc..) . 38 EXAMPLE 11: High-throughput screening assay SPA references : N. Bosworth, P. Towers, "Scintillation proximity assay" Nature 341, 167-168 (1989) . N.D. Cook, "Scintillation proximity assay- a versatile high throughput screening technology" Drug Discovery Today, 1, 287-294 (1996) . "Determination of DNA helicase activity using a [ 3 H] scintillation proximity assay (SPA) system" Proximity News, July 1996. This assay is similar to that in Example 9. The radiolabeled DNA substrate for this assay consists of a 19- base oligonucleotide (TTC CCA GTC ACG ACG TTG T) (SEQ ID NO 12) annealed to single-stranded M13mpl8 plasmid. The Klenow fragment is used to extend the partial duplex to 24 bases, using four [ 33 P] dATP and one unlabeled dCTP. Helicase reactions are run by mixing 10 μL each of the following components : 1) a substrate cocktail comprising radiolabeled DNA substrate, ATP, and magnesium acetate; 2) inhibitors dissolved in buffer plus 18% DMSO; 3) HPV-11 El purified as in Example 4. Assay buffer, used for all dilutions, consisted of 20 mM MES, pH 7.0, 10% glycerol, 1.0 mM DTT, and 0.05 mM EDTA. Final concentrations in the assay are 0.8 μM (concentration in nucleotides), 1.0 mM ATP, 1.0 mM magnesium acetate, 6% DMSO. Sufficient El is used to give approximately 20% unwinding (as determined in Example 9) . Reaction mixtures are incubated at 37° for 2 h in Microfluor® 96-well plates (Dynex) . 30 μL of a "stop" buffer is then added, which consists of 100 mM HEPES, pH 7.5, 300 mM NaCl, 20 mM EDTA, 1% SDS, and a biotinylated 39 oligonucleotide (complementary to the substrate oligonucleotide) at 20 nM. After 1.5 h at room temperature, 50 μL of a suspension of streptavidin-coated polyvinyl toluene SPA beads (1.25 mg/mL in 50 mM HEPES, pH 7.5, 0.02% NaN 3 ) is added, followed by a further 0.5 h incubation at room temperature. Assay plates are then centrifuged briefly to pellet the SPA beads and the amount of reaction product is detected by scintillation counting using a Packard Topcount™ . EXAMPLE 12: Inhibition of El helicase activity (IC 50 curves) To determine the potency of potential inhibitors, El helicase SPA reactions (Example 11) were run in the presence of serially diluted inhibitors. The concentrations of both M13 and ethidium bromide ranged from 0.04 to 20 μM. Reaction controls with no inhibitor, and blanks with no inhibitor and no enzyme, were run simultaneously. Unwinding was detected as described above and results were fit to a logistic using the SAS software package. [SAS is a registered trademark of the SAS Institute, Inc. of Cary, North Carolina] For both Figures 7 and 8, data points are graphed as the percent inhibition at each inhibitor concentration. Concentration is expressed in μM on a log scale. Percent inhibition at each inhibitor concentration ( [I] ) is determined from the following formula: 100 - 1 00 x (activitv a t Til -blank ) (control activity-blank) The solid line shows the best fit to the data determined by SAS. Some data points are out of range and are not shown in the figures . From Figures 7 and 8, it can be approximated that the IC 50 are 3 and 4 μM respectively for M13 and ethidium bromide. 40 EXAMPLE 13: HPV-6 El EXPRESSION Construction of recombinant plasmid Recombinant baculovirus construct (Bac-to-Bac system) : El gene from HPV type 6a was PCR-amplified using recombinant plasmid pCR3.1-E1 (6a) as DNA template previously constructed in our lab from DNA isolated from a clinical sample. The forward primer was 5'-CGC GGA TCC AGG ATG CAT CAC CAT CAC CAT CACGCG GAC GAT TCA CGT ACA GAA AAT GAG 3' (SEQ ID NO.l ) and the reverse one was GG CTG AAT TCA TAA AGT TCT AAC AAC T (SEQ ID NO.2) . The resulting PCR fragment was then purified and restricted with EcoRI and BamHI and ligated with donor plasmid pFASTBACl linearized with the same enzymes. HPV-6 El protein expression HIS-El-pFASTBAC was then transformed in E. coli DHlOBac® strain for transposition following the manufacturer's instructions (Gibco-BRL) . White colonies were first selected and transposition confirmed by analytical PCR using primers flanking the insertion site in bacmid (baculovirus circular DNA) . Mini-preparation of recombinant bacmids was conducted and then transfected in SF9 cell. 72 h post-transfection, baculovirus-containing supematants were collected and infected cells resuspended in 2X Leammli buffer for expression analysis by Western using K72 polyclonal antibody. Recombinant baculovirus confirmed to express E1HIS was reamplified and further used to infect SF21 cells for large scale production. EXAMPLE 14: HPV-11 and HPV-6a His-El extraction using different concentrations of salt El extraction. SF21 insect cells infected with El-pFASTBAC recombinant baculovirus were harvested from 5 L culture in SF- 900 II SFM medium to give a cell pellet of 65 ml which has 41 been frozen rapidly in dry ice. Frozen pellet was then thawed rapidly and cells resuspended in 65 ml of cell lysis buffer A (20 mM tris, pH 8.0, 1 mM DTT, 1 mM EDTA, 5 mM KCl, ImM MgCl 2 - antipain, leupeptin and pepstatin each at 1 μg/ml - 1 mM Pefabloc™) . Following 15 min incubation on ice, cells were broken with a Dounce homogenizer («5 min, pestle B) and then centrifuged at 2500 g, 20 min, 4°. Supernatant was re- centrifuged at 148 000 g, 4° for 45 min and this second supernatant was kept as "cytosol" . Glycerol was added to this supernatant to 10% final concentration and this sample was frozen rapidly on dry ice and stored at -80°. Pelleted nuclei were resuspended to 90 ml with extraction buffer A (20 mM Tris pH 8, 5 mM β-mercaptoethanol, 2 mM Pefabloc™, 2 μg/ml pepstatin, 2 μg/ml leupeptin, and 2 μg/ml antipain) and distributed in 18 ml aliquots in 5 tubes. 18 ml of extraction buffer B (20 mM Tris pH 8, 5 mM β-mercaptoethanol, and 0.02% Triton X-100) was added and NaCl concentration was adjusted with a 5M solution to the conditions listed below. Samples were incubated at 4° with rocking for 30 min and centrifuged at 148000 g, 4° for 45 min. Supematants were finally recovered and glycerol was added to the supernatant to 10% final concentration and the extract was frozen rapidly on dry ice and stored at -80°. Example 15: HPV-11 and HPV-6a His-El purification. Nuclear extracts were thawed rapidly and the NaCl concentration adjusted to 500 mM before the preparation was loaded on 1 ml Hi-Trap™ chelating column previously charged with NiS0 4 according to the Manufacturer's instructions (Pharmacia, Biotech) . The column was then pre-equilibrated in equilibration buffer (20 mM Tris pH 8, 5 mM β-mercaptoethanol, 500 mM NaCl, 10 mM imidazole, and 10% glycerol) and the flow- through collected for analysis. The column was washed first 42 with 5-6 volumes of equilibration buffer and then with 5-6 volumes of washing buffer (equilibration buffer but with 50 mM imidazole) before His-El was eluted (1 mL fractions 1 to 10) with elution buffer (equilibration buffer but with 180 mM imidazole) . El proteins were then dialyzed in dialysis buffer (20 mM MES pH 7.0, 500 mM NaCl, 1 mM DTT, 0.05 mM EDTA, and 10 % glycerol) before being frozen on dry ice and stored at -80°. For load and flow-through samples, 4 μl of each fractions were run on 10% SDS-PAGE) . For each comparison experiment, 1 gel was stained with Coomassie Blue (Figure 9A) and another one transferred for the membrane to be hybridized with anti-El K72 polyclonal antibody and detected with "western blot chemiluminescent reagent" (DuPont NEN, Boston, MA) and the emitted light was captured on autoradiography film (Figure 9B) . Legends of Figures 9A (Coomassie) and 9B (Western blot) : A: load on Hi-Trap column (Crude Extract) B: Flow through from Hi-Trap column Lane 1: HPV-11 El extracted in absence of NaCl Lane 2: HPV-11 El extracted with 50mM NaCl Lane 3: HPV-11 El extracted with lOOmM NaCl Lane 4: HPV-11 El extracted with 250mM NaCl Lane 5: HPV-11 El extracted with 500mM NaCl Lane 6: HPV-11 El extracted from cytosol Figure 9A shows a Coomassie blue-stained gel of crude El extract obtained from the protocol of Example 14. Lanes 4 and 5 reveal that there is a lot more material extracted at 250 and 500 mM salt but most of that material is not retained on the column, indicating that the majority of the material extracted at these salt concentrations is not El. Figure 9B allowed us to see that most of the El bound to the column. Once again the results of this experiment are in 43 agreement with example 4. Bradford protein assay was performed on elution fractions and SDS-PAGE and western blot of purified El were done with a content amount of total protein (lμg for SDS-PAGE; 0.2 μg for western) (Figures 10A, 10B) . Figures 10A Legend: SDS-PAGE purified El Lane 1: HPV-11 El extracted in absence of NaCl Lane 2: HPV-11 El extracted with 50mM NaCl Lane 3: HPV- 11 El extracted with lOOmM NaCl Lane 4: HPV-11 El extracted with 250mM NaCl Lane 5: HPV- 11 El extracted with 500mM NaCl Lane 6: HPV-11 El extracted from cytosol Lane 7: HPV-6a El extracted in absence of NaCl Figures 10B Legend: Western Blot of purified El Lane 1: HPV-11 El extracted in absence of NaCl Lane 2: HPV-11 El extracted with 50mM NaCl Lane 3: HPV-11 El extracted with lOOmM NaCl Lane 4: HPV-11 El extracted with 250mM NaCl Lane 5: HPV-11 El extracted with 500mM NaCl Lane 6: HPV-11 El extracted from cytosol Lane 7: HPV-6a El extracted in absence of NaCl Figures 10A and 10B reproduce and extend the results of example 3 where, as salt concentrations increase, the nuclear preparation is less pure and the preparation from the column is also less pure. In absence of salt and at 50 mM almost all of the El protein is already extracted. Concentrations of 100 mM and over do not improve the extraction of El. Lane 6 also shows clearly that the extraction and purification of HPV-6 El is as effective in the absence of salt. 44 It was therefore established that the conditions for routine extraction would be performed at a salt concentration equal or lower than 300 mM for optimal results, preferably in hypotonic conditions equal or lower than 100 mM, more preferably equal or lower than 50 mM, and most preferably in the absence thereof . The El proteins from HPV-11 and HPV-6a were sequenced and showed minor amino acid changes from the published literature. Our sequences are presented in Figure 11 (as SEQ ID NO.26) for HPV-11, and in Figure 12 (SEQ ID NO.27) for HPV- 6a.

Claims: 45 CLAIMS 1. A method for isolating the papillomavirus El protein or a functional derivative thereof having viral helicase activity comprising the steps of: - producing an El recombinant protein in a eukaryotic expression system and isolating a nuclei preparation thereof; and - extracting El protein from said nuclei preparation in a buffer comprising salt at a concentration lower than 300 mM. 2. The method of claim 1, further comprising the step of: - purifying El protein from said nuclear extract by affinity chromatography. 3. The method of claim 1, wherein said HPV El is purified from a nuclei preparation in the presence of 0-100 mM salt. 4. The method of claim 1, wherein said HPV El is purified from a nuclei preparation in the presence of 0-50 mM salt. 5. The method of claim 1, wherein said HPV El is purified from a nuclei preparation in the absence of salt. 6. The method according to claim 1, wherein said salt is NaCl. 7. The method of claim 1, wherein said El protein is the El helicase from cottontail rabbit papillomavirus (CRPV) , bovine papillomavirus (BPV) or human papillomavirus (HPV) . 8. The method of claim 7, wherein said El protein is from HPV low risk or high risk types. 46 9. The method of claim 7, wherein said recombinant HPV El protein is from a low risk type HPV selected from the group consisting of: type 6, type 11 and type 13. 10. The method of claim 7, wherein said low risk HPV is type 11 or type 6. 11. The method of claim 8 , wherein said recombinant HPV El protein is from a high risk type HPV selected from the group consisting of types 16, 18, 31, 33, 35, 45, 52, or 58. 12. The method of claim 11, wherein said high risk HPV is type 16. 13. The method of claim 1, wherein said eukaryotic expression system is selected from the group consisting of: baculovirus in insect cells; Vaccinia, Sindbis, Semliki forest viruses, or Adenovirus in mammalian cells; and plasmid in yeast expression systems . 14. The method of claim 13, wherein said eukaryotic expression system is insect cells infected with a baculovirus . 15. The method of claim 1, wherein said El protein comprises an affinity label . 16. The method of claim 15, wherein said affinity label is selected from the group consisting of: histidine tag, glutathione-S-transferase, and maltose-binding-protein. 17. ' The method of claim 16, wherein said affinity label is recognized by an affinity ligand selected from the group consisting of: antibody, metal, maltose and glutathione. 47 18. The method of claim 17, wherein said affinity label is positioned at the N-terminus of said El protein. 19. The method of claim 17, wherein said antibody is a monoclonal or a polyclonal antibody. 20. The method of claim 17, wherein said El protein is labeled with a histidine-tag and said metal affinity ligand is a nickel column. 21. A preparation of recombinant papillomavirus El protein from a eukaryotic expression system, said El having viral helicase activity, wherein the El protein is extracted from a nuclei preparation in the presence of salt at a concentration less than 300 mM, and purified by affinity chromatography. 22. The HPV El preparation of claim 21, wherein said HPV El is purified from a nuclei preparation in the presence of 0- 100 mM salt. 23. The HPV El preparation of claim 22, wherein said HPV El is purified from a nuclei preparation in the presence of 0- 50 mM salt. 24. The HPV El preparation of claim 23 , wherein said HPV El is purified from a nuclei preparation in the absence of salt. 25. The El preparation of claim 20, wherein said El protein is substantially purified. 26. The El preparation of claim 20, wherein said El protein is substantially pure. 27. The El preparation of claim 20, wherein said El protein is essentially pure. 48 28. The El preparation of claim 20, wherein said El protein is the El helicase from cottontail rabbit papillomavirus (CRPV) , bovine papillomavirus (BPV) or human papillomavirus (HPV) . 29. The El preparation of claim 28, wherein said El protein is from HPV low risk or high risk type. 30. The HPV El preparation of claim 29, wherein said recombinant HPV El protein is from a low risk type HPV selected from the group consisting of: type 6, type 11 and type 13. 31. The HPV El preparation of claim 30, wherein said low risk HPV is type 11 and type 6. 32. The HPV El preparation of claim 29, wherein said recombinant HPV El protein is from a high risk type HPV selected from the group consisting of: of types 16, 18, 31, 33, 35, 45, 52, or 58. 33. The HPV El preparation of claim 32, wherein said high risk HPV is type 16. 34. The HPV El preparation of claim 21, wherein said El protein is selected from the group consisting of: SEQ ID NO. 13; SEQ ID NO. 14; SEQ ID NO. 15; SEQ ID NO. 16; SEQ ID NO. 17; SEQ ID NO. 18; SEQ ID NO. 19; SEQ ID NO. 20; SEQ ID NO. 26; and SEQ ID NO. 27. 35. A method for assaying the specific viral helicase activity of papillomavirus El protein, said method comprising the steps of: -incubating a mixture of said El protein preparation according to claim 1, and a suitable substrate for said 49 viral helicase enzymatic activity; and - measuring the amount of specific helicase activity of said El protein. 36. The assay according to claim 35, wherein said papillomavirus is HPV-11 or HPV-6. 37. The assay of claim 35, wherein said El protein is selected from the group consisting of: SEQ ID NO. 13; SEQ ID NO. 14; SEQ ID NO. 15; SEQ ID NO. 16; SEQ ID NO. 17; SEQ ID NO. 18; SEQ ID NO. 19; SEQ ID NO. 20; SEQ ID NO. 26; and SEQ ID NO. 27. 38. A method for identifying agents capable of modulating said helicase activity, said method comprising the steps of: a) assaying the activity of said El helicase in the absence of said agent by the method according to claim 35; b) assaying the activity of said helicase in the presence of said agent by the method of claim 35, wherein said agent is added to said helicase and substrate mixture during said incubation; and c) comparing the result of step a) with the result of step b) . 39. The assay according to claim 38, wherein said papillomavirus is HPV-11 or HPV-6. 40. The assay of claim 38, wherein said El protein is selected from the group consisting of: SEQ ID NO. 13; SEQ ID NO. 14; SEQ ID NO. 15; SEQ ID NO. 16; SEQ ID NO. 17; SEQ ID NO. 18; SEQ ID NO. 19; SEQ ID NO. 20; SEQ ID NO. 26; and SEQ ID NO. 27. 50 41. A method for assaying papillomavirus DNA replication, said method comprising the steps of: - incubating a candidate agent with a mixture of El protein preparation according to claim 1, with E2 protein and a suitable DNA origin of replication; and - measuring the amount of DNA unwinding. 42. The assay according to claim 41, wherein said papillomavirus is HPV-11 or HPV-6. 43. The assay of claim 41, wherein said El protein is selected from the group consisting of: SEQ ID NO. 13; SEQ ID NO. 14; SEQ ID NO. 15; SEQ ID NO. 16; SEQ ID NO. 17; SEQ ID NO. 18; SEQ ID NO. 19; SEQ ID NO. 20; SEQ ID NO. 26; and SEQ ID NO. 27. 44. A method for identifying an agent capable of modulating papillomavirus DNA replication, said method comprising the steps of: a) assaying said DNA replication activity in the absence of said agent by the method according to claim 41; b) assaying said DNA replication activity in the presence of said agent by the method of claim 41, wherein said agent is added to said mixture during said incubation; and c) comparing the result of step a) with the result of step b) . 45. The assay according to claim 44, wherein said papillomavirus is HPV-11 or HPV-6. 51 46. The assay of claim 44, wherein said El protein is selected from the group consisting of: SEQ ID NO. 13; SEQ ID NO. 14; SEQ ID NO. 15; SEQ ID NO. 16; SEQ ID NO. 17; SEQ ID NO. 18; SEQ ID NO. 19; SEQ ID NO. 20; SEQ ID NO. 26; and SEQ ID NO. 27.

Inventors: FARNET CHRIS M, PELLETIER ALEX, FARNET, CHRIS, M., PELLETIER, ALEX
Applicants: BOEHRINGER INGELHEIM CA LTD, FARNET CHRIS M, PELLETIER ALEX, BOEHRINGER INGELHEIM (CANADA) LTD., FARNET, CHRIS, M., PELLETIER, ALEX
Application year: 1999
Main classification: C12N15
IPC codes: A61K38, A61K39, A61P17, A61P31, A61P43, C07K1, C07K14, C07K19, C12N9, C12N15, C12Q1