RICIN-LIKE TOXIN VARIANTS FOR TREATMENT OF CANCER, VIRAL OR PARASITIC INFECTIONS

Abstract: The present invention provides a protein having an A chain of a ricin-like toxin, a B chain of a ricin-like toxin and a heterologous linker amino acid sequence, linking the A and B chains. The linker sequence contains a cleavage recognition site for a disease specific protease such as a cancer, fungal, viral or parasitic protease. The invention also relates to a nucleic acid molecule encoding the protein and to expression vectors incorporating the nucleic acid molecule. Also provided is a method of inhibiting or destroying mammalian cancer cells, cells infected with a virus, a fungus, or parasite, or parasites utilizing the nucleic acid molecules and proteins of the invention and pharmaceutical compositions for treating human cancer, viral infection, fungal infection, or parasitic infection.

Description: Title: RICIN-LIKE TOXIN VARIANTS FOR TREATMENT OF CANCER, VIRAL OR PARASITIC INFECTIONS FIELD OF THE INVENTION The invention relates to proteins useful as therapeutics against cancer, viral infections, parasitic and fungal infections. The proteins contain A and B chains of a ricin-like toxin linked by a linker sequence that is specifically cleaved and activated by proteases specific to disease-associated pathogens or cells. BACKGROUND OF THE INVENTION Bacteria and plants are known to produce cytotoxic proteins which may consist of one, two or several polypeptides or subunits. Those proteins having a single subunit may be loosely classified as Type I proteins. Many of the cytotoxins which have evolved two subunit structures are referred to as type II proteins (Saelinger, C.B. in Trafficking of Bacterial Toxins (eds. Saelinger, C.B.) 1-13 (CRC Press Inc., Boca Raton, Florida, 1990). One subunit, the A chain, possesses the toxic activity whereas the second subunit, the B chain, binds cell surfaces and mediates entry of the toxin into a target cell. A subset of these toxins kill target cells by inhibiting protein biosynthesis. For example, bacterial toxins such as diphtheria toxin or Pseudomonas exotoxin inhibit protein synthesis by inactivating elongation factor 2. Plant toxins such as ricin, abrin, and bacterial toxin Shiga toxin, inhibit protein synthesis by directly inactivating the ribosomes (Olsnes, S. & Phil, A. in Molecular action of toxins and viruses (eds. Cohen, P. & vanHeyningen, S.) 51-105 Elsevier Biomedical Press, Amsterdam, 1982). Ricin, derived from the seeds of Ricinus communis (castor oil plant), may be the most potent of the plant toxins. It is estimated that a single ricin A chain is able to inactivate ribosomes at a rate of 1500 ribosomes /minute. Consequently, a single molecule of ricin is enough to kill a cell (Olsnes, S. & Phil, A. in Molecular action of toxins and viruses (eds. Cohen, P. & vanHeyningen, S.) (Elsevier Biomedical Press, Amsterdam, 1982). The ricin toxin is a glycosylated heterodimer consisting of A and B chains with molecular masses of 30,625 Da and 31,431 Da linked by a disulphide bond. The A chain of ricin has an N-glycosidase activity and catalyzes the excision of a specific adenine residue from the 28S rRNA of eukaryotic ribosomes (Endo, Y. & Tsurugi, K. J., Biol. Chem. 262:8128 (1987)). The B chain of ricin, although not toxic in itself, promotes the toxicity of the A chain by binding to galactose residues on the surface of eukaryotic cells and stimulating receptor-mediated endocytosis of the toxin molecule (Simmons et al., Biol. Chem. 261:7912 (1986)). Once the toxin molecule consisting of the A and B chains is internalized into the cell via clathrin-dependent or independent mechanisms, the greater reduction potential within the cell induces a release of the active A chain, eliciting its inhibitory effect on protein synthesis and its cytotoxicity (Emmanuel, F. et al, Anal. Biochem. 173: 134-141 (1988); Blum, J.S. et al, /. Biol. Chem. 266: 22091-22095 (1991); Fiani, M.L. et al, Arch. Biochem. Biophys. 307: 225-230 (1993)). Empirical evidence suggests that activated toxin (e.g. ricin, shiga toxin and others) in the endosomes is transcytosed through the trans-Golgi network to the endoplasmic reticulum by retrograde transport before the A chain is translocated into the cytoplasm to elicit its action (Sandvig, K. & van Deurs, B., FEBS Lett. 346: 99-102 (1994). Protein toxins are initially produced in an inactive, precursor form. Ricin is initially produced as a single polypeptide (preproricin) with a 35 amino acid N-terminal presequence and 12 amino acid linker between the A and B chains. The pre-sequence is removed during translocation of the ricin precursor into the endoplasmic reticulum (Lord, J.M., Eur. J. Biochem. 146:403-409 (1985) and Lord, J.M., Eur. J. Biochem. 146:411-416 (1985)). The proricin is then translocated into specialized organelles called protein bodies where a plant protease cleaves the protein at a linker region between the A and B chains (Lord, J.M. et al., FASAB Journal 8:201-208 (1994)). The two chains, however, remain covalently attached by an interchain disulfide bond (cysteine 259 in the A chain to cysteine 4 in the B chain) and mature disulfide linked ricin is stored in protein bodies inside the plant cells. The A chain is inactive in proricin (O'Hare, M. et al., FEBS Lett. 273:200-204 (1990)) and it is inactive in the disulfide-linked mature ricin (Richardson, P.T. et al, FEBS Lett. 255:15-20 (1989)). The ribosomes of the castor bean plant are themselves susceptible to inactivation by ricin A chain; however, as there is no cell surface galactose to permit B chain recognition the A chain cannot re-enter the cell. The exact mechanism of A chain release and activation in target cell cytoplasm is not known (Lord, J.M. et al., FASAB Journal 8:201-208 (1994)). However, it is known that for activation to take place the disulfide bond between the A and B chains must be reduced and, hence, the linkage between subunits broken. Diphtheria toxin is produced by Corynebacterium diphtheriae as a 535 amino acid polypeptide with a molecular weight of approximately 58kD (Greenfield, L. et al., Proc. Natl. Acad. Sci. USA 80:6853-6857 (1983); Pastan, I. et al., Annu. Rev. Biochem. 61:331-354 (1992); Collier, RJ. & Kandel, J., /. Biol. Chem. 246:1496-1503 (1971)). It is secreted as a single-chain polypeptide consisting of 2 functional domains. Similar to proricin, the N-terminal domain (A-chain) contains the cytotoxic moiety whereas the C-terminal domain (B-chain) is responsible for binding to the cells and facilitates toxin endocytosis. Conversely, the mechanism of cytotoxicity for diphtheria toxin is based on ADP-ribosylation of EF-2 thereby blocking protein synthesis and producing cell death. The 2 functional domains in diphtheria toxin are linked by an arginine-rich peptide sequence as well as a disulphide bond. Once the diphtheria toxin is internalized into the cell, the arginine-rich peptide linker is cleaved by trypsin-like enzymes and the disulphide bond (Cys 186-201) is reduced. The cytotoxic domain is subsequently translocated into the cytosol substantially as described above for ricin and elicits ribosomal inhibition and cytotoxicity. Pseudomonas exotoxin is also a 66kD single-chain toxin protein secreted by Pseudomonas aeruginosa with a similar mechanism of cytotoxicity to that of diphtheria toxin (Pastan, I. et al., Annu. Rev. Biochem. 61:331-354 (1992); Ogata, M. et al., /. Biol. Chem. 267:25396- 25401 (1992); Vagil, M.L. et al., Infect. Immunol. 16:353-361 (1977)). Pseudomonas exotoxin consists of 3 conjoint functional domains. The first domain la (amino acids 1-252) is responsible for cell binding and toxin endocytosis, a second domain II (amino acids 253-364) is responsible for toxin translocation from the endocytic vesicle to the cytosol, and a third domain III (amino acids 400-613) is responsible for protein synthesis inhibition and cytotoxicity. After Pseudomonas exotoxin enters the cell, the liberation of the cytotoxic domain is effected by both proteolytic cleavage of a polypeptide sequence in the second domain (near Arg 279) and the reduction of the disulphide bond (Cys 265-287) in the endocytic vesicles. In essence, the overall pathway to cytotoxicity is analogous to diphtheria toxin with the exception that the toxin translocation domain in Pseudomonas exotoxin is structurally distinct. Other toxins possessing distinct functional domains for cytotoxicity and cell binding/toxin translocation include abrin, modeccin and volkensin (Sandvig, K. et al., Biochem. Soc. Trans. 21:707- 711 (1993)). Some toxins such as Shiga toxin and cholera toxin also have multiple polypeptide chains responsible for receptor binding and endocytosis. The ricin gene has been cloned and sequenced, and the X-ray crystal structures of the A and B chains have been described (Rutenber, E. et al. Proteins 10:240-250 (1991); Weston et al, Mol. Bio. 244:410-422, 1994; Lamb and Lord, Eur. J. Biochem. 14:265 (1985); Hailing, K. et al. Nucleic Acids Res. 13:8019 (1985)). Similarly, the genes for diptheria toxin and Pseudomonas exotoxin have been cloned and sequenced, and the 3-dimensional structures of the toxin proteins have been elucidated and described (Columblatti, M. et al., /. Biol. Chem. 261:3030-3035 (1986); Allured, V.S. et al., Proc. Natl. Acad. Sci. USA 83:1320-1324 (1986); Gray, G.L. et al., Proc. Natl. Acad. Sci. USA 81:2645- 2649 (1984); Greenfield, L. et al., Proc. Natl. Acad. Sci. USA 80:6853-6857 (1983); Collier, RJ. et al., /. Biol. Chem. 257:5283-5285 (1982)). The potential of bacterial and plant toxins for inhibiting mammalian retroviruses, particularly acquired immunodeficiency syndrome (AIDS), has been investigated. Bacterial toxins such as Pseudomonas exotoxin-A and subunit A of diphtheria toxin; dual chain ribosomal inhibitory plant toxins such as ricin, and single chain ribosomal inhibitory proteins such as trichosanthin and pokeweed antiviral protein have been used for the elimination of HIV infected cells (Olson et al., AIDS Res. and Human Retroviruses 7:1025-1030 (1991)). The high toxicity of these toxins for mammalian cells, combined with a lack of specificity of action poses a major problem to the development of pharmaceuticals incorporating the toxins, such as immunotoxins. Due to their extreme toxicity there has been much interest in making ricin-based immunotoxins as therapeutic agents for specifically destroying or inhibiting infected or tumourous cells or tissues (Vitetta et al., Science 238:1098-1104(1987)). An immunotoxin is a conjugate of a specific cell binding component, such as a monoclonal antibody or growth factor and the toxin in which the two protein components are covalently linked. Generally, the components are chemically coupled. However, the linkage may also be a peptide or disulfide bond. The antibody directs the toxin to cell types presenting a specific antigen thereby providing a specificity of action not possible with the natural toxin. Immunotoxins have been made both with the entire ricin molecule (i.e. both chains) and with the ricin A chain alone (Spooner et al., Mol. Immunol. 31:117-125, (1994)). Immunotoxins made with the ricin dimer (IT-Rs) are more potent toxins than those made with only the A chain (IT- As). The increased toxicity of IT-Rs is thought to be attributed to the dual role of the B chains in binding to the cell surface and in translocating the A chain to the cytosolic compartment of the target cell (Vitetta et al., Science 238:1098-1104 (1987); Vitetta & Thorpe, Seminars in Cell Biology 2:47-58 (1991)). However, the presence of the B chain in these conjugates also promotes the entry of the immunotoxin into nontarget cells. Even small amounts of B chain may override the specificity of the cell-binding component as the B chain will bind nonspecifically to galactose associated with N-linked carbohydrates, which is present on most cells. IT-As are more specific and safer to use than IT-Rs. However, in the absence of the B chain the A chain has greatly reduced toxicity. Due to the reduced potency of IT-As as compared to IT-Rs, large doses of IT-As must be administered to patients. The large doses frequently cause immune responses and production of neutralizing antibodies in patients (Vitetta et al., Science 238:1098-1104 (1987)). IT-As and IT-Rs both suffer from reduced toxicity as the A chain is not released from the conjugate into the target cell cytoplasm. A number of immunotoxins have been designed to recognize antigens on the surfaces of tumour cells and cells of the immune system (Pastan et al., Annals New York Academy of Sciences 758:345-353 (1995)). A major problem with the use of such immunotoxins is that the antibody component is its only targeting mechanism and the target antigen is often found on non-target cells (Vitetta et al., Immunology Today 14:252-259 (1993)). Also, the preparation of a suitable specific cell binding component may be problematic. For example, antigens specific for the target cell may not be available and many potential target cells and infective organisms can alter their antigenic make up rapidly to avoid immune recognition. In view of the extreme toxicity of proteins such as ricin, the lack of specificity of the immunotoxins may severely limit their usefulness as therapeutics for the treatment of cancer and infectious diseases. The insertion of intramolecular protease cleavage sites between the cytotoxic and cell-binding components of a toxin can mimic the way that the natural toxin is activated. European patent application no. 466,222 describes the use of maize-derived pro-proteins which can be converted into active form by cleavage with extracellular blood enzymes such as factor Xa, thrombin or collagenase. Garred, O. et al. (/. Biol. Chem. 270:10817-10821 (1995)) documented the use of a ubiquitous calcium-dependent serine protease, furin, to activate shiga toxin by cleavage of the trypsin-sensitive linkage between the cytotoxic A-chain and the pentamer of cell-binding B-units. Westby et al. (Bioconjugate Chem. 3:375-381 (1992)) documented fusion proteins which have a specific cell binding component and proricin with a protease sensitive cleavage site specific for factor Xa within the linker sequence. O'Hare et al. (FEBS Lett. 273:200-204 (1990)) also described a recombinant fusion protein of RTA and staphylococcal protein A joined by a trypsin-sensitive cleavage site. In view of the ubiquitous nature of the extracellular proteases utilized in these approaches, such artificial activation of the toxin precursor or immunotoxin does not confer a mechanism for intracellular toxin activation and the problems of target specificity and adverse immunological reactions to the cell-binding component of the immunotoxin remain. In a variation of the approach of insertion of intramolecular protease cleavage sites on proteins which combine a binding chain and a toxic chain, Leppla, S.H. et al. (Bacterial Protein Toxins zbl.bakt.suppl. 24:431-442 (1994)) suggest the replacement of the native cleavage site of the protective antigen (PA) produced by Bacillus anthracis with a cleavage site that is recognized by cells that contain a particular protease. PA, recognizes, binds, and thereby assists in the internalization of lethal factor (LF) and edema toxin (ET). also produced by Bacillus anthracis. However, this approach is wholly dependent on the availability of LF, or ET and PA all being localized to cells wherein the modified PA can be activated by the specific protease. It does not confer a mechanism for intracellular toxin activation and presents a problem of ensuring sufficient quantities of toxin for internalization in target cells. The in vitro activation of a Staphylococcus-deriveά pore- forming toxin, α-hemolysin by extracellular tumour-associated proteases has been documented (Panchel, R.G. et al., Na tu re Biotechnology 14:852-857 (1996)). Artificial activation of α-hemolysin in vitro by said proteases was reported but the actual activity and utility of α-hemolysin in the destruction of target cells were not demonstrated. Hemolysin does not inhibit protein synthesis but is a heptameric transmembrane pore which acts as a channel to allow leakage of molecules up to 3 kD thereby disrupting the ionic balances of the living cell. The α-hemolysin activation domain is likely located on the outside of the target cell (for activation by extracellular proteases). The triggering mechanism in the disclosed hemolysin precursor does not involve the intracellular proteolytic cleavage of 2 functionally distinct domains. Also, the proteases used for the α-hemolysin activation are ubitquitiously secreted extracellular proteases and toxin activation would not be confined to activation in the vicinity of diseased cells. Such widespread activation of the toxin does not confer target specificity and limits the usefulness of said α-hemolysin toxin as therapeutics due to systemic toxicity. A variety of proteases specifically associated with malignancy, viral infections and parasitic infections have been identified and described. For example, cathepsin is a family of serine, cysteine or aspartic endopeptidases and exopeptidases which has been implicated to play a primary role in cancer metastasis (Schwartz, M.K., Clin. Chim. Ada 237:67-78 (1995); Spiess, E. et al., /. Histochem. Cytochem. 42:917-929 (1994); Scarborough, P.E. et al., Protein Sci. 2:264- 276 (1993); Sloane, B.F. et al., Proc. Natl. Acad. Sci. USA 83:2483-2487 (1986); Mikkelsen, T. et al., /. Neurosurge 83:285-290 (1995)). Matrix metalloproteinases (MMPs or matrixins) are zinc-dependent proteinases consisting of collagenases, matrilysin, stromelysins, gelatinases and macrophage elastase (Krane, S.M., Ann. N.Y. Acad. Sci. 732:1-10 (1994); Woessner, J.F., Ann. N.Y. Acad. Sci. 732:11-21 (1994); Carvalho, K. et al., Biochem. Biophys. Res. Comm. 191:172-179 (1993); Nakano, A. et al. /. of Neurosurge, 83:298-307 (1995); Peng, K-W, et al. Human Gene Therapy, 8:729-738 (1997); More, D.H. et al. Gynaecologic Oncology, 65:78-82 (1997)). These proteases are involved in pathological matrix remodeling. Under normal physiological conditions, regulation of matrixin activity is effected at the level of gene expression. Enzymatic activity is also controlled stringently by tissue inhibitors of metalloproteinases (TIMPs) (Murphy, G. et al., Ann. N.Y. Acad. Sci. 732:31-41 (1994)). The expression of MMP genes is reported to be activated in inflammatory disorders (e.g. rheumatoid arthritis) and malignancy. In malaria, parasitic serine and aspartic proteases are involved in host erythrocyte invasion by the Plasmodium parasite and in hemoglobin catabolism by intraerythrocytic malaria (O'Dea, K.P. et al., Mol. Biochem. Parasitol. 72:111-119 (1995); Blackman, MJ. et al, Mol. Biochem. Parasitol. 62:103-114 (1993); Cooper, J.A. et al, Mol. Biochem. Parasitol. 56:151-160 (1992); Goldberg, D.E. et al., /. Exp. Med. 173:961-969 (1991)). Schistosoma mansoni is also a pathogenic parasite which causes schistosomiasis or bilharzia. Elastinolytic proteinases have been associated specifically with the virulence of this particular parasite (McKerrow, J.H. et al., /. Biol. Chem. 260:3703-3707 (1985)). Welch, A.R. et al. (Proc. Natl. Acad. Sci. USA 88:10797- 10800 (1991)) has described a series of viral proteases which are specifically associated with human cytomegalovirus, human herpesviruses, Epstein-Barr virus, varicella zoster virus-I. and infectious laryngotracheitis virus. These proteases possess similar substrate specificity and play an integral role in viral scaffold protein restructuring in capsid assembly and virus maturation. Other viral proteases serving similar functions have also been documented for human T-cell leukemia virus (Blaha, I. et al., FEBS Lett. 309:389-393 (1992); Pettit, S.C. et al., /. Biol. Chem. 266:14539-14547 (1991)), hepatitis viruses (Hirowatari, Y. et al., Anal. Biochem. 225:113-120 (1995); Hirowatari, Y. et al, Arch. Virol. 133:349-356 (1993); Jewell, D.A. et al., Biochemistry 31:7862-7869 (1992)), poliomyelitis virus (Weidner, J.R. et al., Arch. Biochem. Biophys. 286:402-408 (1991)), and human rhinovirus (Long, A.C. et al., FEBS Lett. 258:75-78 (1989)). Candida yeasts are dimorphic fungi which are responsible for a majority of opportunistic infections in AIDS patients (Holmberg, K. and Myer, R., Scand. J. Infect. Dis. 18:179-192 (1986)). Aspartic proteinases have been associated specifically with numerous virulent strains of Candida including Candida albican, Candida tropicalis, and Candida parapsilosis (Abad-Zapatero, C. et al., Protein Sci. 5:640-652 (1996); Cutfield, S.M. et al., Biochemistry 35:398-410 (1995); Ruchel, R. et al, Zentralbl. Bakteriol. Mikrobiol Hyg. I Abt. Orig. A. 255:537-548 (1983); Remold, H. et al, Biochim. Biophys. Acta 167:399-406 (1968)), and the levels of these enzymes have been correlated with the lethality of the strain (Schreiber, B, et al., Diagn. Microbiol. Infect. Dis. 3:1-5 (1985)). SUMMARY OF THE INVENTION The invention relates to novel recombinant toxic proteins which are specifically toxic to diseased cells but do not depend for their specificity of action on a specific cell binding component. The recombinant proteins of the invention have an A chain of a ricin-like toxin linked to a B chain by a synthetic linker sequence which may be cleaved specifically by a protease localised in cells or tissues affected by a specific disease to liberate the toxic A chain thereby selectively inhibiting or destroying the diseased cells or tissues. The term diseased cells as used herein, includes cells affected by cancer, or infected by fungi, or viruses, including retroviruses, or parasites. Toxin targeting using the recombinant toxic proteins of the invention takes advantage of the fact that many DNA viruses exploit host cellular transport mechanisms to escape immunological destruction. This is achieved by enhancing the retrograde translocation of host major histocompatibility complex (MHC) type I molecules from the endoplasmic reticulum into the cytoplasm (Bonifacino, J.S., Nature 384: 405-406 (1996); Wiertz, E.J. et al., Nature 384: 432-438 (1996)). The facilitation of retrograde transport in diseased cells by the virus can enhance the transcytosis and cytotoxicity of a recombinant toxic protein of the present invention thereby further reducing non-specific cytotoxicity and improving the overall safety of the product. The recombinant toxic proteins of the present invention may be used to treat diseases including various forms of cancer such as T- and B-cell lymphoproliferative diseases, ovarian cancer, pancreatic cancer, head and neck cancer, squamous cell carcinoma, gastrointestinal cancer, breast cancer, prostate cancer, non small cell lung cancer, malaria, and diverse viral disease states associated with infection with human cytomegalovirus, hepatitis virus, herpes virus, human rhinovirus, infectious laryngotracheitis virus, poliomyelitis virus, or varicella zoster virus. In one aspect, the present invention provides a purified and isolated nucleic acid having a nucleotide sequence encoding an A chain of a ricin-like toxin, a B chain of a ricin-like toxin and a heterologous linker amino acid sequence, linking the A and B chains. The linker sequence is not a native linker sequence of a ricin-like toxin, but rather a synthetic heterologous linker sequence containing a cleavage recognition site for a disease-specific protease. The A and or the B chain may be those of ricin. In an embodiment, of the invention the cleavage recognition site is the cleavage recognition site for a cancer-associated protease. In particular embodiments, the linker amino acid sequence comprises SLLKSRMVPNFN or SLLIARRMPNFN cleaved by cathepsin B; SKLVQASASGVN or SSYLKASDAPDN cleaved by an Epstein-Barr virus protease; RPKPQQFFGLMN cleaved by MMP-3 (stromelysin); SLRPLALWRSFN cleaved by MMP-7 (matrilysin); SPQGIAGQRNFN cleaved by MMP-9; DVDERDVRGFASFL cleaved by a thermolysin-like MMP; SLPLGLWAPNFN cleaved by matrix metalloproteinase 2(MMP- 2) ; SLLIFRSWANFN cleaved by cathespin L; SGVVIATVIVIT cleaved by cathespin D; SLGPQGIWGQFN cleaved by matrix metalloproteinase l(MMP-l); KKSPGRVVGGSV cleaved by urokinase-type plasminogen activator; PQGLLGAPGILG cleaved by membrane type 1 matrixmetalloproteinase (MT-MMP); HGPEGLRVGFYESDVMGRGHARLVHVEEPHT cleaved by stromelysin 3 (or MMP-11), thermolysin, f ibroblast collagenase and stromelysin- 1; GPQGLAGQRGIV cleaved by matrix metalloproteinase 13 (collagenase- 3); GGSGQRGRKALE cleaved by tissue-type plasminogen activator(tPA); SLSALLSSDIFN cleaved by human prostate-specific antigen; SLPRFKIIGGFN cleaved by kallikrein (hK3); SLLGIAVPGNFN cleaved by neutrophil elastase; and FFKNIVTPRTPP cleaved by calpain (calcium activated neutral protease). The nucleic acid sequences for ricin A and B chains with each of the linker sequences are shown in Figures 2D, 35C, 3D, 4D, 5D, 6D, 16D, 17D, 34C, 36C , 37C, 38C , 39C, 40C, 41C, 42C , 43C, 44C, 45C, 46C and 47C, respectively. In another embodiment, the cleavage recognition site is the cleavage recognition site for a protease associated with the malaria parasite, Plasmodium falciparum. In particular embodiments, the linker amino acid sequence comprises QVVQLQNYDEED; LPIFGESEDNDE; QVVTGEAISVTM; ALERTFLSFPTN or KFQDMLNISQHQ. The nucleic nucleotide sequences for ricin A and B chains with each of the linker sequences are shown in Figures 7D, 8D, 9D, 10D, and 11D. In a another embodiment, the cleavage recognition site is the cleavage recognition site for a viral protease. The linker sequences preferably comprise the sequence Y-X-Y-A-Z wherein X is valine or leucine, Y is a polar amino acid, and Z is serine, asparagine or valine. In particular embodiments, the linker amino acid sequence comprises SGVVNASCRLAN or SSYVKASVSPEN cleaved by a human cytomegalovirus protease; SALVNASSAHVN or STYLQASEKFKN cleaved by a herpes simplex 1 virus protease; SSILNASVPNFN cleaved by a human herpes virus 6 protease; SQDVNAVEASSN or SVYLQASTGYGN cleaved by a varicella zoster virus protease; or SKYLQANEVITN cleaved by an infectious laryngotracheitis virus protease. The nucleic nucleotide sequences for ricin A and B chains with each of the linker sequences are shown in Figures 12D, 13D, 14D, 15D, 18D, 19D, 20D, and 22D. In another embodiment, the cleavage recognition site is the cleavage recognition site for a hepatitis A viral protease. In particular embodiments, the linker amino acid sequence comprises SELRTQSFSNWN or SELWSQGIDDDN cleaved by a hepatitis A virus protease. The nucleic nucleotide sequences for ricin A and B chains with each of the linker sequences are shown in Figures 23D or 24D. In another embodiment, the cleavage recognition site is the cleavage recognition site for a hepatitis C viral protease. In particular embodiments, the linker amino acid sequence comprises DLEVVTSTWVFN, DEMEECASHLFN, EDVVCCSMSYFN or KGWRLLAPITAY cleaved by a hepatitis C virus protease. The nucleic nucleotide sequences for ricin A and B chains with each of the linker sequences are shown in Figures 30C, 31C, 32C and 33C. In another embodiment, the cleavage recognition site is the cleavage recognition site for a Candida fungal protease. In particular embodiments, the linker amino acid sequence is SKPAKFFRLNFN, SKPIEFFRLNFN or SKPAEFFALNFN cleaved by Candida aspartic protease. The nucleic nucleotide sequences for ricin A and B chains with the first linker sequence are shown in Figures 25D. The present invention also provides a plasmid incorporating the nucleic acid of the invention. In an embodiment, the plasmid has the restriction map as shown in Figures 2A, 3A, 4A, 5A, 6A, 7A, 8A, 9A, 10A, HA, 12A, 13A, 14A, 15A, 16A, 17A, 18A, 19A, 20A, 21A, 22A, 23A, 24A, or 25A. In another embodiment, the present invention provides a baculovirus transfer vector incorporating the nucleic acid of the invention. In particular embodiments, the invention provides a baculovirus transfer vector having the DNA sequence as shown in Figure 1. In a further embodiment, the present invention provides a baculovirus transfer vector incorporating the nucleic acid of the invention. In particular embodiments, the invention provides a baculovirus transfer vector having the restriction map as shown in Figures 2C, 3C, 4C, 5C, 6C, 7C, 8C, 9C, IOC, 11C, 12C, 13C, 14C, 15C, 16C, 17C, 18C, 19C, 20C, 21C, 22C, 23C, 24C, 25C, 30A, 31A, 32A, 33A, 34A, 35A, 36A, 37A, 38A, 39A, 40A, 41A, 42A, 43A, 44A, 45A, 46A, or 47A. or having the DNA sequence as shown in Figure 1. In a further aspect, the present invention provides a recombinant protein comprising an A chain of a ricin-like toxin, a B chain of a ricin-like toxin and a heterologous linker amino acid sequence, linking the A and B chains, wherein the linker sequence contains a cleavage recognition site for a disease-specific protease (e.g.. a cancer, viral, parasitic, or fungal protease). The A and /or the B chain may be those of ricin. In an embodiment, the cleavage recognition site is the cleavage recognition site for a cancer, viral or parasitic protease substantially as described above. In a particular embodiment, the cancer is T-cell or B-cell lymphoproliferative disease. In another particular embodiment, the virus is human cytomegalovirus, Epstein-Barr virus, hepatitis virus, herpes virus, human rhinovirus, infectious laryngotracheitis virus, poliomyelitis virus, or varicella zoster virus. In a further particular embodiment, the parasite is Plas modiu m falciparum. In a further aspect, the invention provides a pharmaceutical composition for treating a fungal infection, such as Candida, in a mammal comprising the recombinant protein of the invention and a pharmaceutically acceptable carrier, diluent or excipient. In yet another aspect, the invention provides a method of inhibiting or destroying cells affected by a disease, which cells are associated with a disease specific protease, including cancer or infection with a virus, fungus, or a parasite each of which has a specific protease, comprising the steps of preparing a recombinant protein of the invention having a heterologous linker sequence which contains a cleavage recognition site for the disease-specific protease and administering the recombinant protein to the cells. In an embodiment, the cancer is T-cell or B-cell lymphoproliferative disease, ovarian cancer, pancreatic cancer, head and neck cancer, squamous cell carcinoma, gastrointestinal cancer, breast cancer, prostate cancer, non small cell lung cancer. In another embodiment, the virus is human cytomegalovirus, Epstein-Barr virus, hepatitis virus, herpes virus, human rhinovirus, human T-cell leukemia virus, infectious laryngotracheitis virus, poliomyelitis virus, or varicella zoster virus. In another embodiment, the parasite is Plasmodium falciparum. The present invention also relates to a method of treating a mammal with disease wherein cells affected by the disease are associated with a disease specific protease, including cancer or infection with a virus, fungus, or a parasite each of which has a specific protease by administering an effective amount of one or more recombinant proteins of the invention to said mammal. Still further, a process is provided for preparing a pharmaceutical for treating a mammal with disease wherein cells affected by the disease are associated with a disease specific protease, including cancer or infection with a virus, fungus, or a parasite each of which has a specific protease comprising the steps of preparing a purified and isolated nucleic acid having a nucleotide sequence encoding an A chain of a ricin-like toxin, a B chain of a ricin-like toxin and a heterologous linker amino acid sequence, linking the A and B chains, wherein the linker sequence contains a cleavage recognition site for the disease-specific protease; introducing the nucleic acid into a host cell; expressing the nucleic acid in the host cell to obtain a recombinant protein comprising an A chain of a ricin-like toxin, a B chain of a ricin-like toxin and a heterologous linker amino acid sequence, linking the A and B chains wherein the linker sequence contains the cleavage recognition site for the disease-specific protease; and suspending the protein in a pharmaceutically acceptable carrier, diluent or excipient. In an embodiment, a process is provided for preparing a pharmaceutical for treating a mammal with disease wherein cells affected by the disease are associated with a disease specific protease, including cancer or infection with a virus, fungus, or a parasite each of which has a specific protease comprising the steps of identifying a cleavage recognition site for the protease; preparing a recombinant protein comprising an A chain of a ricin-like toxin, a B chain of a ricin-like toxin and a heterologous linker amino acid sequence, linking the A and B chains wherein the linker sequence contains the cleavage recognition site for the protease and suspending the protein in a pharmaceutically acceptable carrier, diluent or excipient. In a further aspect, the invention provides a pharmaceutical composition for treating for treating a mammal with disease wherein cells affected by the disease are associated with a disease specific protease, including cancer or infection with a virus, fungus, or a parasite comprising the recombinant protein of the invention and a pharmaceutically acceptable carrier, diluent or excipient. Other features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples while indicating preferred embodiments of the invention are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. DESCRIPTION OF THE DRAWINGS The invention will be better understood with reference to the drawings in which: Figure 1 shows the DNA sequence of the baculovirus transfer vector, pVL1393; Figure 2A summarizes the cloning strategy used to generate the pAP-213 construct; Figure 2B shows the nucleotide sequence of the Cathepsin B linker regions of pAP-213; Figure 2C shows the subcloning of the Cathepsin B linker variant into a baculovirus transfer vector; Figure 2D shows the DNA sequence of the pAP-214 insert containing ricin and the Cathepsin B linker; Figure 3A summarizes the cloning strategy used to generate the pAP-215 construct; Figure 3B shows the nucleotide sequence of the MMP-3 linker regions of pAP-215; Figure 3C shows the subcloning of the MMP-3 linker variant into a baculovirus transfer vector; Figure 3D shows the DNA sequence of the pAP-216 insert containing ricin and the MMP-3 linker; Figure 4A summarizes the cloning strategy used to generate the pAP-217 construct; Figure 4B shows the nucleotide sequence of the MMP-7 linker regions of pAP-217; Figure 4C shows the subcloning of the MMP-7 linker variant into a baculovirus transfer vector; Figure 4D shows the DNA sequence of the pAP-218 insert containing ricin and the MMP-7 linker; Figure 5A summarizes the cloning strategy used to generate the pAP-219 construct; Figure 5B shows the nucleotide sequence of the MMP-9 linker regions of pAP-219; Figure 5C shows the subcloning of the MMP-9 linker variant into a baculovirus transfer vector; Figure 5D shows the DNA sequence of the pAP-220 insert containing ricin and the MMP-9 linker. Figure 6A summarizes the cloning strategy used to generate the pAP-221 construct; Figure 6B shows the nucleotide sequence of the thermolysin-like MMP linker regions of pAP-221; Figure 6C shows the subcloning of the thermolysin-like MMP linker variant into a baculovirus transfer vector. Figure 6D shows the DNA sequence of the pAP-222 insert containing ricin and the thermolysin-like MMP linker; Figure 7A summarizes the cloning strategy used to generate the pAP-223 construct; Figure 7B shows the nucleotide sequence of the Plasmodium falciparum-A linker regions of pAP-223; Figure 7C shows the subcloning of the Plasmodium falciparum-A linker variant into a baculovirus transfer vector; Figure 7D shows the DNA sequence of the pAP-224 insert containing ricin and the Plasmodium falciparum-A linker; Figure 8A summarizes the cloning strategy used to generate the pAP-225 construct; Figure 8B shows the nucleotide sequence of the Plasmodium falciparum-B linker regions of pAP-225; Figure 8C shows the subcloning of the Plasmodium falciparum-B linker variant into a baculovirus transfer vector; Figure 8D shows the DNA sequence of the pAP-226 insert containing ricin and the Plasmodium falciparum-B linker; Figure 9A summarizes the cloning strategy used to generate the pAP-227 construct; Figure 9B shows the nucleotide sequence of the Plasmodium falciparum-C linker regions of pAP-227; Figure 9C shows the subcloning of the Plasmodium falciparum-C linker variant into a baculovirus transfer vector; Figure 9D shows the DNA sequence of the pAP-228 insert containing ricin and the Plasmodium falciparum-C linker; Figure 10A summarizes the cloning strategy used to generate the pAP-229 construct; Figure 10B shows the nucleotide sequence of the Plasmodium falciparum-D linker regions of pAP-229; Figure IOC shows the subcloning of the Plasmodium falciparum-D linker variant into a baculovirus transfer vector; Figure 10D shows the DNA sequence of the pAP-230 insert containing ricin and the Plasmodium falciparum-D linker; Figure HA summarizes the cloning strategy used to generate the pAP-231 construct; Figure 11B shows the nucleotide sequence of the Plasmodium falciparum-E linker regions of pAP-231; Figure 11C shows the subcloning of the Plasmodium falciparum-E linker variant into a baculovirus transfer vector; Figure 11D shows the DNA sequence of the pAP-232 insert containing ricin and the Plasmodium falciparum-E linker; Figure 12A summarizes the cloning strategy used to generate the pAP-233 construct; Figure 12B shows the nucleotide sequence of the HSV-A linker regions of pAP-233; Figure 12C shows the subcloning of the HSV-A linker variant into a baculovirus transfer vector; Figure 12D shows the DNA sequence of the pAP-234 insert containing ricin and the HSV-A linker; Figure 13A summarizes the cloning strategy used to generate the pAP-235 construct; Figure 13B shows the nucleotide sequence of the HSV-B linker regions of pAP-235; Figure 13C shows the subcloning of the HSV-B linker variant into a baculovirus transfer vector- Figure 13D shows the DNA sequence of the pAP-236 insert containing ricin and the HSV-B linker; Figure 14A summarizes the cloning strategy used to generate the pAP-237 construct; Figure 14B shows the nucleotide sequence of the VZV-A linker regions of pAP-237; Figure 14C shows the subcloning of the VZV-A linker variant into a baculovirus transfer vector; Figure 14D shows the DNA sequence of the pAP-238 insert containing ricin and the VZV-A linker; Figure 15A summarizes the cloning strategy used to generate the pAP-239 construct; Figure 15B shows the nucleotide sequence of the VZV-B linker regions of pAP-239; Figure 15C shows the subcloning of the VZV-B linker variant into a baculovirus transfer vector; Figure 15D shows the DNA sequence of the pAP-240 insert containing ricin and the VZV-B linker; Figure 16A summarizes the cloning strategy used to generate the pAP-241 construct; Figure 16B shows the nucleotide sequence of the EBV-A linker regions of pAP-241; Figure 16C shows the subcloning of the EBV-A linker variant into a baculovirus transfer vector; Figure 16D shows the DNA sequence of the pAP-242 insert containing ricin and the EBV-A linker; Figure 17A summarizes the cloning strategy used to generate the pAP-243 construct; Figure 17B shows the nucleotide sequence of the EBV-B linker regions of pAP-243; Figure 17C shows the subcloning of the EBV-B linker variant into a baculovirus transfer vector; Figure 17D shows the DNA sequence of the pAP-244 insert containing ricin and the EBV-B linker; Figure 18 A summarizes the cloning strategy used to generate the pAP-245 construct; Figure 18B shows the nucleotide sequence of the CMV-A linker regions of pAP-245; Figure 18C shows the subcloning of the CMV-A linker variant into a baculovirus transfer vector; Figure 18D shows the DNA sequence of the pAP-246 insert containing ricin and the CMV-A linker; Figure 19A summarizes the cloning strategy used to generate the pAP-247 construct; Figure 19B shows the nucleotide sequence of the CMV-B linker regions of pAP-247; Figure 19C shows the subcloning of the CMV-B linker variant into a baculovirus transfer vector; Figure 19D shows the DNA sequence of the pAP-248 insert containing ricin and the CMV-B linker. Figure 20A summarizes the cloning strategy used to generate the pAP-249 construct; Figure 20B shows the nucleotide sequence of the HHV-6 linker regions of pAP-249; Figure 20C shows the subcloning of the HHV-6 linker variant into a baculovirus transfer vector; Figure 20D shows the DNA sequence of the pAP-250 insert containing ricin and the HHV-6 linker; Figure 21 shows the amino acid sequences of the wild type ricin linker and cancer protease-sensitive amino acid linkers contained in pAP-213 to pAP-222 and linkers pAP-241 to pAP-244; Figure 22A summarizes the cloning strategy used to generate the pAP-253 construct; Figure 22B shows the nucleotide sequence of the ILV linker regions of pAP-253; Figure 22C shows the subcloning of the ILV linker variant into a baculovirus transfer vector; Figure 22D shows the DNA sequence of the pAP-254 insert containing ricin and the ILV linker; Figure 23A summarizes the cloning strategy used to generate the pAP-257 construct; Figure 23B shows the nucleotide sequence of the HAV-A linker regions of pAP-257; Figure 23C shows the subcloning of the HAV-A linker variant into a baculovirus transfer vector; Figure 23D shows the DNA sequence of the pAP-258 insert containing ricin and the HAV-A linker; Figure 24A summarizes the cloning strategy used to generate the pAP-255 construct; Figure 24B shows the nucleotide sequence of the HAV-B linker regions of pAP-255; Figure 24C shows the subcloning of the HAV-B linker variant into a baculovirus transfer vector; Figure 24D shows the DNA sequence of the pAP-256 insert containing ricin and the HAV-B linker; Figure 25A summarizes the cloning strategy used to generate the pAP-259 construct; Figure 25B shows the nucleotide sequence of the CAN linker regions of pAP-259; Figure 25C shows the subcloning of the CAN linker variant into a baculovirus transfer vector; Figure 25D shows the DNA sequence of the pAP-260 insert containing ricin and the CAN linker; Figure 26 shows the amino acid sequences of the wild type ricin linker and Plasmodium falciparum protease-sensitive amino acid linkers contained in linkers pAP-223 to pAP-232; Figure 27 shows the amino acid sequences of the wild type ricin linker and the viral protease-sensitive amino acid linkers contained in pAP-233 to pAP-240, ρAP-245-pAP-248, pAP-253 to pAP- 258; Figure 28 shows the amino acid sequences of the wild type ricin linker and the Candida aspartic protease-sensitive amino acid linker contained in pAP-259 to pAP-264; Figure 29 describes an alternative mutagenesis and subcloning strategy to provide a baculovirus transfer vector containing the ricin-like toxin variant gene; and Figure 30A summarizes the cloning strategy used to generate the pAP-262 construct; Figure 30B shows the nucleotide sequence of the HCV-A linker region of pAP-262; Figure 30C shows the DNA sequence of the pAP-262 insert; Figure 30D shows the amino acid sequence comparison of mutant preproricin linker region HCV-A to wild type; Figure 31A summarizes the cloning strategy used to generate the pAP-264 construct; Figure 31B shows the nucleotide sequence of the HCV-B linker region of pAP-264; Figure 31C shows the DNA sequence of the pAP-264 insert; Figure 31D shows the amino acid sequence comparison of mutant preproricin linker region HCV-B to wild type; Figure 32A summarizes the cloning strategy used to generate the pAP-266 construct; Figure 32B shows the nucleotide sequence of the HCV-C linker region of pAP-266; Figure 32C shows the DNA sequence of the pAP-266 insert; Figure 32D shows the amino acid sequence comparison of mutant preproricin linker region HCV-C to wild type; Figure 33A summarizes the cloning strategy used to generate the pAP-268 construct; Figure 33B shows the nucleotide sequence of the HCV-D linker region of pAP-268; Figure 33C shows the DNA sequence of the pAP-268 insert; Figure 33D shows the amino acid sequence comparison of mutant preproricin linker region HCV-D to wild type; Figure 34A summarizes the cloning strategy used to generate the pAP-270 construct; Figure 34B shows the nucleotide sequence of the MMP-2 linker region of pAP-270; Figure 34C shows the DNA sequence of the pAP-270 insert; Figure 34D shows the amino acid sequence comparison of mutant preproricin linker region of MMP-2 to wild type; Figure 35A summarizes the cloning strategy used to generate the pAP-272 construct; Figure 35B shows the nucleotide sequence of the Cathepsin B (Site 2) linker region of pAP-272; Figure 35C shows the DNA sequence of the pAP-272 insert; Figure 35D shows the amino acid sequence comparison of mutant preproricin linker region of Cathepsin B (Site 2) to wild type; Figure 36A summarizes the cloning strategy used to generate the pAP-274 construct; Figure 36B shows the nucleotide sequence of the Cathepsin L linker region of pAP-274; Figure 36C shows the DNA sequence of the pAP-274 insert; Figure 36D shows the amino acid sequence comparison of mutant preproricin linker region of Cathepsin L to wild type; Figure 37A summarizes the cloning strategy used to generate the pAP-276 construct; Figure 37B shows the nucleotide sequence of the Cathepsin D linker region of pAP-276; Figure 37C shows the DNA sequence of the pAP-276 insert; Figure 37D shows the amino acid sequence comparison of mutant preproricin linker region of Cathepsin D to wild type; Figure 38A summarizes the cloning strategy used to generate the pAP-278 construct; Figure 38B shows the nucleotide sequence of the MMP-1 linker region of pAP-278; Figure 38C shows the DNA sequence of the pAP-278 insert; Figure 38D shows the amino acid sequence comparison of mutant preproricin linker region of MMP-1 to wild type; Figure 39A summarizes the cloning strategy used to generate the pAP-280 construct; Figure 39B shows the nucleotide sequence of the Urokinase-Type Plasminogen Activator linker region of pAP-280; Figure 39C shows the DNA sequence of the pAP-280 insert; Figure 39D shows the amino acid sequence comparison of mutant preproricin linker region of Urokinase-Type Plasminogen Activator to wild type; Figure 40A summarizes the cloning strategy used to generate the pAP-282 construct; Figure 40B shows the nucleotide sequence of the MT- MMP linker region of pAP-282; Figure 40C shows the DNA sequence of the pAP-282 insert; Figure 40D shows the amino acid sequence comparison of mutant preproricin linker region of MT-MMP to wild type; Figure 41A summarizes the cloning strategy used to generate the pAP-284 construct; Figure 41B shows the nucleotide sequence of the MMP-11 linker region of pAP-284; Figure 41C shows the DNA sequence of the pAP-284 insert; Figure 41D shows the amino acid sequence comparison of mutant preproricin linker region of MMP-11 to wild type; Figure 42A summarizes the cloning strategy used to generate the pAP-286 construct; Figure 42B shows the nucleotide sequence of the MMP-13 linker region of pAP-286; Figure 42C shows the DNA sequence of the pAP-286 insert; Figure 42D shows the amino acid sequence comparison of mutant preproricin linker region of MMP-13 to wild type; Figure 43A summarizes the cloning strategy used to generate the pAP-288 construct; Figure 43B shows the nucleotide sequence of the Tissue- type Plasminogen Activator linker region of pAP-288; Figure 43C shows the DNA sequence of the pAP-288 insert; Figure 43D shows the amino acid sequence comparison of mutant preproricin linker region of Tissue-type Plasminogen Activator to wild type; Figure 44A summarizes the cloning strategy used to generate the pAP-290 construct; Figure 44B shows the nucleotide sequence of the human Prostate-Specific Antigen linker region of pAP-290; Figure 44C shows the DNA sequence of the pAP-290 insert; Figure 44D shows the amino acid sequence comparison of mutant preproricin linker region of the human Prostate-Specific Antigen to wild type; Figure 45A summarizes the cloning strategy used to generate the pAP-292 construct; Figure 45B shows the nucleotide sequence of the kallikrein linker region of pAP-292; Figure 45C shows the DNA sequence of the pAP-292 insert; Figure 45D shows the amino acid sequence comparison of mutant preproricin linker region of the kallikrein to wild type; Figure 46A summarizes the cloning strategy used to generate the pAP-294 construct; Figure 46B shows the nucleotide sequence of the neutrophil elastase linker region of pAP-294; Figure 46C shows the DNA sequence of the pAP-294 insert; Figure 46D shows the amino acid sequence comparison of mutant preproricin linker region of neutrophil elastase to wild type; Figure 47A summarizes the cloning strategy used to generate the pAP-296 construct; Figure 47B shows the nucleotide sequence of the calpain linker region of pAP-296; Figure 47C shows the DNA sequence of the pAP-296 insert; Figure 47D shows the amino acid sequence comparison of mutant preproricin linker region of calpain to wild type; Figure 48 is a blot showing cleavage of pAP-214 by Cathepsin B; Figure 49 is a blot showing cleavage of pAP-220 with MMP-9; Figure 50 is a blot showing activation of pAP-214; and Figure 51 is a blot showing activation of pAP-220. Figure 52 is a blot showing cleavage of pAP-248 with HCMV. Figure 53 is a blot showing activation of pAP-248. Figure 54 is a blot showing cleavage of pAP-256 by HAV 3C Figure 55 is a blot showing activation of pAP-256. Figure 56 is a semi-logithmic graph illustrating the cytotoxicity to COS-1 cells of undigested pAP-214 and pAP-214 digested with Cathepsin B. Figure 57 is a semi-logithmic graph illustrating the cytotoxicity of pAP-220 digested with MMP-9 compared to freshly thawed pAP-220 and ricin on COS-1 cells. Figure 58 is a blot showing cleavage of pAP-270 with MMP-2. Figure 59 is a blot showing activation of pAP-270. Figure 60 is a blot showing cleavage of pAP-288 by t-PA. Figure 61 is a blot showing activation of pAP-288. Figure 62 is a blot showing cleavage of pAP-294 with human neutrophil elastase. Figure 63 is a blot showing activation of pAP-294. Figure 64 is a blot showing cleavage of pAP-296 with calpain. Figure 65 is a blot showing activation of pAP-296. Figure 66 is a blot showing cleavage of pAP-222 with MMP-2. Figure 67 is a blot showing activation of pAP-222. DETAILED DESCRIPTION OF THE INVENTION Nucleic Acid Molecules of the Invention As mentioned above, the present invention relates to novel nucleic acid molecules comprising a nucleotide sequence encoding an A chain of a ricin-like toxin, a B chain of a ricin- like toxin and a heterologous linker amino acid sequence, linking the A and B chains. The heterologous linker sequence contains a cleavage recognition site for a disease-specific protease (e.g. a viral protease, parasitic protease, cancer-associated protease, or a fungal protease). The term "isolated and purified" as used herein refers to a nucleic acid substantially free of cellular material or culture medium when produced by recombinant DNA techniques, or chemical precursors, or other chemicals when chemically synthesized. An "isolated and purified" nucleic acid is also substantially free of sequences which naturally flank the nucleic acid (i.e. sequences located at the 5' and 3' ends of the nucleic acid) from which the nucleic acid is derived. The term "nucleic acid" is intended to include DNA and RNA and can be either double stranded or single stranded. The term "linker sequence" as used herein refers to an internal amino acid sequence within the protein encoded by the nucleic acid molecule of the invention which contains residues linking the A and B chain so as to render the A chain incapable of exerting its toxic effect, for example catalytically inhibiting translation of a eukaryotic ribosome. By heterologous is meant that the linker sequence is not a sequence native to the A or B chain of a ricin-like toxin or precursor thereof. However, preferably, the linker sequence may be of a similar length to the linker sequence of a ricin-like toxin and should not interfere with the role of the B chain in cell binding and transport into the cytoplasm. When the linker sequence is cleaved the A chain becomes active or toxic. The nucleic acid molecule of the invention is cloned by subjecting a preproricin cDNA clone to site-directed mutagenesis in order to generate a series of variants differing only in the sequence between the A and B chains (linker region). Oligonucleotides, corresponding to the extreme 5' and 3' ends of the preproricin gene are synthesized and used to PCR amplify the gene. Using the cDNA sequence for preproricin (Lamb et al., Eur. J. Biochem. 145:266-270 (1985)), several oligonucleotide primers are designed to flank the start and stop codons of the preproricin open reading frame. The preproricin cDNA is amplified using the upstream primer Ricin-99 or Ricin-109 and the downstream primer Ricinl729C with Vent DNA polymerase (New England Biolabs) using standard procedures (Sambrook et al., Molecular Cloning: A Laboratory Manual, Second Edition, (Cold Spring Harbor Laboratory Press, 1989)). The purified PCR fragment encoding the preproricin cDNA is then ligated into an Eco Rl-digested pBluescript II SK plasmid (Stratagene), and is used to transform competent XLl-Blue cells (Stratagene). The cloned PCR product containing the putative preproricin gene is confirmed by DNA sequencing of the entire cDNA clone . The sequences and location of oligonucleotide primers used for sequencing are shown in Table 1. The preproricin cDNA clone is subjected to site directed mutagenesis in order to generate a series of variants differing only in the sequence between the A and B chains (linker region). The wild-type preproricin linker region is replaced with the heterogenous linker sequences that are cleaved by the various disease-specific proteases as shown in Figures 21, 26, 27, 28, and Part D of Figures 30-47. Linker identification as used herein in connection with the sequences provided in these figures have been assigned the sequence ID numbers as discussed below. The linker regions of the variants encode a cleavage recognition sequence for a disease-specific protease associated with for example, cancer, viruses, parasites, or fungii. The mutagenesis and cloning strategy used to generate the disease-specific protease-sensitive linker variants are summarized in Part A of Figures 2-20, and Part A of Figures 22-25. The first step involves a DNA amplification using a set of mutagenic primers in combination with the two flanking primers Richin-99Eco or Ricin-109Eco and Ricinl729C Pst I. Restriction digested PCR fragments are gel purified and then ligated with PBluescript SK which has been digested with Eco RI and Pst I. Ligation reactions are used to transform competent XLl-Blue cells (Stratagene). Recombinant clones are identified by restriction digests of plasmid miniprep DNA and the mutant linker sequences are confirmed by DNA sequencing. With respect to the nucleotide sequences and amino acid sequences prepared as a result of the implementation of this strategy the following sequences have been assigned the sequence ID numbers as indicated. SEQ ID NO. 1 is used herein in connection with the DNA sequence of the baculovirus transfer vector, pVL1393. The nucleotide sequence of Cathepsin B linker regions of pAP-213 are referred to herein as SEQ ID NO. 2. The nucleotide sequence of Cathepsin B linker regions of pAP-214 are referred to herein as SEQ ID NO. 3. The nucleotide sequence of MMP-3 linker regions of pAP- 215 are referred to herein as SEQ ID NO. 4. The DNA sequence of the pAP-216 insert containing ricin and the MMP-3 linker are referred to herein as SEQ ID NO. 5. The nucleotide sequence of MMP-7 linker regions of pAP- 217 are referred to herein as SEQ ID NO. 6. The DNA sequence of the pAP-218 insert containing ricin and the MMP-7 linker are referred to herein as SEQ ID NO. 7. The nucleotide sequence of MMP-9 linker regions of pAP- 219 are referred to herein as SEQ ID NO. 8. The DNA sequence of the pAP-220 insert containing ricin and the MMP-9 are referred to herein as SEQ ID NO. 9. The nucleotide sequence of thermolysin-like MMP linker regions of pAP-221 are referred to herein as SEQ ID NO. 10. The DNA sequence of of pAP-222 insert containing ricin and the thermolysin-like MMP linker are referred to herein as SEQ ID NO. 11. The nucleotide sequence of Plasmodium falciparum-A linker regions of pAP-223 are referred to herein as SEQ ID NO. 12. The DNA sequence of the pAP-224 insert containing ricin and the Plasmodium falciparum-A linker are referred to herein as SEQ ID NO. 13. The nucleotide sequence of Plasmodium falciparum-B linker regions of pAP-225 are referred to herein as SEQ ID NO. 14. The DNA sequence of the pAP-226 insert containing ricin and the Plasmodium falciparum-B linker are referred to herein as SEQ ID NO. 15. The nucleotide sequence of Plasmodium falciparum-C linker regions of pAP-227 are referred to herein as SEQ ID NO. 16. The DNA sequence of the pAP-228 insert containing ricin and the Plasmodium falciparum-C linker are referred to herein as SEQ ID NO. 17. The nucleotide sequence of the the Plasmodium falciparum-D linker regions of pAP-229 is referred to herein as SEQ ID NO. 18. The DNA sequence of the pAP-230 insert containing ricin and the Plasmodium falciparum-D linker is referred to herein as SEQ ID NO. 19. The nucleotide sequence of the Plasmodium falciparum- E linker regions of pAP-231 is referred to herein as SEQ ID NO. 20. The DNA sequence of the pAP-232 insert containing ricin and the Plasmodium falciparum-E linker is referred to herein as SEQ ID NO. 21. The nucleotide sequence of the HSV-A linker regions of pAP-233 is referred to herein as SEQ ID NO. 22. The DNA sequence of the pAP-234 insert containing ricin and the HSV-A linker is referred to herein as SEQ ID NO. 23. The nucleotide sequence of the HSV-B linker regions of pAP-235 is referred to herein as SEQ ID NO. 24. The DNA sequence of the pAP-236 insert containing ricin and the HSV-B linker is referred to herein as SEQ ID NO. 25. The nucleotide sequence of the VZV-A linker regions of pAP-237 are referred to herein as SEQ ID NO. 26. The DNA sequence of the pAP-238 insert containing ricin and the VZV-A linker are referred to herein as SEQ ID NO. 27. The nucleotide sequence of the VZV-B linker regions of PAP-239 is referred to herein as SEQ ID NO. 28. The DNA sequence of the pAP-240 insert containing ricin and the VZV-B linker is referred to herein as SEQ ID NO. 29. The nucleotide sequence of the EBV-A linker regions of pAP-241 is referred to herein as SEQ ID NO. 30. The DNA sequence of the pAP-242 insert containing ricin and the EBV-A linker is referred to herein as SEQ ID NO. 31. The nucleotide sequence of the EBV-B linker regions of pAP-243 is referred to herein as SEQ ID NO. 32. The DNA sequence of the pAP-244 insert containing ricin and the EBV-B linker is referred to herein as SEQ ID NO. 33. The nucleotide sequence of the CMV-A linker regions of pAP-245 is referred to herein as SEQ ID NO. 34. The DNA sequence of the pAP-246 insert containing ricin and the CMV-A linker is referred to herein as SEQ ID NO. 35. The nucleotide sequence of the CMV-B linker regions of pAP-247 is referred to herein as SEQ ID NO. 36. The DNA sequence of the pAP-248 insert containing ricin and the CMV-B linker is referred to herein as SEQ ID NO. 37. The nucleotide sequence of the HHV-6 linker regions of pAP-249 is referred to herein as SEQ ID NO. 38. The DNA sequence of the pAP-250 insert containing ricin and the HHV-6 linker is referred to herein as SEQ ID NO. 39. The amino acid sequences of the cancer protease-sensitive amino acid linkers contained in the following pAP proteins have the sequence ID numbers as indicated: ρAP-213 and pAP-214 (SEQ ID NO. 40); pAP-215 and pAP-216 (SEQ ID NO. 41); pAP-217 and pAP-218; (SEQ ID NO. 42); pAP-219 and pAP-220 (SEQ ID NO. 43); and pAP-221 and pAP-222 (SEQ ID NO.44). The amino acid sequences of the following cancer protease-sensitive linkers are referred to herein with the corresponding sequence ID numbers: pAP-241 and pAP-242 (SEQ ID NO. 45); and pAP- 243 and pAP-244 (SEQ ID NO. 46). The nucleotide sequence of the ILV linker regions of pAP-253 is referred to herein as SEQ ID NO. 47. The DNA sequence of the pAP-254 insert containing ricin and the ILV linker is referred to herein as SEQ ID NO. 48. The nucleotide sequence of the HAV-A linker regions of pAP-257 is referred to herein as SEQ ID NO. 49. The DNA sequence of the pAP-258 insert containing ricin and HAV-A linker is referred to herein as SEQ ID NO. 50. The nucleotide sequence of the HAV-B linker regions of ρAP-255 is referred to herein as SEQ ID NO. 51. The DNA sequence of the pAP-256 insert containing ricin and the HAV-B linker is referred to herein as SEQ ID NO. 52. The nucleotide sequence of the CAN linker regions of pAP-259 is referred to herein as SEQ ID NO. 53. The DNA sequence of the pAP-260 insert containing ricin and the CAN linker is referred to herein as SEQ ID NO. 54. The amino acid sequences of Plasmodium falciparum protease-sensitive linkers are referred to herein by the sequence ID numbers as follows: pAP-223 and ρAP-224 (SEQ ID NO 55); pAP-225 and pAP-226 (SEQ ID NO 56); pAP-227 and pAP-228 (SEQ ID NO 57); pAP-229 and pAP-230 (SEQ ID NO 58); and pAP-231 and pAP-232 (SEQ ID NO 59) (see Figure 26). The amino acid sequences of the viral protease-sensitive linkers which follow are referred to herein by the sequence ID numbers indicated: pAP-233 and pAP 234 (SEQ ID NO 60); pAP-235 and pAP-236 (SEQ ID NO 61); and pAP-249 and pAP-250 (SEQ ID NO 62) (see Figure 27). The amino acid sequences of the viral protease-sensitive linkers which follow are referred to herein by the sequence ID numbers indicated: pAP-245 and pAP-246 (SEQ ID NO 63) ; and pAP-247 and pAP- 248 (SEQ ID NO 64) (see Figure 27). The amino acid sequences of the viral protease-sensitive linkers which follow are referred to herein by the sequence ID numbers indicated: pAP-237 and pAP-238 (SEQ ID NO 65); and pAP-239 and pAP- 240 (SEQ ID NO 66); pAP-253 and pAP-254 (SEQ ID NO 67); pAP-255 and pAP-256 (SEQ ID NO 68); and pAP-257 and pAP-258 (SEQ ID NO 69) (see Figure 27). The amino acid sequences of the Candida aspartic protease-sensitive linkers are referred to herein by the sequence ID numbers indicated: pAP-259 and pAP-260 (SEQ ID NO 70); pAP-261 and pAP-262 (SEQ ID NO 71); and pAP-263 and pAP-264 (SEQ ID NO 72 ). An alternative mutagenesis and cloning strategy that can be used to generate the disease-specific protease-sensitive linker variants is summarized in Figure 29. The first step of this method involves a DNA amplification using a set of mutagenic primers in combination with the two flanking primers Ricin-109Eco and Ricinl729Pst. Restriction digested PCR fragments (Eco RI and Pst I) are gel purified. Preproricin variants produced from this method can be subcloned directly into the baculovirus transfer vector digested with Eco RI and Pst I and intermediate ligation steps involving pBluescript SK and pSB2 are circumvented. The cloning strategies used to generate disease-specific protease-sensitive linker variants are summarized in Part A of Figures 30 to 47. With respect to the nucleotide sequences and amino acid sequences prepared as a result of the implementation of this strategy the following sequences have been assigned the sequence ID numbers as indicated. The nucleotide sequence of the HCV-A linker region of pAP-262 is referred to herein as SEQ ID NO. 73. The DNA sequence of the pAP-262 insert is referred to herein as SEQ ID NO. 74. The amino acid sequence of the mutant preproricin linker region for HCV-A, pAP-262, is referred to herein as SEQ ID NO. 75. The nucleotide sequence of the HCV-B linker region of pAP-264 is referred to herein as SEQ ID NO. 76. The DNA sequence of the pAP-264 insert is referred to herein as SEQ ID NO. 77. The amino acid sequence of the mutant preproricin linker region for HCV-B, pAP-264, is referred to herein as SEQ ID NO. 78. The nucleotide sequence of the HCV-C linker region of pAP-266 is referred to herein as SEQ ID NO. 79. The DNA sequence of the pAP-266 insert is referred to herein as SEQ ID NO. 80. The amino acid sequence of the mutant preproricin linker region for HCV-C, pAP-266, is referred to herein as SEQ ID NO. 81. The nucleotide sequence of the HCV-D linker region of ρAP-268 is referred to herein as SEQ ID NO. 82. The DNA sequence of the pAP-268 insert is referred to herein as SEQ ID NO. 83. The amino acid sequence of the mutant preproricin linker region for HCV-D , pAP-268, is referred to herein as SEQ ID NO. 84. The nucleotide sequence of the MMP-2 linker region of ρAP-270 is referred to herein as SEQ ID NO. 85. The DNA sequence of the pAP-270 insert is referred to herein as SEQ ID NO. 86. The amino acid sequence of the mutant preproricin linker region for MMP-2, pAP-270, is referred to herein as SEQ ID NO. 87. The nucleotide acid sequence of the Cathepsin B (Site 2) linker region of pAP-272 is referred to herein as SEQ ID NO. 88. The DNA sequence of the pAP-272 insert is referred to herein as SEQ ID NO. 89. The amino acid sequence of the mutant preproricin linker region for Cathepsin B (Site 2), pAP-272, is referred to herein as SEQ ID NO. 90. The nucleotide sequence of the Cathepsin L linker region of pAP-274 is referred to herein as SEQ ID NO. 91. The DNA sequence of the pAP-274 insert is referred to herein as SEQ ID NO. 92. The amino acid sequence of the mutant preproricin linker region of Cathepsin L, pAP-274, is referred to herein as SEQ ID NO. 93. The nucleotide sequence of Cathepsin D linker region of pAP-276 is referred to herein as SEQ ID NO. 94. The DNA sequence of the pAP-276 insert is referred to herein as SEQ ID NO. 95. The amino acid sequence of the mutant preproricin linker region for Cathepsin D, pAP-276, is referred to herein as SEQ ID NO. 96. The nucleotide sequence of the MMP-1 linker region of pAP-278 is referred to herein as SEQ ID NO. 97. The DNA sequence of the pAP-278 insert is referred to herein as SEQ ID NO. 98. The amino acid sequence of the mutant preproricin linker region for MMP-1, pAP-278, is referred to herein as SEQ ID NO. 99. The nucleotide sequence of the Urokinase-Type Plasminogen Activator linker region of pAP-280 is referred to herein as SEQ ID NO. 100. The DNA sequene of the pAP-280 insert is referred to herein as SEQ ID NO. 101. The amino acid sequence of the mutant preproricin linker region for Urokinase-Type Plasminogen Activator, pAP-280, is referred to herein as SEQ ID NO. 102. The nucleotide sequence of MT-MMP linker region of pAP-282 is referred to herein as SEQ ID NO. 103. The DNA sequence of the pAP-282 insert is referred to herein as SEQ ID NO. 104. The amino acid sequence of the mutant preproricin linker region for MT-MMP, pAP-282, is referred to herein as SEQ ID NO. 105. The nucleotide sequence of the MMP-11 linker region of pAP-284 is referred to herein as SEQ ID NO. 106. The DNA sequence of the pAP-284 insert is referred to herein as SEQ ID NO. 107. The amino acid sequence of the mutant preproricin linker region for MMP-11, pAP-284, is referred to herein as SEQ ID NO. 108. The nucleotide sequence of the MMP-13 linker region of pAP-286 is referred to herein as SEQ ID NO. 109. The DNA sequence of the pAP-286 insert is referred to herein as SEQ ID NO. 110. The amino acid sequence of the mutant preproricin linker region for MMP-13, pAP-286, is referred to herein as SEQ ID NO. 111. The nucleotide sequence of the Tissue-type Plasminogen Activator linker region of pAP-288 is referred to herein as SEQ ID NO. 112. The DNA sequence of the pAP-288 insert is referred to herein as SEQ ID NO. 113. The amino acid sequence of the mutant preproricin linker region for Tissue-type Plasminogen Activator, pAP-288, is referred to herein as SEQ ID NO. 114. The nucleotide sequence of the human Prostate-Specific Antigen linker region of pAP-290 is referred to herein as SEQ ID NO. 115. The DNA sequence of the pAP-290 insert is referred to herein as SEQ ID NO. 116. The amino acid sequence of the mutant preproricin linker region for the human Prostate-Specific Antigen, pAP-290, is referred to herein as SEQ ID NO. 117. The nucleotide sequence of the kallikrein linker region of pAP-292 is referred to herein as SEQ ID NO. 118. The DNA sequence of the pAP-292 insert is referred to herein as SEQ ID NO. 119. The amino acid sequence of the mutant preproricin linker region for the kallikrein, pAP-292, is referred to herein as SEQ ID NO. 120. The nucleotide sequence of the neutrophil elastase linker region of ρAP-294 is referred to herein as SEQ ID NO. 121. The DNA sequence of the pAP-294 insert is referred to herein as SEQ ID NO. 122. The amino acid sequence of the mutant preproricin linker region for neutrophil elastase, pAP-294, is referred to herein as SEQ ID NO. 123. The nucleotide sequence of the calpain linker region of pAP-296 is referred to herein as SEQ ID NO. 124. The DNA sequence of the pAP-296 insert is referred to herein as SEQ ID NO. 125. The amino acid sequence of the mutant preproricin linker region for calpain, pAP-296, is referred to herein as SEQ ID NO. 126. The amino acid sequence of the wild type linker region is referred to herein as SEQ ID NO. 127. The nucleic acid molecule of the invention has sequences encoding an A chain of a ricin-like toxin, a B chain of a ricin-like toxin and a heterologous linker sequence containing a cleavage recognition site for a disease-specific protease. The nucleic acid may be expressed to provide a recombinant protein having an A chain of a ricin-like toxin, a B chain of a ricin-like toxin and a heterologous linker sequence containing a cleavage recognition site for a disease-specific protease. The nucleic acid molecule may comprise the A and /or B chain of ricin. The ricin gene has been cloned and sequenced, and the X-ray crystal structures of the A and B chains are published (Rutenber, E., et al. Proteins 10:240-250 (1991); Weston et al, Mol. Biol. 244:410-422 (1994); Lamb and Lord, Eur. J. Biochem. 14:265 (1985); Hailing, K., et al, Nucleic Acids Res. 13:8019 (1985)). It will be appreciated that the invention includes nucleic acid molecules encoding truncations of A and B chains of ricin like proteins and analogs and homologs of A and B chains of ricin-like proteins and truncations thereof (i.e., ricin-like proteins), as described herein. It will further be appreciated that variant forms of the nucleic acid molecules of the invention which arise by alternative splicing of an mRNA corresponding to a cDNA of the invention are encompassed by the invention. Another aspect of the invention provides a nucleotide sequence which hybridizes under high stringency conditions to a nucleotide sequence encoding the A and /or B chains of a ricin-like protein. Appropriate stringency conditions which promote DNA hybridization are known to those skilled in the art, or can be found in Current Protocols in Molecular Biology, John Wiley & Sons, N.Y. (1989), 6.3.1 6.3.6. For example, 6.0 x sodium chloride /sodium citrate (SSC) at about 45°C, followed by a wash of 2.0 x SSC at 50°C may be employed. The stringency may be selected based on the conditions used in the wash step. By way of example, the salt concentration in the wash step can be selected from a high stringency of about 0.2 x SSC at 50°C. In addition, the temperature in the wash step can be at high stringency conditions, at about 65°C. The nucleic acid molecule may comprise the A and /or B chain of a ricin-like toxin. Methods for cloning ricin-like toxins are known in the art and are described, for example, in E.P. 466,222. Sequences encoding ricin or ricin-like A and B chains may be obtained by selective amplification of a coding region, using sets of degenerative primers or probes for selectively amplifying the coding region in a genomic or cDNA library. Appropriate primers may be selected from the nucleic acid sequence of A and B chains of ricin or ricin-like toxins. It is also possible to design synthetic oligonucleotide primers from the nucleotide sequences for use in PCR. Suitable primers may be selected from the sequences encoding regions of ricin-like proteins which are highly conserved, as described for example in U.S. Patent No 5,101,025 and E.P. 466,222. A nucleic acid can be amplified from cDNA or genomic DNA using these oligonucleotide primers and standard PCR amplification techniques. The nucleic acid so amplified can be cloned into an appropriate vector and characterized by DNA sequence analysis. It will be appreciated that cDNA may be prepared from mRNA, by isolating total cellular mRNA by a variety of techniques, for example, by using the guanidinium-thiocyanate extraction procedure of Chirgwin et al, Biochemistry 18, 5294-5299 (1979). cDNA is then synthesized from the mRNA using reverse transcriptase (for example, Moloney MLV reverse transcriptase available from Gibco /BRL, Bethesda, MD, or AMV reverse transcriptase available from Seikagaku America, Inc., St. Petersburg, FL). It will be appreciated that the methods described above may be used to obtain the coding sequence from plants, bacteria or fungi, preferably plants, which produce known ricin-like proteins and also to screen for the presence of genes encoding as yet unknown ricin-like proteins. A sequence containing a cleavage recognition site for a specific protease may be selected based on the disease or the pathogen which is to be targeted by the recombinant protein. The cleavage recognition site may be selected from sequences known to encode a cleavage recognition site for the cancer, viral or parasitic protease. Sequences encoding cleavage recognition sites may be identified by testing the expression product of the sequence for susceptibility to cleavage by the respective protease. A sequence containing a cleavage recognition site for a viral, fungal, parasitic or cancer associated protease may be selected based on the retrovirus which is to be targeted by the recombinant protein. The cleavage recognition site may be selected from sequences known to encode a cleavage recognition site for the viral, fungal, parasitic or cancer associated protease. Sequences encoding cleavage recognition sites may be identified by testing the expression product of the sequence for susceptibility to cleavage by a viral, fungal, parasitic or cancer associated protease. A polypeptide containing the suspected cleavage recognition site may be incubated with a protease and the amount of cleavage product determined (Dilannit, 1990, J. Biol. Chem. 285: 17345-17354 (1990)). The protease may be prepared by methods known in the art and used to test suspected cleavage recognition sites. In one embodiment, the preparation of tumour- associated cathepsin B, its substrates and enzymatic activity assay methodology have been described by Sloane, B.F. et al. (Proc. Natl. Acad. Sci. USA 83:2483-2487 (1986)), Schwartz, M.K. (Clin. Chim. Acta 237:67-78 (1995)), and Panchal, R.G. et al. (Nature Biotechnol. 14:852-856 (1996)). The preparation of Epstein-Barr virus protease, its substrates and enzymatic activity assay methodology have been described by Welch, A.R. (Proc. Natl. Acad. Sci. USA 88:10792-10796 (1991)). In another embodiment, the preparation of Plasmodium falciparum proteases, their substrates and enzymatic activity assay methodology have been described by Goldberg, D.E. et al. (J. Exp. Med. 173:961-969 (1991)), Cooper & Bujard (Mol. Biochem. Parasitol. 56:151- 160 (1992)), Nwagwu, M. et al. (Exp. Parasitol. 75:399-414 (1992)), Rosenthal, PJ. et al. (/. Clin. Invest. 91:1052-1056 (1993)), Blackman, M.J. et al. (Mol. Biochem. Parasitol. 62:103-114 (1995)). In a further embodiment, the preparation of proteases from human cytomegalovirus, human herpes virus, varicalla zoster virus and infectious laryngotracheitis virus have been taught by Liu F. & Roizman, B. (/. Virol. 65:5149-5156 (1991)) and Welch, A.R. (Proc. Natl. Acad. Sci. USA 88:10792-10796 (1991)). In addition, their respective substrates and enzymatic activity assay methodologies are also described. In another embodiment, the preparation of hepatitis A virus protease, its substrates and enzymatic activity assay methodology have been described by Jewell, D.A. et al. (Biochemistry 31:7862-7869 (1992)). The preparation of poliovirus protease, its substrates and enzymatic activity assay methodology have been described by Weidner, J.R. et al. (Arch. Biochem. Biophys. 286:402-408 (1991)). The preparation of human rhinovirus protease, its substrates and enzymatic activity assay methodology have been described by Long, A.C. et al. (FEBS Lett. 258:75-78 (1989)). In another embodiment of the invention, the preparation of proteases associated with Candida yeasts their substrates and enzymatic activity are contemplated, including the aspartic proteinases which have been associated specifically with numerous virulent strains of Candida including Candida albican, Candida tropicalis, and Candida parapsilosis (Abad-Zapatero, C. et al., Protein Sci. 5:640-652 (1996); Cutfield, S.M. et al, Biochemistry 35:398-410 (1995); Ruchel, R. et al, Zentralbl. Bakteriol. Mikrobiol Hyg. I Abt. Orig. A. 255:537-548 (1983); Remold, H. et al, Biochim. Biophys. Acta 167:399-406 (1968)). The nucleic acid molecule of the invention may be prepared by site directed mutagenesis. For example, the cleavage site of a disease-specific protease may be prepared by site directed mutagenesis of the homologous linker sequence of a proricin-like toxin. Procedures for cloning proricin-like genes, encoding a linker sequence are described in EP 466,222. Site directed mutagenesis may be accomplished by DNA amplification of mutagenic primers in combination with flanking primers. Suitable procedures using the mutagenic primers are shown in Parts A and B of Figures 1-4, Figures 13-16, Figures 18-36, Figures 38- 41, and Figures 50-67. The nucleic acid molecule of the invention may also encode a fusion protein. A sequence encoding a heterologous linker sequence containing a cleavage recognition site for a disease-specific protease may be cloned from a cDNA or genomic library or chemically synthesized based on the known sequence of such cleavage sites. The heterologous linker sequence may then be fused in frame with the sequences encoding the A and B chains of the ricin-like toxin for expression as a fusion protein. It will be appreciated that a nucleic acid molecule encoding a fusion protein may contain a sequence encoding an A chain and a B chain from the same ricin-like toxin or the encoded A and B chains may be from different toxins. For example, the A chain may be derived from ricin and the B chain may be derived from abrin. A protein may also be prepared by chemical conjugation of the A and B chains and linker sequence using conventional coupling agents for covalent attachment. An isolated and purified nucleic acid molecule of the invention which is RNA can be isolated by cloning a cDNA encoding an A and B chain and a linker into an appropriate vector which allows for transcription of the cDNA to produce an RNA molecule which encodes a protein of the invention. For example, a cDNA can be cloned downstream of a bacteriophage promoter, (e.g. a T7 promoter) in a vector, cDNA can be transcribed in vitro with T7 polymerase, and the resultant RNA can be isolated by standard techniques. Recombinant Protein of the Invention As previously mentioned, the invention provides novel recombinant proteins which incorporate the A and B chains of a ricin like toxin linked by a heterologous linker sequence containing a cleavage recognition site for a disease-specific protease. It is an advantage of the recombinant proteins of the invention that they are non-toxic until the A chain is liberated from the B chain by specific cleavage of the linker by the target protease. Thus the protein may be used to specifically target cancer cells or cells infected with a virus or parasite in the absence of additional specific cell-binding components to target infected cells. It is a further advantage that the disease-specific protease cleaves the heterologous linker intracellularly thereby releasing the toxic A chain directly into the cytoplasm of the cancer cell or infected cell. As a result, said cells are specifically targeted and non-infected normal cells are not directly exposed to the activated free A chain. Ricin is a plant derived ribosome inhibiting protein which blocks protein synthesis in eukaryotic cells. Ricin may be derived from the seeds of Ricinus communis (castor oil plant).The ricin toxin is a glycosylated heterodimer with A and B chain molecular masses of 30,625 Da and 31,431 Da respectively. The A chain of ricin has an N-glycosidase activity and catalyzes the excision of a specific adenine residue from the 28S rRNA of eukaryotic ribosomes (Endo, Y; & Tsurugi, K. J. Biol. Chem. 262:8128 (1987)). The B chain of ricin, although not toxic in itself, promotes the toxicity of the A chain by binding to galactose residues on the surface of eukaryotic cells and stimulating receptor-mediated endocytosis of the toxin molecule (Simmons et al., Biol. Chem. 261:7912 (1986)). All protein toxins are initially produced in an inactive, precursor form. Ricin is initially produced as a single polypeptide (preproricin) with a 35 amino acid N-terminal presequence and 12 amino acid linker between the A and B chains. The pre-sequence is removed during translocation of the ricin precursor into the endoplasmic reticulum (Lord, J.M., Eur. J. Biochem. 146:403-409 (1985) and Lord, J.M., Eur. J. Biochem. 146:411-416 (1985)). The proricin is then translocated into specialized organelles called protein bodies where a plant protease cleaves the protein at a linker region between the A and B chains (Lord, J.M. et al, FASAB Journal 8:201-208 (1994)). The two chains, however, remain covalently attached by an interchain disulfide bond (cysteine 259 in the A chain to cysteine 4 in the B chain) and mature disulfide linked ricin is stored in protein bodies inside plant cells. The A chain is inactive in the proricin (O'Hare, M., et al., FEBS Lett. 273:200-204 (1990)) and it is inactive in the disulfide-linked mature ricin (Richardson, P.T. et al, FEBS Lett. 255:15-20 (1989)). The ribosomes of the castor bean plant are themselves susceptible to inactivation by ricin A chain; however, as there is no cell surface galactose to permit B chain recognition the A chain cannot re-enter the cell. Ricin-like proteins include, but are not limited to, bacterial, fungal and plant toxins which have A and B chains and inactivate ribosomes and inhibit protein synthesis. The A chain is an active polypeptide subunit which is responsible for the pharmacologic effect of the toxin. In most cases the active component of the A chain is an enzyme. The B chain is responsible for binding the toxin to the cell surface and is thought to facilitate entry of the A chain into the cell cytoplasm. The A and B chains in the mature toxins are linked by disulfide bonds. The toxins most similar in structure to ricin are plant toxins which have one A chain and one B chain. Examples of such toxins include abrin which may be isolated from the seeds of Abrus precatorius and modeccin. Ricin-like bacterial proteins include diphtheria toxin, which is produced by Corynebacterium diphtheriae, Pseudomonas enterotoxin A and cholera toxin. It will be appreciated that the term ricin-like toxins is also intended to include the A chain of those toxins which have only an A chain. The recombinant proteins of the invention could include the A chain of these toxins conjugated to, or expressed as, a recombinant protein with the B chain of another toxin. Examples of plant toxins having only an A chain include trichosanthin, MMC and pokeweed antiviral proteins, dianthin 30, dianthin 32, crotin II, curcin II and wheat germ inhibitor. Examples of fungal toxins having only an A chain include alpha-sarcin, restrictocin, mitogillin, enomycin, phenomycin. Examples of bacterial toxins having only an A chain include cytotoxin from Shigella dysenteriae and related Shiga-like toxins. Recombinant trichosanthin and the coding sequence thereof is disclosed in U.S. Patents 5,101,025 and 5,128,460. In addition to the entire A or B chains of a ricin-like toxin, it will be appreciated that the recombinant protein of the invention may contain only that portion of the A chain which is necessary for exerting its cytotoxic effect. For example, the first 30 amino acids of the ricin A chain may be removed resulting in a truncated A chain which retains toxic activity. The truncated ricin or ricin-like A chain may be prepared by expression of a truncated gene or by proteolytic degradation, for example with Nagarase (Funmatsu et al., Jap. J. Med. Sci. Biol. 23:264-267 (1970)). Similarly, the recombinant protein of the invention may contain only that portion of the B chain necessary for galactose recognition, cell binding and transport into the cell cytoplasm. Truncated B chains are described for example in E.P. 145,111. The A and B chains may be glycosylated or non-glycosylated. Glycosylated A and B chains may be obtained by expression in the appropriate host cell capable of glycosylation. Non-glycosylated chains may be obtained by expression in nonglycosylating host cells or by treatment to remove or destroy the carbohydrate moieties. The proteins of the invention may be prepared using recombinant DNA methods. Accordingly, the nucleic acid molecules of the present invention may be incorporated in a known manner into an appropriate expression vector which ensures good expression of the protein. Possible expression vectors include but are not limited to cosmids, plasmids, or modified viruses (e.g. replication defective retroviruses, adenoviruses and adeno-associated viruses), so long as the vector is compatible with the host cell used. The expression vectors are "suitable for transformation of a host cell", which means that the expression vectors contain a nucleic acid molecule of the invention and regulatory sequences selected on the basis of the host cells to be used for expression, which is operatively linked to the nucleic acid molecule. Operatively linked is intended to mean that the nucleic acid is linked to regulatory sequences in a manner which allows expression of the nucleic acid. The invention therefore contemplates a recombinant expression vector of the invention containing a nucleic acid molecule of the invention, or a fragment thereof, and the necessary regulatory sequences for the transcription and translation of the inserted protein- sequence. Suitable regulatory sequences may be derived from a variety of sources, including bacterial, fungal, viral, mammalian, or insect genes (For example, see the regulatory sequences described in Goeddel, Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, CA (1990). Selection of appropriate regulatory sequences is dependent on the host cell chosen as discussed below, and may be readily accomplished by one of ordinary skill in the art. Examples of such regulatory sequences include: a transcriptional promoter and enhancer or RNA polymerase binding sequence, a ribosomal binding sequence, including a translation initiation signal. Additionally, depending on the host cell chosen and the vector employed, other sequences, such as an origin of replication, additional DNA restriction sites, enhancers, and sequences conferring inducibility of transcription may be incorporated into the expression vector. It will also be appreciated that the necessary regulatory sequences may be supplied by the native A and B chains and /or its flanking regions. The recombinant expression vectors of the invention may also contain a selectable marker gene which facilitates the selection of host cells transformed or transfected with a recombinant molecule of the invention. Examples of selectable marker genes are genes encoding a protein such as G418 and hygromycin which confer resistance to certain drugs, β-galactosidase, chloramphenicol acetyltransferase, firefly luciferase, or an immunoglobulin or portion thereof such as the Fc portion of an immunoglobulin preferably IgG. Transcription of the selectable marker gene is monitored by changes in the concentration of the selectable marker protein such as β-galactosidase, chloramphenicol acetyltransferase, or firefly luciferase. If the selectable marker gene encodes a protein conferring antibiotic resistance such as neomycin resistance transformant cells can be selected with G418. Cells that have incorporated the selectable marker gene will survive, while the other cells die. This makes it possible to visualize and assay for expression of recombinant expression vectors of the invention and in particular to determine the effect of a mutation on expression and phenotype. It will be appreciated that selectable markers can be introduced on a separate vector from the nucleic acid of interest. The recombinant expression vectors may also contain genes which encode a fusion moiety which provides increased expression of the recombinant protein; increased solubility of the recombinant protein; and aid in the purification of the target recombinant protein by acting as a ligand in affinity purification. For example, a proteolytic cleavage site may be added to the target recombinant protein to allow separation of the recombinant protein from the fusion moiety subsequent to purification of the fusion protein. Typical fusion expression vectors include pGEX (Amrad Corp., Melbourne, Australia), pMAL (New England Biolabs, Beverly, MA) and pRIT5 (Pharmacia, Piscataway, NJ) which fuse glutathione S-transferase (GST), maltose E binding protein, or protein A, respectively, to the recombinant protein. Recombinant expression vectors can be introduced into host cells to produce a transformant host cell. The term "transformant host cell" is intended to include prokaryotic and eukaryotic cells which have been transformed or transfected with a recombinant expression vector of the invention. The terms "transformed with", "transfected with", "transformation" and "transfection" are intended to encompass introduction of nucleic acid (e.g. a vector) into a cell by one of many possible techniques known in the art. Prokaryotic cells can be transformed with nucleic acid by, for example, electroporation or calcium-chloride mediated transformation. Nucleic acid can be introduced into mammalian cells via conventional techniques such as calcium phosphate or calcium chloride co-precipitation, DEAE-dextran mediated transfection, lipofectin, electroporation or microinjection. Suitable methods for transforming and transfecting host cells can be found in Sambrook et al. (Molecular Cloning: A Laboratory Manual, 2nd Edition, Cold Spring Harbor Laboratory press (1989)), and other laboratory textbooks. Suitable host cells include a wide variety of prokaryotic and eukaryotic host cells. For example, the proteins of the invention may be expressed in bacterial cells such as E. coli, insect cells (using baculovirus), yeast cells or mammalian cells. Other suitable host cells can be found in Goeddel, Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, CA (1991). More particularly, bacterial host cells suitable for carrying out the present invention include E. coli, B. subtilis, Salmonella typhimurium, and various species within the genus' Pseudomonas, Streptomyces, and Staphylococcus, as well as many other bacterial species well known to one of ordinary skill in the art. Suitable bacterial expression vectors preferably comprise a promoter which functions in the host cell, one or more selectable phenotypic markers, and a bacterial origin of replication. Representative promoters include the β-lactamase (penicillinase) and lactose promoter system (see Chang et al., Nature 275:615 (1978)), the trp promoter (Nichols and Yanofsky, Meth in Enzymology 101:155, (1983) and the tac promoter (Russell et al., Gene 20: 231, (1982)). Representative selectable markers include various antibiotic resistance markers such as the kanamycin or ampicillin resistance genes. Suitable expression vectors include but are not limited to bacteriophages such as lambda derivatives or plasmids such as pBR322 (Bolivar et al, Gene 2:9S, (1977)), the pUC plasmids pUC18, pUC19, pUC118, pUC119 (see Messing, Meth in Enzymology 101:20-77, 1983 and Vieira and Messing, Gene 19:259-268 (1982)), and pNH8A, pNH16a, pNH18a, and Bluescript M13 (Stratagene, La Jolla, Calif.). Typical fusion expression vectors which may be used are discussed above, e.g. pGEX (Amrad Corp., Melbourne, Australia), pMAL (New England Biolabs, Beverly, MA) and pRIT5 (Pharmacia, Piscataway, NJ). Examples of inducible non-fusion expression vectors include pTrc (Amann et al., Gene 69:301-315 (1988)) and pET lid (Studier et al, Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, California, 60-89 (1990)). Yeast and fungi host cells suitable for carrying out the present invention include, but are not limited to Saccharomyces cerevisae, the genera Pichia or Kluyveromyces and various species of the genus Aspergillus. Examples of vectors for expression in yeast S. cerivisae include pYepSecl (Baldari. et al., Embo J. 6:229-234 (1987)), pMFa (Kurjan and Herskowitz, Cell 30:933-943 (1982)), pJRY88 (Schultz et al., Gene 54:113-123 (1987)), and ρYES2 (Invitrogen Corporation, San Diego, CA). Protocols for the transformation of yeast and fungi are well known to those of ordinary skill in the art.(see Hinnen et al., Proc. Natl. Acad. Sci. USA 75:1929 (1978); Itoh et al., /. Bacteriology 153:163 (1983), and Cullen et al. (Bio/Technology 5:369 (1987)). Mammalian cells suitable for carrying out the present invention include, among others: COS (e.g., ATCC No. CRL 1650 or 1651), BHK (e.g. ATCC No. CRL 6281), CHO (ATCC No. CCL 61), HeLa (e.g., ATCC No. CCL 2), 293 (ATCC No. 1573) and NS-1 cells. Suitable expression vectors for directing expression in mammalian cells generally include a promoter (e.g., derived from viral material such as polyoma, Adenovirus 2, cytomegalovirus and Simian Virus 40), as well as other transcriptional and translational control sequences. Examples of mammalian expression vectors include pCDMδ (Seed, B., Nature 329:840 (1987)) and pMT2PC (Kaufman et al., EMBO J. 6:187-195 (1987)). Given the teachings provided herein, promoters, terminators, and methods for introducing expression vectors of an appropriate type into plant, avian, and insect cells may also be readily accomplished. For example, within one embodiment, the proteins of the invention may be expressed from plant cells (see Sinkar et al., /. Biosci (Bangalore) 11:47-58 (1987), which reviews the use of Agrobacterium rhizogenes vectors; see also Zambryski et al., Genetic Engineering, Principles and Methods, Hollaender and Setlow (eds.), Vol. VI, pp. 253-278, Plenum Press, New York (1984), which describes the use of expression vectors for plant cells, including, among others, pAS2022, pAS2023, and pAS2034). Insect cells suitable for carrying out the present invention include cells and cell lines from Bombyx, Trichoplusia or Spodotera species. Baculovirus vectors available for expression of proteins in cultured insect cells (SF 9 cells) include the pAc series (Smith et al., Mol. Cell Biol. 3:2156-2165 (1983)) and the pVL series (Lucklow, V.A., and Summers, M.D., Virology 170:31-39 (1989)). Some baculovirus-insect cell expression systems suitable for expression of the recombinant proteins of the invention are described in PCT /US/ 02442. Alternatively, the proteins of the invention may also be expressed in non-human transgenic animals such as, rats, rabbits, sheep and pigs (Hammer et al. Nature 315:680-683 (1985); Palmiter et al. Science 222:809-814 (1983); Brinster et al. Proc. Natl. Acad. Sci. USA 82:4438-4442 (1985); Palmiter and Brinster Cell 41:343-345 (1985) and U.S. Patent No. 4,736,866). The proteins of the invention may also be prepared by chemical synthesis using techniques well known in the chemistry of proteins such as solid phase synthesis (Merrifield, /. Am. Chem. Assoc. 85:2149-2154 (1964)) or synthesis in homogenous solution (Houbenweyl, Methods of Organic Chemistry, ed. E. Wansch, Vol. 15 I and II, Thieme, Stuttgart (1987)). The present invention also provides proteins comprising an A chain of a ricin-like toxin, a B chain of a ricin-like toxin and a heterologous linker amino acid sequence linking the A and B chains, wherein the linker sequence contains a cleavage recognition site for a disease-specific protease. Such a protein could be prepared other than by recombinant means, for example by chemical synthesis or by conjugation of A and B chains and a linker sequence isolated and purified from their natural plant, fungal or bacterial source. Such A and B chains could be prepared having the glycosylation pattern of the native ricin-like toxin. N-terminal or C-terminal fusion proteins comprising the protein of the invention conjugated with other molecules, such as proteins may be prepared by fusing, through recombinant techniques. The resultant fusion proteins contain a protein of the invention fused to the selected protein or marker protein as described herein. The recombinant protein of the invention may also be conjugated to other proteins by known techniques. For example, the proteins may be coupled using heterobifunctional thiol-containing linkers as described in WO 90/10457, N-succinimidyl-3-(2-pyridyldithio-proprionate) or N-succinimidyl-5 thioacetate. Examples of proteins which may be used to prepare fusion proteins or conjugates include cell binding proteins such as immunoglobulins, hormones, growth factors, lectins, insulin, low density lipoprotein, glucagon, endorphins, transferrin, bombesin, asialoglycoprotein glutathione-S-transferase (GST), hemagglutinin (HA), and truncated myc. Utility of the Nucleic Acid Molecules and Proteins of the Invention The proteins of the invention may be used to specifically inhibit or destroy mammalian cells affected by a disease or infection which have associated with such cells a specific protease, i.e., disease- specific, for example cancer cells or cells infected with a virus, fungus or parasite, all of which are encompased within the term "disease-specific." It is an advantage of the recombinant proteins of the invention that they have specificity for said cells without the need for a cell binding component. The ricin-like B chain of the recombinant proteins recognize galactose moieties on the cell surface and ensure that the protein is taken up by the diseased cell and released into the cytoplasm. When the protein is internalized into a non-infected cell, cleavage of the heterologous linker would not occur in the absence of the disease- specific protease and the A chain will remain inactive bound to the B chain. Conversely, when the protein is internalized into a diseased cell, the disease-specific protease will cleave the cleavage recognition site in the linker thereby releasing the toxic A chain. The specificity of a recombinant protein of the invention may be tested by treating the protein with the disease-specific protease which is thought to be specific for the cleavage recognition site of the linker and assaying for cleavage products. Disease-specific proteases may be isolated from cancer cells or infected cells, or they may be prepared recombinantly, for example following the procedures in Darket et al. (J. Biol. Chem. 254:2307-2312 (1988)). The cleavage products may be identified for example based on size, antigenicity or activity. The toxicity of the recombinant protein may be investigated by subjecting the cleavage products to an in vitro translation assay in cell lysates, for example using Brome Mosaic Virus mRNA as a template. Toxicity of the cleavage products may be determined using a ribosomal inactivation assay (Westby et al., Bioconjugate Chem. 3:377-382 (1992)). The effect of the cleavage products on protein synthesis may be measured in standardized assays of in vitro translation utilizing partially defined cell free systems composed for example of a reticulocyte lysate preparation as a source of ribosomes and various essential cof actors, such as mRNA template and amino acids. Use of radiolabelled amino acids in the mixture allows quantitation of incorporation of free amino acid precursors into trichloroacetic acid precipitable proteins. Rabbit reticulocyte lysates may be conveniently used (O'Hare, FEBS Lett. 273:200-204 (1990)). The ability of the recombinant proteins of the invention to selectively inhibit or destroy animal cancer cells or cells infected with a virus or parasite may be readily tested in vitro using animal cancer cell lines or cell cultures infected with the virus or parasite of interest. The selective inhibitory effect of the recombinant proteins of the invention may be determined, for example, by demonstrating the selective inhibition of viral antigen expression in infected mammalian cells, the selective inhibition of general mRNA translation and protein synthesis in diseased cells, or selective inhibition of cellular proliferation in cancer cells or infected cells. Toxicity may also be measured based on cell viability, for example the viability of infected and non-infected cell cultures exposed to the recombinant protein may be compared. Cell viability may be assessed by known techniques, such as trypan blue exclusion assays. In another example, a number of models may be used to test the cytotoxicity of recombinant proteins having a heterologous linker sequence containing a cleavage recognition site for a cancer- associated matrix metalloprotease. Thompson, E.W. et al. (Breast Cancer Res. Treatment 31:357-370 (1994)) has described a model for the determination of invasiveness of human breast cancer cells in vitro by measuring tumour cell-mediated proteolysis of extracellular matrix and tumour cell invasion of reconstituted basement membrane (collagen, laminin, fibronectin, Matrigel or gelatin). Other applicable cancer cell models include cultured ovarian adenocarcinoma cells (Young, T.N. et al. Gynecol. Oncol. 62:89-99 (1996); Moore, D.H. et al. Gynecol. Oncol. 65:78-82 (1997)), human follicular thyroid cancer cells (Demeure, M.J. et al, World J. Surg. 16:770-776 (1992)), human melanoma (A-2058) and fibrosarcoma (HT-1080) cell lines (Mackay, A.R. et al. Lab. Invest. 70:781- 783 (1994)), and lung squamous (HS-24) and adenocarcinoma (SB-3) cell lines (Spiess, E. et al. /. Histochem. Cytochem. 42:917-929 (1994)). An i n vivo test system involving the implantation of tumours and measurement of tumour growth and metastasis in athymic nude mice has also been described (Thompson, E.W. et al., Breast Cancer Res. Treatment 31:357-370 (1994); Shi, Y.E. et al., Cancer Res. 53:1409-1415 (1993)). A further model may be used to test the cytotoxicity of recombinant proteins having a heterologous linker sequence containing a cleavage recognition site for a cancer-associated Cathepsin B protease is provided in human glioma (Mikkelsen, T. et al. J. Neurosurge, 83:285-290 (1995)). Similarly, the cytotoxicity of recombinant proteins having a heterologous linker sequence containing a cleavage recognition site for a malarial protease may be tested by a Plasmodium invasion assay using human erythrocytes infected with mature-stage merozoite parasites as described by McPherson, R.A. et al. (Mol. Biochem. Parasitol. 62:233-242 (1993)). Alternatively, in vitro cultures of human hepatic parenchymal cells may be used to evaluate schizont infectivity and Plasmodium merozoite generation. With respect to models of viral infection and replication, suitable animal cells which can be cultured in vitro and which are capable of maintaining viral replication can be used as hosts. The toxicity of the recombinant protein for infected and non-infected cultures may then be compared. The ability of the recombinant protein of the invention to inhibit the expression of these viral antigens may be an important indicator of the ability of the protein to inhibit viral replication. Levels of these antigens may be measured in assays using labelled antibodies having specificity for the antigens. Inhibition of viral antigen expression has been correlated with inhibition of viral replication (U.S. Patent No. 4,869,903). Toxicity may also be assessed based on a decrease in protein synthesis in target cells, which may be measured by known techniques, such as incorporation of labelled amino acids, such as [3H] leucine (O'Hare et al, FEBS Lett. 273:200-204 (1990)). Infected cells may also be pulsed with radiolabelled thymidine and incorporation of the radioactive label into cellular DNA may be taken as a measure of cellular proliferation. Toxicity may also be measured based on cell death or lysis, for example, the viability of infected and non-infected cell cultures exposed to the recombinant protein may be compared. Cell viability may be assessed by known techniques, such as trypan blue exclusion assays. Although the primary specificity of the proteins of the invention for diseased cells is mediated by the specific cleavage of the cleavage recognition site of the linker, it will be appreciated that specific cell binding components may optionally be conjugated to the proteins of the invention. Such cell binding components may be expressed as fusion proteins with the proteins of the invention or the cell binding component may be physically or chemically coupled to the protein component. Examples of suitable cell binding components include antibodies to cancer, viral or parasitic proteins. Antibodies having specificity for a cell surface protein may be prepared by conventional methods. A mammal, (e.g. a mouse, hamster, or rabbit) can be immunized with an immunogenic form of the peptide which elicits an antibody response in the mammal. Techniques for conferring immunogenicity on a peptide include conjugation to carriers or other techniques well known in the art. For example, the peptide can be administered in the presence of adjuvant. The progress of immunization can be monitored by detection of antibody titers in plasma or serum. Standard ELISA or other immunoassay procedures can be used with the immunogen as antigen to assess the levels of antibodies. Following immunization, antisera can be obtained and, if desired, polyclonal antibodies isolated from the sera. To produce monoclonal antibodies, antibody producing cells (lymphocytes) can be harvested from an immunized animal and fused with myeloma cells by standard somatic cell fusion procedures thus immortalizing these cells and yielding hybridoma cells. Such techniques are well known in the art, (e.g. the hybridoma technique originally developed by Kohler and Milstein (Nature 256:495-497 (1975)) as well as other techniques such as the human B-cell hybridoma technique (Kozbor et al., Immunol. Today 4:72 (1983)), the EBV-hybridoma technique to produce human monoclonal antibodies (Cole et al., Monoclonal Antibodies in Cancer Therapy Allen R., Bliss, Inc, pages 77-96 (1985)), and screening of combinatorial antibody libraries (Huse et al., Science 246:1275 (1989)). Hybridoma cells can be screened immunochemically for production of antibodies specifically reactive with the peptide and the monoclonal antibodies can be isolated. The term "antibody" as used herein is intended to include fragments thereof which also specifically react with a cell surface component. Antibodies can be fragmented using conventional techniques and the fragments screened for utility in the same manner as described above. For example, F(ab')2 fragments can be generated by treating antibody with pepsin. The resulting F(ab')2 fragment can be treated to reduce disulfide bridges to produce Fab' fragments. Chimeric antibody derivatives, i.e., antibody molecules that combine a non-human animal variable region and a human constant region are also contemplated within the scope of the invention. Chimeric antibody molecules can include, for example, the antigen binding domain from an antibody of a mouse, rat, or other species, with human constant regions. Conventional methods may be used to make chimeric antibodies containing the immunoglobulin variable region which recognizes a cell surface antigen (See, for example, Morrison et al., Proc. Natl Acad. Sci. U.S.A. 81:6851 (1985); Takeda et al., Nature 314:452 (1985), Cabilly et al., U.S. Patent No. 4,816,567; Boss et al., U.S. Patent No. 4,816,397; Tanaguchi et al., E.P. Patent No. 171,496; European Patent No. 173,494, United Kingdom Patent No. GB 2177096B). It is expected that chimeric antibodies would be less immunogenic in a human subject than the corresponding non-chimeric antibody. Monoclonal or chimeric antibodies specifically reactive against cell surface components can be further humanized by producing human constant region chimeras, in which parts of the variable regions, particularly the conserved framework regions of the antigen-binding domain, are of human origin and only the hypervariable regions are of non-human origin. Such immunoglobulin molecules may be made by techniques known in the art, (e.g. Teng et al., Proc. Natl. Acad. Sci. U.S.A., 80:7308-7312 (1983); Kozbor et al., Immunology Today 4:7279 (1983); Olsson et al., Meth. Enzymol., 92:3-16 (1982), and PCT Publication WO92/06193 or EP 239,400). Humanized antibodies can also be commercially produced (Scotgen Limited, 2 Holly Road, Twickenham, Middlesex, Great Britain.) Specific antibodies, or antibody fragments, reactive against cell surface components may also be generated by screening expression libraries encoding immunoglobulin genes, or portions thereof, expressed in bacteria with cell surface components. For example, complete Fab fragments, VH regions and FV regions can be expressed in bacteria using phage expression libraries (See for example Ward et al, Nature 341:544-546 (1989); Huse et al., Science 246:1275-1281 (1989); and McCafferty et al, Nature 348:552-554 (1990)). Alternatively, a SCID-hu mouse, for example the model developed by Genpharm, can be used to produce antibodies, or fragments thereof. The proteins of the invention may be formulated into pharmaceutical compositions for adminstration to subjects in a biologically compatible form suitable for administration in vivo. By "biologically compatible form suitable for administration in vivo" is meant a form of the substance to be administered in which any toxic effects are outweighed by the therapeutic effects. The substances may be administered to living organisms including humans, and animals. Administration of a therapeutically active amount of the pharmaceutical compositions of the present invention is defined as an amount effective, at dosages and for periods of time necessary to achieve the desired result. For example, a therapeutically active amount of a substance may vary according to factors such as the disease state, age, sex, and weight of the individual, and the ability of antibody to elicit a desired response in the individual. Dosage regime may be adjusted to provide the optimum therapeutic response. For example, several divided doses may be administered daily or the dose may be proportionally reduced as indicated by the exigencies of the therapeutic situation. The nucleic acid molecules of the invention may be formulated into pharmaceutical compositions for adminstration to subjects in a biologically compatible form suitable for administration in vivo. By "biologically compatible form suitable for administration in vivo" is meant a form of the substance to be administered in which any toxic effects are outweighed by the therapeutic effects. The substances may be administered to living organisms including humans, and animals. Administration of a therapeutically active amount of the pharmaceutical compositions of the present invention is defined as an amount effective, at dosages and for periods of time necessary to achieve the desired result. For example, a therapeutically active amount of a substance may vary according to factors such as the disease state, age, sex, and weight of the individual, and the ability of antibody to elicit a desired response in the individual. Dosage regime may be adjusted to provide the optimum therapeutic response. For example, several divided doses may be administered daily or the dose may be proportionally reduced as indicated by the exigencies of the therapeutic situation. The active substance may be administered in a convenient manner such as by injection (subcutaneous, intravenous, intramuscular, etc.), oral administration, inhalation, transdermal administration (such as topical cream or ointment, etc.), or suppository applications. Depending on the route of administration, the active substance may be coated in a material to protect the compound from the action of enzymes, acids and other natural conditions which may inactivate the compound. The compositions described herein can be prepared by per se known methods for the preparation of pharmaceutically acceptable compositions which can be administered to subjects, such that an effective quantity of the active substance is combined in a mixture with a pharmaceutically acceptable vehicle. Suitable vehicles are described, for example, in Remington's Pharmaceutical Sciences (Remington's Pharmaceutical Sciences, Mack Publishing Company, Easton, Pa., USA 1985). On this basis, the compositions include, albeit not exclusively, solutions of the substances in association with one or more pharmaceutically acceptable vehicles or diluents, and contained in buffered solutions with a suitable pH and iso-osmotic with the physiological fluids. The pharmaceutical compositions may be used in methods for treating animals, including mammals, preferably humans, with cancer or infected with a virus or a parasite. It is anticipated that the compositions will be particularly useful for treating patients with B- cell lymphoproliferative disease, (melanoma), mononucleosis, cytomegalic inclusion disease, malaria, herpes, shingles, hepatitis, poliomyelitis, or infectious laryngotracheitis. The dosage and type of recombinant protein to be administered will depend on a variety of factors which may be readily monitored in human subjects. Such factors include the etiology and severity (grade and stage) of neoplasia, the stage of malarial infection (e.g. exoerythrocytic vs. erythrocytic), or antigen levels associated with viral load in patient tissues or circulation. As mentioned above, the novel recombinant toxic proteins and nucleic acid molecules of the present invention are useful in treating cancerous or infected cells wherein the cells contain a specific protease that can cleave the linker region of the recombinant toxic protein. One skilled in the art can appreciate that many different recombinant toxic proteins can be prepared once a disease associated protease has been identified. For example,the novel recombinant toxic proteins and nucleic acid molecules of the invention may be used to treat CNS tumors. Muller et al. (1993) describe increased activity of Insulin-type Growth Factor Binding Protein-3 (IGFBP-3) protease in the Cerebral Spinal Fluid of patients with CNS tumors. Cohen et al. (1992) claim that prostate-specific antigen (PSA) is an IGFBP-3 protease. The pAP290 construct described above is a substrate for PSA. Conover et al. (1994) claim that cathepsin D is IGFBP-3 protease. The pAP276 described herein is a substrate for cathepsin D. Another example of a specific use of the invention is treatment of human glioma which has been shown to produce cathepsin D (Mikkelsen, T. et al. /. Neurosurge, 83:285-290 (1995)). The pAP 214 and 272 define herein are substrates for cathepsin B. In addition, the novel proteins and nucleic acid molecules of the present invention may be used to treat cystic fibrosis. Hansen et al. (1995) describe how CF airway disease is characterized by neutrophil-dominated chronic inflammation with an excess of uninhibited neutrophil elastase (NE). NE levels in CF sputum are 350 times higher than that found in normal sputum. The pAP294 described herein is a substrate for neutrophil elastase. As well, the novel proteins and nucleic acid molecules of the present invention may also be used to treat multiple sclerosis. Bever Jr. et al. (1994) implicate cathepsin B (possibly from inflammatory cells of hematogenous origin) in the demyelination found in multiple sclerosis. pAPs 214 and 272 defined herein present substrates for cathepsin B. The term "animal" as used herein includes all members of the animal kingdom including mammals, preferably humans. The following non-limiting examples are illustrative of the present invention: EXAMPLES Example 1 Cloning and Expression of Proricin Variants Activated by Disease- Specific Proteases Isolation of total RNA The preproricin gene was cloned from new foliage of the castor bean plant. Total messenger RNA was isolated according to established procedures (Sambrook et al., Molecular Cloning: A Lab Manual (Cold Spring Harbour Press, Cold Spring Harbour, (1989)) and cDNA generated using reverse transcriptase. cDNA Synthesis: Oligonucleotides, corresponding to the extreme 5' and 3' ends of the preproricin gene were synthesized and used to PCR amplify the gene. Using the cDNA sequence for preproricin (Lamb et al., Eur. J. Biochem., 145:266-270, 1985), several oligonucleotide primers were designed to flank the start and stop codons of the preproricin open reading frame. The oligonucleotides were synthesized using an Applied Biosystems Model 392 DNA/RNA Synthesizer. First strand cDNA synthesis was primed using the oligonucleotide Ricinl729C (Table 1). Three micrograms of total RNA was used as a template for oligo Ricinl729C primed synthesis of cDNA using Superscript II Reverse Transcriptase (BRL) following the manufacturer's protocol. DNA Amplification and Cloning The first strand cDNA synthesis reaction was used as template for DNA amplification by the polymerase chain reaction (PCR). The preproricin cDNA was amplified using the upstream primer Ricin-99 and the downstream primer Ricinl729C with Vent DNA polymerase (New England Biolabs) using standard procedures (Sambrook et al., Molecular Cloning: A Laboratory Manual, Second Edition, (Cold Spring Harbor Laboratory Press, 1989)). Amplification was carried out in a Biometra thermal cycler (TRIO-Thermalcycler) using the following cycling parameters: denaturation 95°C for 1 min., annealing 52°C for 1 min., and extension 72°C for 2 min., (33 cycles), followed by a final extension cycle at 72°C for 10 min. The 1846bρ amplified product was fractionated on an agarose gel (Sambrook et al., Molecular Cloning: A Laboratory Manual, Second Edition, (Cold Spring Harbor Laboratory Press, 1989), and the DNA purified from the gel slice using Qiaex resin (Qiagen) following the manufacturer's protocol. The purified PCR fragment encoding the preproricin cDNA was then ligated (Sambrook et al., Molecular Cloning: A Laboratory Manual, Second Edition, (Cold Spring Harbor Laboratory Press, 1989)) into an Eco RV- digested pBluescript II SK plasmid (Stratagene), and used to transform competent XLl-Blue cells (Stratagene). Positive clones were confirmed by restriction digestion of purified plasmid DNA. Plasmid DNA was extracted using a Qiaprep Spin Plasmid Miniprep Kit (Qiagen). DNA Sequencing The cloned PCR product containing the putative preproricin gene was confirmed by DNA sequencing of the entire cDNA clone (pAP-144). Sequencing was performed using an Applied Biosystems 373A Automated DNA Sequencer, and confirmed by double-stranded dideoxy sequencing by the Sanger method using the Sequenase kit (USB). The oligonucleotide primers used for sequencing were as follows: Ricin267, Ricin486, Ricin725, Ricin937, Ricinll51, Ricinil399, Ricinl627, T3 primer (5ΑATTAACCCTCACTAAAGGG-3') (SEQ ID NO. 128) and T7 primer (5'GTAATACGACTCACTATAGGGC-3) (SEQ ID NO. 129). Sequence data was compiled and analyzed using PC Gene software package (intelligenetics). The sequences and location of oligonucleotide primers is shown in Table 1. The oligonucleotide primers shown in Table 1 have been assigned the following sequence ID numbers: Ricin-109 is referred to herein as SEQ ID NO. 130; Ricin-99Eco is referred to herein as SEQ ID NO. 131; Ricin267 is referred to herein as SEQ ID NO. 132; Ricin486 is referred to herein as SEQ ID NO. 133; Ricin725 is referred to herein as SEQ ID NO. 134; Ricin 937 is referred to herein as SEQ ID NO. 135; Ricin 1151 is referred to herein as SEQ ID NO. 136; Ricin 1399 is referred to herein as SEQ ID NO. 137; Ricin 1627 is referred to herein as SEQ ID NO. 138; Ricin 1729C is referred to herein as SEQ ID NO. 139; and Ricin 1729C Xba is referred to herein as SEQ ID NO. 140. Production and Cloning of Linker Variants pAP144 cut with EcoRI was used as target for PCR pairs employing the Ricinl09-Eco oligonucleotide (Ricin-109Eco primer: 5- GGAGGAATCCGGAGATGAAACCGGGAGGAAATACTATTGTAAT-3 (SEQ ID No. 141)) and a mutagenic primer for the 5' half of the linker as well as the Ricinl729PstI primer (Ricinl729-Pstl: 5- GTAGGCGCTGCAGATAACTTGCTGTCCTTTCAG-3 (SEQ ID No. 142)) and a mutagenic primer for the 3' half of the linker. The cycling conditions used for the PCRs were 98 degrees C for 2 min.; 98C 1 min., 52C 1 min., 72C 1 min. 15 sec. (30 cycles); 72 degrees C lOmin.; 4 degrees C soak. The PCR products were then digested by EcoRI and PstI respectively, electrophoresed on an agarose gel, and the bands purified by via glass wool spin columns. Triple ligations comprising the PCR product pairs (corresponding halves of the new linker) and pVL1393 vector digested with EcoRI and PstI were carried out. Recombinant clones were identified by restriction digests of plasmid miniprep DNA and the altered linkers confirmed by DNA sequencing. See Figure 45 as an example of the cloning strategy. Recombinant clones were identified by restriction digests of plasmid miniprep DNA and the altered linkers confirmed by DNA sequencing. Note that since all altered linker variants were cloned directly into the pVL1393 vector odd-numbered pAPs were no longer required or produced. Isolation of Recombinant Baculoviruses Insect cells S. frugiperda (Sf9), and Trichoplusia ni (Tn368 and BTI-TN-581-4 (High Five)) were maintained on EX-CELL 405 medium (JRH Biosciences) supplemented with 10% total calf serum (Summers et al., A Manual of Methods of Baculovirus Vectors and Insect Cell Culture Procedures, (Texas Agricultural Experiment Station, 1987)). Two micrograms of recombinant pVL1393 DNA was co- transfected with 0.5 microgram of BaculoGold AcNPV DNA (Pharmingen) into 2 x 10 6 Tn368 insect cells following the manufacturer's protocol (Gruenwald et al., Baculovirus Expression Vector System: Procedures and Methods Manual, 2nd Edition, (San Diego, CA, 1993)). On day 5 post-transfection, media were centrifuged and the supernatants tested in limiting dilution assays with Tn368 cells (Summers et al., A Manual of Methods of Baculovirus Vectors and Insect Cell Culture Procedures, (Texas Agricultural Experiment Station, 1987)). Recombinant viruses in the supernatants were then amplified by infecting Tn368 cells at a multiplicity of infection (moi) of 0.1, followed by collection of day 3 to 5 supernatants. A total of three rounds of amplification were performed for each recombinant following established procedures (Summers et al., A Manual of Methods of Baculovirus Vectors and Insect Cell Culture Procedures, (Texas Agricultural Experiment Station, 1987 and Gruenwald et al., Baculovirus Expression Vector System: Procedures and Methods Manual, 2nd Edition, (San Diego, CA, 1993)). Expression of Mutant Proricin Recombinant baculoviruses were used to infect 1X10 7 Tn368 or sf9 cells at an moi of 9 in EX-CELL 405 media (JRH Biosciences) with 25mM α-lactose in spinner flasks. Media supernatants containing mutant proricins were collected 3 or 4 days post-infection. EXAMPLE 2 Harvesting and affinity column purification of pro-ricin variants Protein samples were harvested three days post transfection. The cells were removed by centrifuging the media at 8288 g for ten minutesusing a GS3 (Sorvall) centrifuge rotor. The supernatant was further clarified by centrifuging at 25400 g using a SLA- 1500 rotor (Sorvall) for 45 minutes. Protease inhibitor phenylmethylsulfonyl fluoride (Sigma) was slowly added to a final concentration of ImM. The samples were further prepared by adding lactose to a concentration of 20 mM (not including the previous lactose contained in the expression medium). The samples were concentrated to 700 mL using a Prep/Scale-TFF Cartridge (2.5ft, 10K regenerated cellulose (Millipore)) and a Masterflex pump. The samples were then dialysed for 2 days in IX Column Buffer (50 mM Tris, 100 mM NaCl, 0.02% NaN3, pH 7.5) using dialysis tubing (10 K MWCO, 32 mm flat width(Spectra/Por)). Subsequently, the samples were clarified by centri fuging at 25400 g using a SLA-1500 rotor (Sorvall) for 45 minutes. Following centrifugation, the samples were degassed and applied at 4 degrees C to a XK26/20 (Pharmacia) column (attached to a Pharmacia peristaltic pump, Pharmacia Single-path Monitor UV-1 Control and Optical Units, and Bromma LKB 2210 2-Channel Recorder) containing 20 mL of a-Lactose Agarose Resin (Sigma). The column was washed for 3 hours with IX Column buffer. Elution of pro-ricin variant was performed by eluting with buffer (IX Column buffer (0.1% NaN3), 100 mM Lactose) until the baseline was again restored. The samples were concentrated using an Amicon 8050 concentrator (Amicon) with a YM10 76 mm membrane, utilizing argon gas to pressurize the chamber. The samples were further concentrated in Centricon 10 (Millipore) concentrators according to manufacturer's specifications. Purification of Variant pAP-Protein by gel filtration chromatography In order to purify the pro-ricin variant from processed material produced during fermentation, the protein was applied to a SUPERDEX 75 (16/60) column and SUPERDEX 200 (16/60) column (Pharmacia) connected in series equilibrated with 50 mM Tris, lOOmM NaCl, pH 7.5 containing 100 mM Lactose and 0.1% β-mercaptoethanol (βME). The flow rate of the column was 0.15 mL/min and fractions were collected every 25 minutes. The UV (280 nm) trace was used to determine the approximate location of the purified pAP-protein and thus determine the samples for Western analysis. Western analysis of column fractions Fractions eluted from the SUPERDEX columns (Pharmacia) were analyzed for purity using standard Western blotting techniques. An aliquot of lOμL from each fraction was boiled in IX sample buffer (62.6 mM Tris-Cl, pH 6.8, 4.4% βME, 2% sodium dodecyl sulfate (SDS), 5% glycerol (all from Sigma) and 0.002% bromophenol blue (Biorad)) for five minutes. Denatured samples were loaded on 12% Tris-Glycine Gels (Biorad) along with 50 ng of RCA 60 (Sigma) and 5 μL of kaleidoscope prestained standards (Biorad). Electrophoresis was carried out for ninety minutes at 100V in 25 mM Tris-Cl, pH 8.3, 0.1% SDS, and 192 mM glycine using the BioRad Mini Protean II cells (Biorad). Following electrophoresis gels were equilibrated in transfer buffer (48 mM Tris, 39 mM glycine, 0.0375% SDS, and 20% Methanol) for a few minutes. PVDF Biorad membrane was presoaked for one minute in 100% methanol, rinsed in ddH 2 0 and two minutes in transfer buffer. Whatman paper was soaked briefly in transfer buffer. Five pieces of Whatman paper, membrane, gel, and another five pieces of Whatman paper were arranged on the bottom cathode (anode) of the Pharmacia Novablot transfer apparatus (Pharmacia). Transfer was for one hour at constant current (2 mA/cm 2 ). Transfer was confirmed by checking for the appearance of the prestained standards on the membrane. Non-specific sites on the membrane were blocked by incubating the blot for thirty minutes in IX Phosphate Buffered Saline (IX PBS; 137 mM NaCl, 2.7 mM KCI, 8 mM Na 2 HP0 4 , 1.5 mM KH 2 P0 4 , pH 7.4) with 5% skim milk powder (Carnation). Primary antibody (Rabbit α-ricin, Sigma) was diluted 1:3000 in IX PBS containing 0.1% Tween 20 (Sigma) and 2.5% skim milk and incubated with blot for forty five minutes on a orbital shaker (VWR). Non-specifically bound primary antibody was removed by washing the blot for ten minutes with IX PBS containing 0.2% Tween 20. This was repeated four times. Secondary antibody donkey anti-rabbit (Amersham) was incubated with the blot under the same conditions as the primary antibody. Excess secondary antibody was washed as described above. Blots were developed with the ECL Western Blotting detection reagents according to the manufacturer's instructions. Blots were exposed to Medtec's Full Speed Blue Film (Medtee) or Amersham's ECL Hyperfilm (Amersham) for one second to five minutes. Film was developed in a KODAK Automatic Developer. Determination of lectin binding ability of pro-ricin variant An Immulon 2 plate (VDVR) was coated with 100 μl per well of lOμg/ml of asialofetuin and left overnight at 4°C. The plate was washed with 3X 300 μL per well with ddH 2 0 using an automated plate washer (BioRad). The plate was blocked for one hour at 37°C by adding 300 μL per well of PBS containing 1% ovalbumin. The plate was washed again as above. Pro-ricin variant pAP-protein was added to the plate in various dilutions in IX Baculo. A standard curve of RCA 60 (Sigma) from 1-10 ng was also included. The plate was incubated for 1 h at 37°C. The plate was washed as above. Anti-ricin monoclonal antibody (Sigma) was diluted 1:3000 in IX PBS containing 0.5% ovalbumin and 0.1% tween-20, added at 100 μL per well and incubated for 1 h at 37°C. The plate was washed as above. Donkey-anti rabbity polyclonal antibody was diluted 1:3000 in IX PBS containing 0.5% ovalbumin, 0.1% Tween-20, and added at lOOμL per well and incubated for 1 h at 37°C. The plate was given a final wash as described above. Substrate was added to plate at lOOμL per well (1 mg/ml o- phenylenediamine (Sigma), 1 μL/ml H 2 0 2 , 25 μL of stop solution (20% H 2 S0 ) was added and the absorbance read (A490nm-A630nm) using a SPECTRA MAX 340 plate reader (Molecular Devices). Determination of pAP -Protein activity using the rabbit reticulocyte assay Ricin samples were prepared for reduction. A) RCA 6fJ = 3,500 ng/μL of RCA 6fJ + 997 μL lxEndo buffer (25mM Tris, 25mM KCl,5mM MGC1 2 , pH 7.6) Reduction = 95 μL of lOng/μL + 5 μL β-mercaptoethanol B) Ricin variants Reduction = 40 μL variant + 2 μL β-mercaptoethanol The ricin standard and the variants were incubated for 30 minutes at room temperature. Ricin - Rabbit Reticulocyte lysate reaction The required number of 0.5 mL tubes were labelled. (2 tubes for each sample, + and - aniline). To each of the sample tubes 20 μL of IX endo buffer was added, and 30 μL of buffer was added to the controls. To the sample tubes either 10 μL of lOng/μL Ricin or lOμL of variant was added. Finally, 30μL of rabbit reticulocyte lysate was added to all the tubes. The samples were incubated for 30 minutes at 30°C using the thermal block. Samples were removed from the eppendorf tube and contents added into a 1.5 mL tube containing 1 mL of TRIZOL (Gibco). Samples were incubated for 15 minutes at room temperature. After the incubation, 200 μL of chloroform was added, and the sample was vortexed and spun at 12,000 g for 15 minutes at 4°C. The top aqueous layer from the samples was removed and contents added to a 1 mL tube containing 500 μL of isopropanol. Samples were incubated for 15 minutes at room temperature and then centrifuged at 12,000 for 15 minutes at 4°C. Supernatant was removed and the pellets were washed with 1 mL of 70% ethanol. Centrifugation at 12,000 g for 5 minutes at 4°C precipitated the RNA. All but approximately 20 μL of the supernatant was removed and air dried. Pellets from the other samples (+aniline samples) were dissolved in 20 μL of DEPC treated ddH 2 0. An 80 μL aliquot of 1 M aniline (distilled) with 2.8 M acetic acid was added to these RNA samples and transferred to a fresh 0.5 mL tube. The samples were incubated in the dark for 3 minutes at 60°C. RNA was precipitated by adding 100 μL of 95% ethanol and 5μL of 3M sodium acetate, pH 5.2 to each tube and centrifuging at 12,000 g for 30 minutes at 4°C. Pellets were washed with 1 mL 70% ethanol and centrifuged again at 12,000g for 5 minutes at 4°C to precipitate RNA. The supernatant was removed and air dried. These pellets were dissolved in lOμL of 0.1 X E buffer. To all samples, 10 μL of formamide loading dye was added. The RNA ladder (8 μL of ladder + 8 μL of loading dye) was also included. Samples were incubated for 2 minutes at 70°C on the thermal block. Electrophoresis was carried out on the samples using 1.2% agarose, 50% formamide gels in 0.1X E buffer + 0.2% SDS. The gel was run for 90 minutes at 75 watts. RNA was visualized by staining the gel in 1 μg/μL ethidium bromide in running buffer for 45 minutes. The gel was examined on a 302 nm UV box, photographed using the gel documentation system and saved to a computer disk. Results: Protein Expression Yields Aliquots were taken at each stop of the harvesting/purification and tested. Yields of functional ricin variant were determined by ELISA. Typical results of an 2400 mL prep of infected T. ni cells are given below. Aliquot μg pAP 220 Before concentration and dialysis 6000 After concentration and dialysis 4931 alpha- Lactose agarose column flow through 219 alpha- Lactose agarose column elution 1058 Yield: 1058/6000 = 17.6% Purification of pAP -Protein and Western Analysis of column fractions Partially purfied pAP-protein was applied to Superdex 75 and 200 (16/60) columns connected in series in order to remove the contaminating non-specifically processed pAP-protein. Eluted fractions were tested via Western analysis as described above and the fractions containing the most pure protein were pooled, concentrated and re- applied to the column. The variant was applied a total of three times to the column. Final purified pAP-protein has less than 1% processed variant. The purified pAP-protein was tested for susceptibility to cleavage by the particular protease and for activation of the A-chain of the proricin variant, (inhibition of protein synthesis). Typically, pAP-protein was incubated with and without protease for a specified time period and then electrophoresed and blotted. Cleaved pAP will run as two 30 kDa proteins (B is slightly larger) under reducing (SDS-PAGE) conditions. Unprocessed pAP-protein, which contains the linker region, will run at 60 kDa. Activation of pAP -Protein variant with Specific Protease Activation of protease treated pAP-protein is based on the method of May et al. (EMBO Journal. 8 301-8, 1989). Activation of ricin A chain upon cleavage of the intermediary linker results in catalytic depurination of the adenosine 4325 residue of 28S or 26S rRNA. This depurination renders the molecule susceptible to amine-catalyzed hydrolysis by aniline of the phosphodiester bond on either side of the modification site. The result is a diagnostic 390 base band. As such, reticulocyte ribosomes incubated with biochemically purified ricin A chain, released the characteristic RNA fragment upon aniline treatment of isolated rRNA (May, MJ. et al. Embo. Journal, 8:301-308 at 302-303 (1989)). It is on this basis that the assay allows for the determination of activity of a ricin A chain which has been cleaved from the intact unit containing a particular variant linker sequence. EXAMPLE 3 In Vitro Protease Digestion of Proricin Variants: Affinity-purified proricin variant is treated with individual disease-specific proteases to confirm specific cleavage in the linker region. Ricin-like toxin variants are eluted from the lactose-agarose matrix in protease digestion buffer (50mM NaCl, 50mM Na-acetate, pH 5.5, ImM dithiothreitol) containing lOOmM lactose. Proricin substrate is then incubated at 37°C for 60 minutes with a disease-specific protease. The cleavage products consisting ricin A and B chains are identified using SDS/PAGE (Sambrook et al., Molecular Cloning: a Laboratory Manual, 2nd. ed., Cold Spring Harbor Press, 1989), followed by Western blot analysis using anti-ricin antibodies (Sigma). Cathepsin B may be obtained from Medcor or Calbiochem. Matrix metalloproteinases may be prepared substantially as described by Lark, M.W. et al. (Proceedings of the 4th International Conference of the Imflammation Research Association Abstract 145 (1988)) and Welch, A.R. et al. (Arch. Biochem. Biophys. 324:59-64 (1995)). Candida acid protease may be prepared substantially as described in Remold, H.H. et al. (Biochim. Biophys. Acta 167:399-406 (1968)), Ray, T.L. and Payne, CD. (Infect. Immunol. 58:508-514 (1990)) and Fusek, M. et al. (FEBS Lett. 327:108-112 (1993)). Hepatitis A protease may be prepared as described in Jewell, D.A. et al. (Biochemistry 31:7862-7869 (1992)). Plasmodium proteases may be prepared as described in Goldberg, D.E. et al. (/. Exp. Med. 173:961-969 (1991)) and Cooper, J.A. and Bujard, H. (Mol. Biochem. Parasitol. 56:151-160 (1992)). In Vitro Cytotoxicity Assay: Human ovarian cancer cells (e.g. MA148) are seeded in 96-well flat-bottom plates and are exposed to ricin-like toxin variants or control medium at 37°C for 16 h. The viability of the cancer cells is determined by measuring [ 35 S] methionine incorporation and is significantly lower in wells treated with the toxin variants than those with control medium. In Vivo Tumour Growth Inhibition Assay: Human breast cancer (e.g. MCF-7) cells are maintained in suitable medium containing 10% fetal calf serum. The cells are grown, harvested and subsequently injected subcutaneously into ovariectomized athymic nude mice. Tumour size is determined at intervals by measuring two right-angle measurements using calipers. In animals that received ricin-like toxin variants containing the matrix metalloproteinase-sensitive linkers, tumour size and the rate of tumour growth are lower than animals in the control group. In Vivo Tumour Metastasis Assay: The metastasis study is performed substantially as described in Honn, K.V. et al. (Biochem. Pharmacol. 34:235-241 (1985)). Viable B16a melanoma tumour cells are prepared and injected subcutaneously into the left axillary region of syngeneic mice. The extent of tumour metastasis is measured after 4 weeks. The lungs are removed from the animals and are fixed in Bouin's solution and macroscopic pulmonary metastases are counted using a dissecting microscope. In general without therapeutic intervention, injection of 10 5 viable tumour cells forms approximately 40-50 pulmonary metastases. The number of metastases in animal treated with proricin variants containing cathepsin B-sensitive linkers is substantially lower. EXAMPLE 4 In Vitro Protease Digestion of Proricin Variants by Cancer Proteases Cathepsin B or MMP-9 The general protocol for proricin digestion by cancer proteases is described in Examples 2 and 3. In Vitro Protease Digestion of Cathepsin B Proricin Variant Affinity-purified mutant proricin is treated with individual disease-specific proteases to confirm specific cleavage in the linker region. The proricin substrate is digested in a Cathepsin B protease buffer (50 mM Sodium acetate, 2 mM EDTA, 0.05% Triton) at 40°C Two hours and overnight (16 hr) digestion reactions are carried out using lOOng of proricin substrate and 100 and 618 ng of Cathepsin B protease per reaction (CALBIOCHEM, USA). The cleavage products of proricin (ricin A and B chains) are identified using SDS/PAGE (Sambrook et al., Molecular cloning: a laboratory Manual, 2nd. ed., Cold Spring Harbor Press, 1989), followed by Western blot analysis using anti-ricin antibodies (Sigma). In Vitro Protease Digestion of MMP-9 Proricin Variant Affinity-purified mutant proricin is treated with individual disease-specific proteases to confirm specific cleavage in the linker region. The proricin substrate is digested in IX column buffer (100 mM NaCl, 50 mM Tris, PH 7.5) at 37°C Two hours and overnight (16 hr) digestion reactions are set up using 50 ng of MMP-9 proricin substrate and 20 and 200 ng of MMP-9 protease per reaction (CALBIOCHEM, USA). The cleavage products of proricin (ricin A and B chains) are identified using SDS /PAGE (Sambrook et al., Molecular cloning: a laboratory Manual, 2nd. ed., Cold Spring Harbor Press, 1989), followed by Western blot analysis using anti-ricin antibodies (Sigma). The protocol for Western analysis of ricin chains is described in Example 2. Results Figures 48 and 49 illustrate Western blots showing the cleavage of the protease-sensitive linkers by cathepsin B (pAP 214) and MMP-9 (pAP 220) respectively. Without protease digestion, the proricin variant appears as a single band at approximately 60 kDa (Lane B of Figure 48 and Lane A of Figure 49). Wild type ricin A chain and B chain appear as two disparate bands at approximately 30 kDa (Lane A of Figure 48 and Lane E of Figure 49). Increasing extent of proricin cleavage can clearly be observed with increasing protease concentration (Lanes C and D of Figure 48 and Lanes B-C of Figure 49). EXAMPLE 5 In vitro protease digestion of various proricin variants by their corresponding proteases. The general protocol for proricin digestion by coresponding proteases was as desribed in Examples 2 and 3 and should be considered in connection with the digestions described below. Cleavage of pAP-222 protein with the Matrix Metalloproteinase 2 (MMP-2) Affinity-purified mutant proricin is treated with individual disease-specific proteases to confirm specific cleavage in the linker region. The pAP-222 protein sample (1.0 ug) was digested with the MMP-2 protease (1.0 ug) overnight at 37° C. The total volume of the digestion reaction was 21.5 ul, and 0.250 ug of the reaction sample was loaded on a protein gel. The MMP-2 protease was purchased from Calbiochem-Novabiochem Corporation, USA. Cleavage of pAP-248 protein with the Human Cytomegalovirus (HCMV) protease Affinity-purified mutant proricin is treated with individual disease-specific proteases to confirm specific cleavage in the linker region. The pAP-248 protein sample (1.19 ug) was digested with the HCMV protease (1.13 ug) overnight at 37°C The total volume of the digestion was 10.5 ul, and 0.279 ug of the reaction sample was loaded on a protein gel. The HCMV was purchased from BACHEM Bioscience Inc., USA. Cleavage of ρAP-256 protein with the Hepatitis A virus 3C (HAV 3C) protease Affinity-purified mutant proricin is treated with individual disease-specific proteases to confirm specific cleavage in the linker region. The pAP-256 protein sample (1.26 ug) was digested with the HAV 3C protease (5 ug) overnight at 37°C The total volume of the digestion was 12.5 ul, and 0.302 ug of the digestion sample was loaded on a protein gel. The HAV 3C protease was a gift from Dr. G. Lawson from Bates Collage, Main, USA. Cleavage of pAP-270 protein with the Matrix Metalloproteinase 2 (MMP-2) Affinity-purified mutant proricin is treated with individual disease-specific proteases to confirm specific cleavage in the linker region. The pAP-270 protein sample (0.120 ug) was digested with the MMP-2 protease (0.25 ug) overnight at 37° C. The total volume of the digestion reaction was 22.5 ul, and 0.106 ug of the reaction sample was loaded on a protein gel. The MMP-2 protease was purchased from Calbiochem-Novabiochem Corporation, USA. Cleavage of pAP-288 protein with tPA plasminogen tissue activator Affinity-purified mutant proricin is treated with individual disease-specific proteases to confirm specific cleavage in the linker region. The pAP-288 protein sample (1.65 ug) was digested with the t-PA protease (0.5 ug) overnight at 37° C. The total volume of the digestion reaction was 55 ul, and 0.6 ug of the reaction sample was loaded on a protein gel. The t-PA was purchased from Sigma Chemical Co., USA. Cleavage of pAP-294 protein with human neutraphil elastase Affinity-purified mutant proricin is treated with individual disease-specific proteases to confirm specific cleavage in the linker region. The pAP-256 protein sample (0.6 ug) was digested with the Elastase protease (5 ug) at 25° C for one hour. The total volume of the digestion reaction was 52.5 ul, and 0.171 ug of the digestion sample was loaded on a protein gel. The Human Neutrophil Elastase protease was purchased from Cedarlane Laboratories Limited, Canada. Cleavage of pAP -296 protein with calpain Affinity-purified mutant proricin is treated with individual disease-specific proteases to confirm specific cleavage in the linker region. The pAP-296 protein sample (2.05 ug) was digested with the Calpain protease (10 ug) overnight at 37° C. The total volume of the digestion reaction was 35 ul and 0.761 ug of the reaction sample was loaded on a protein gel. The Calpain protease was purchased from Sigma Chemical Co., USA Results Figures 52, 54, 58 & 66(MMP-2), 60, 64 and 62 show the cleavage of proteases of linkers by HCMV, HAV 3C, MMP-2, t-PA, calpain, and human neutraphil elastase respectively. Without protease digestion, the proricin variants appear as a single band at approximately 60kDA (Lane A in connection with Figure 52; Lane B of Figure 54; Lane A of Figure 58; Lane B of Figure 60; and Lane C of Figure 62; lane B of Figure 64 and lane B of Figure 66). Wild type ricin chain A and B appear as two bands at approximately 30kDA (see for example Lanes C and D of Figure 52) proricin cleavage can clearly be obvserved with the appearance of 30kDA bands in connection with the protein which has been digested by the respective protease (see Lane B of Figure 52; Lane C of Figure 54; or Lane B of Figure 58 for examples). EXAMPLE 6 In Vitro Translation Assay (Activation by Cancer Proteases Cathepsin B or MMP-9 The general protocol for the rabbit retoculocyte lysate reaction to test the cytotoxicity of cancer protease-activiated proricin is described briefly in Example 3 and is described in more detail in Example 2. Results Activation of pAP 214 and pAP 220 proricin variants by cathepsin B and MMP-9, based on the method of May et al. (EMBO J. 8:301-308, 1989), is illustrated in Figures 50 and 51 respectively. The appearance of the 390 base pair product (positive control) is observed in Lane F of Figure 50 and Lane G of Figure 51. This 390 base pair product is absent in the negative control lanes. Without cathepsin or MMP-9 activation, no or minimal N-glycosidase activity in the pAP 214 variant (Lanes H to L, Figure 50) or the pAP 220 variant (Lanes A to E, Figure 51) was observed. When the pAP 214 variant and the pAP 220 variant were activated by cathepsin or MMP-9 respectively, appearance of the 390 base pair product was observed in a proricin concentration-dependent manner (Lanes A to E of Figure 50 and Lanes H to L of Figure 51). The present experimental series demonstrated the successful and selective activation of proricin variants by cancer-associated proteases. EXAMPLE 7 The general protocol for the rabbit retoculocyte lysate reaction is described briefly in Example 3 and is described in more detail in Example 2, all of which compliments the description below. Depurination of Rabbit Reticulocyte 28S Ribosomal RNA by Digested and Undigested Ricin Variants Affinity-purified mutant proricin mutants which were previously digested with the disease-specific protease, were reduced with 5% 2-mercaρtoethanol then diluted to lOOng, 14.2ng,2.0ng,291pg, and 41.7pg with 1 X ENDO buffer(25mM Tris pH 7.6, 25mM KCI, 5mM MgCl 2 ) and incubated with rabbit reticulocyte lysate, untreated (Promega) for 30minutes at 30(C. To compare the digested with the undigested proricin variant, the proricin in digestion buffer (according to the specific digestion protocol) was treated in the same manner as the digested sample. As a positive and negative control, lOng of ricin A chain and 1 X ENDO buffer consecutively, was incubated with rabbit reticulocyte lysate, untreated, for 30 min at 30°C Aniline Cleavage of rRNA and Gel Fractionation Total RNA was then extracted from reticulocyte lysate translation mixtures with Trizol reagent (Gibco-BRL) as per manufacturer's instructions. The RNA was incubated with 80ul of IM aniline (distilled) with 2.8M acetic acid for 3 min at 60(C in the dark. Ethanol-precipitated RNA samples were dissolved in 20ul of 50% formamide, 0.1X E buffer (3.6mM Tris, 3mM NaH 2 P0 4 , 0.2mM EDTA), and 0.05% xylene cyanol. lOul of this was heated to 70(C for 2 minutes, loaded and electrophoresed in 1.2% agarose, 0.1X E buffer, and 50% formamide gel with RNA running buffer (0.1 X E buffer, 0.2% SDS). Results Activation of pAP-248 proricin variant by HCMV; pAP-256 by HAV3C protease; pAP-270 by MMP-2 protease; pAP-288 by t-PA protease; pAP-294 by human neutrophil elastase; pAP-296 by calpain; and pAP- 222 by MMP-2 is illustrated in Figures 52, 55, 59, 61, 63, 65, and 67 respectively. The appearance of the 390 base pair product (deposit of control) is obverved in lane L of Figures 53, 55, 61, 63, 65 and 67. The 390 base pair product is observed in lane A of Figures 59 (activation of pAP- 270 by MMP-2). This 390 base pair product is absent in the negative control lanes. Without the specific protease activation, no or minimal activity is seen in the lanes which contained only the proricin variant without digestion (see lane A, B, C, D, and E of Figures 53, 55, 61, 63, 65, and 67). The same observation is made in connection with pAP-270 in Figure 59, however, the undigested lanes appear as H, I, J, K and L. When the variant was activated by its respective protease, there is an appearance of the 390 base pair product in a proricin concentration- dependent manner (see Lanes H, I, J, K and L of Figure 53, 55, 61, 63, 65, and 67 and Lanes A, B, C, D, and E of Figure 59). The present experimental series demonstrate the successful and selective activation of the identified proricin variants by selective corresponding proteases. EXAMPLE 8 Procedure for Examining the Cytotoxicity of Ricin and Ricin Variants on the COS-1 Cell Line Cell Preparation After washing with 1XPBS (0.137 M NaCl, 2.68 mM KCI, 8.10 mM Na 2 HP0 4 , 1.47 mM KH 2 P0 4 ), cells in log phase growth were removed from plates with IX trypsin/EDTA (Gibco/BRL). The cells were centrifuged at 1100 rpm for 3 min, resuspended in Dulbecco's Modified Eagle Medium containing 10%FBS and IX pen/strep, and then counted using a haemocytometer. They were adjusted to a concentration of 5 X 10 4 cells •ml -1 . One hundred microliters per well of cells was added to wells 2B - 2G through to wells 9B - 9G of a Falcon 96 well tissue culture plate. A separate 96 well tissue culture plate was used for each sample of Ricin or Ricin variant. The plates were incubated at 37(C with 5% C0 2 for 24 hours. Toxin Preparation The Ricin and Ricin variants were sterile filtered using a 0.22μm filter (Millipore). The concentration of the sterile samples were then quantified by A 2 so and confirmed by BCA measurements (Pierce). For the variants digested with the protease in vitro, the digests were carried out as described in the digestion procedure for each protease. The digests were then diluted in the 1000 ng^mH dilution and sterile filtered. The Ricin and the undigested pAP214 in the pAP 214 cytotoxicity data were treated in the same manner but without the Cathepsin B treatment. Ricin and Ricin variants were serially diluted to the following concentrations: 1000 ng»mH, 100 ng^ml' 1 , 10 ng«mH, 1 ng»mH, 0.1 ng-mH, 0.01 ng«mH, 0.001 ng^mF with media containing 10%FBS and IX pen/strep. Application of Toxin or Variants to Plates Columns 2 to 9 were labeled: control, 1000 ng»mH, 100 ng*mH, 10 ng»mH, 1 ng«mH, 0.1 ng«mF, 0.01 ng-mk 1 , 0.001 n -mH consecutively. The media was removed from all the sample wells with a multichannel pipettor. For each plate of variant and toxin, 50μl of media was added to wells 2B to 2G as the control, and 50 μl of each sample dilution was added to the corresponding columns. For the pAP220 + MMP-9 data, the plates were incubated for one hour at 37(C with 5% C0 2 , then washed once and replaced with media, then incubated for 48 hours at 37(C with 5% C0 2 . For the pAP 214 + Cathepsin B data, the toxin was left on the plates and incubated for 24 hours at 37(C with 5% C0 2 then 50 μl of media was added to the wells with the toxin and incubated for another 24 hours at 37(C with 5% C0 2 . Sample Application The whole amount of media (and /or toxin) was removed from each well with a multichannel pipettor, and replaced with 100 μl of the substrate mixture (Promega Cell Titer 96 Aqueous Non-Radioactive Cell Proliferation Assay Kit). The plates were incubated at 37(C with 5% C0 2 for 2 to 4 hours, and subsequently read with a Spectramax 340 96 well plate reader at 490nm. The IC 50 values were calculated using the GRAFIT software program. Results In experiments with pAP-214 and Cathepsin B incubated with COS-1 cells, it may be seen that cells incubated with pAP-214 alone, pAP- 214 was ineffective at causing cell death (see Figure 56). However, the cytotoxicity of pAP-214 digested with Cathepsin B behaves similarly to the ricin control in COS-1 cells. This is also illustrated in Figure 56. Similarily, the cytotoxicity of undigested pAP-220 when incubated with COS-1 cells is lower than the cytotoxicity observed with COS-1 cells incubated with pAP-220 digested with MMP-9. Indeed the results suggest that the toxicity of digested pAP-220 is greater than that of ricin. (See Figure 57). EXAMPLE 9 Procedure for Examining the Cytotoxicity of Ricin and Ricin Variants on Various Tissue Culture Cell Lines Cell Preparation After washing with 1XPBS (1.37M NaCl, 26.8mM KCI, 81mM Na 2 HP0 4 , 14.7mM KH 2 P0 4 ), cells in log phase growth were removed from plates with IX trypsin/EDTA (Gibco /BRL). The cells were centrifuged at 1100 rpm for 3 min, resuspended in media containing 10%FBS and IX pen/strep (media used depended on the cell line being tested), and then counted using a haemocytometer. They were adjusted to a concentration of 5 X 10 4 cells •mk 1 (faster growing cell lines were adjusted to 2 X10 4 cells •mH). One hundred microliters per well of cells was added to wells 2B - 2G through to wells 9B - 9G of a Falcon 96 well tissue culture plate. A separate 96 well tissue culture plate was used for each sample of Ricin or Ricin variant. The plates were incubated at 37(C with 5% C0 2 for 24 hours. Toxin Preparation The Ricin and Ricin variants were sterile filtered using a 0.22μm filter (Millipore). The concentration of the sterile samples were then quantified by A 2 go an d confirmed by a BCA measurement (Pierce). Ricin and Ricin variants were serially diluted to the following concentrations: 3000 ng-mk 1 , 300 ng-mk 1 , 30 ng-mk 1 , 3 ng»mH, 0.3 ng«mH, 0.03ng»mH, 0.003 ng-mk 1 with media containing 10%FBS and IX pen /strep. Application of Toxin or Variants to Plates Columns 2 to 9 were labeled: control, 0.001 ng«mH, 0.01 ng-mk 1 , 0.1 ng*mH, lng-mk 1 , 10 ng-mk 1 , 100 ng-mk 1 , 1000 ng«mF consecutively. For each plate of variant and toxin, 50 μl of media was added to wells 2B to 2G as the control, and 50μl of each sample dilution was added to the corresponding columns containing lOOμl per well of cells (i.e. 50 μl of the 3000 ng'ml "1 dilution added to the wells B-G in column 9, labeled 1000 ng^mk 1 ). The plates were incubated for 48 hours at 37(C with 5% C0 2 . Sample Application An amount of 140 μl was removed from each well with a multichannel pipettor, and replaced with 100 μl of the substrate mixture (Promega Cell Titer 96 Aqueous Non-Radioactive Cell Proliferation Assay Kit). The plates were incubated at 37(C with 5% C0 2 for 2 to 4 hours, and subsequently read with a Spectramax 340 96 well plate reader at 490nm. The IC 50 values were calculated using the GRAFIT software program. Results Referring to Table 2, it may be seen that the survival of cells is correlated with the proricin variant and the cell specific protease produced by the cell type. For example, in the HT1080 cell line, both pAP-214 and pAP-220 required only 2-1/2 times the amount of ricin to achieve the same level of cytotoxicity. On the other hand, pAP-224 required 193 times the amount of ricin to achieve the same level of cell death. As well, it may be seen that in the cells where expression of Cathepsin D is found, pAP-214 and 220 were more effective at causing cell death than ricin and more effective than pAP-224. Details concerning the various cells types used in these experiments are outlined below. COS-1 (African Green Monkey Kidney Cells) This is an SV40 transformed cell line which was prepared from established simian cells CV-1. (Reference: Gluzman, Y. (1975) Cell, 23, 175 - 182)(ATCC CRL 1650) HT-1080 Human Fibrosarcoma (ATCC CCL 121) This cell line was shown to produce active MMP-9 in tissue culture. References: Moore et al. (1997) Gynecologic Oncology 65, 83-88. 9L Rat Glioblastoma Glioblastomas are generally associated with cathepsin B expression. Levels of cathepsin B expression correspond to the extent of progression of malignancy i.e. highest levels for glioblastomas over anaplastic astrocytomas over low-grade gliomas and normal brain tissue. The 9L cell line was provided by Dr. William Jia of the B.C. Cancer Agency. References: Mikkelsen et al. (Aug. 1995) Journal of Neurosurgery 83(2), 285-290. Nakano et al. (1995) J. of Neurosurgery 83(2), 298-307. MCF-7 Human Breast Cancer Cell Line (Epithilial) (ATCC CRL 1555) In the absence of estrogen cathepsin B has not been shown to be elevated relative to normal cells. It can be induced with estrogen to produce Cathepsin D. Production of MMP-9 is unknown. Having illustrated and described the principles of the invention in a preferred embodiment, it should be appreciated to those skilled in the art that the invention can be modified in arrangement and detail without departure from such principles. We claim all modifications coming within the scope of the following claims. All publications, patents and patent applications referred to herein are incorporated by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety. FULL CITATIONS FOR CERTAIN REFERENCES REFERRED TO IN THE SPECIFICATION Bever Jr., C.T., Panitch, H.S., and Johnson, K.P. (1994) Neurology 44(4), 745-8. Increased cathepsin B activity in peripheral blood mononuclear cells of multiple sclerosis patients. Cohen, P., Graves, H.C, Peehl, D.M., Kamarei, M., Giudice, L.C, and Rosenfeld, R.G. (1992) Journal of Clinal Endocrinology and Metabolism 75(4), 1046-53. Prostate-specific antigen (PSA) is an insulin-like growth factor binding protein-3 protease found in seminal plasma. Conover, CA. and De Leon, D.D. (1994) J. Biol. Chem. 269(10), 7076-80. Acid activated insulin-like growth factor-binding protein-3 proteolysis in normal and transformed cells. Role of cathepsin D. Hansen, G., Schuster, A., Zubrod, C, and Wahn, V. (1995) Respiration 62(3), 117-24. Alpha 1-proteinase inhibitor abrogates proteolytic and secretagogue activity of cystic fibrosis sputum. Muller, H.L., Oh, Y., Gargosky, S.E., Lehrnbecher, T., Hintz, R.L., and Rosenfeld, R.G. (1993) Journal of Clinical Endocrinology and Metabolism 77(5), 1113-9. Concentrations of insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3), IGF, and IGFBP-3 protease activity in cerebrospinal fluid of children with leukemia, central nervous system tumor, or meningitis. TABLE 1 t underlined sequences inserted for subcloning purposes and not included in final preproricin sequences Table 2: Comparative Toxicities to Selected Cell Lines of Ricin and Ricin Provariants Cell Line IC50RI C J- IC50 P A _- IC50pAP_n IC50pAP1_ (ne/mπ ICSORJ C I,, IC50]u c in ICSORI,.!,, COS-1 0.1 17 22 150 HT1080 0.5 2.46 2.14 193 9L 10.8 1.3 1.7 32.3 MCF-7 0.09 27.8 40 742 (without estrogen)

Claims: I CLAIM: 1. A purified and isolated nucleic acid having a nucleotide sequence encoding an A chain of a ricin-like toxin, a B chain of a ricin-like toxin and a heterologous linker amino acid sequence linking the A and B chains, the heterologous linker sequence containing a cleavage recognition site for a disease-specific protease. 2. The nucleic acid sequence of claim 1 wherein the linker sequence contains a cleavage recognition site recognized by a protease selected from the group consisting of: a cancer associated protease, a viral protease, a fungal protease, and a parasite protease. 3. A nucleic acid sequence of claim 2 wherein the A chain is ricin A chain, abrin toxin A chain, diphtheria toxin A chain, or Domain I of Pseudomonas endotoxin. 4. A nucleic acid sequence of claim 2 wherein the A chain is volkensin toxin A chain, cholera toxin A chain, modeccin toxin A chain or shiga toxin A chain. 5. A nucleic acid sequence of claim 2 wherein the B chain is ricin B chain, abrin toxin A chain, diphtheria toxin B chain, or Domain II of Pseudomonas endotoxin. 6. A nucleic acid sequence of claim 2 wherein the B chain is volkensin toxin B chain, cholera toxin B chain, modeccin toxin B chain or shiga toxin B chain. 7. A nucleic acid sequence of claim 2 wherein the cleavage recognition site is recognized by a cancer-associated protease which is selected from the group consisting of: cathepsin B, an Epstein-Barr virus-specific protease, a matrix metalloproteinase, cathespin L, cathespin D, urokinase-type plasminogen activator, tissue-type plasminogen activator, human prostate-specific antigen, kallikrein, neutrophil elastase, and calpain. 8. A nucleic acid sequence of claim 2 wherein the cleavage recognition site is recognized by a parasitic protease which is a Plasmodium falciparum protease. 9. A nucleic acid sequence of claim 2 wherein the cleavage recognition site is recognized by viral protease which is selected from the group consisting of: human cytomegalovirus, human herpes virus, varicella zoster virus, hepatitis A virus, hepatitis C virus, and infectious laryngotracheitis virus. 10. A nucleic acid sequence of claim 2 wherein the cleavage recognition site is recognized by fungal protease which is a Candida acid protease. 11. A nucleic acid sequence of claim 2 having the nucleotide sequence according to SEQ ID No. 3; SEQ ID No 5; SEQ ID No 7; SEQ ID No 9; SEQ ID No 11; SEQ ID No 13; SEQ ID No 15; SEQ ID No 17; SEQ ID No 19; SEQ ID No 21; SEQ ID No 23; SEQ ID No 25; SEQ ID No 27; SEQ ID No 29; SEQ ID No 31; SEQ ID No 33; SEQ ID No 35; SEQ ID No 37; SEQ ID No 39; SEQ ID No 48; SEQ ID No 50; SEQ ID No 52; SEQ ID No 54; SEQ ID No 74; SEQ ID No 77; SEQ ID No 80; SEQ ID No 83; SEQ ID No 86; SEQ ID No 89; SEQ ID No 92; SEQ ID No 95; SEQ ID No 98; SEQ ID No 101; SEQ ID No 104; SEQ ID No 107; SEQ ID No 110; SEQ ID No 113; SEQ ID No 116; SEQ ID No 119; SEQ ID No 122; or SEQ ID No 125. 12. A plasmid incorporating the nucleic acid of claim 1 to 11. 13. A baculovirus transfer vector incorporating the nucleic acid of claim 1 to 11. 14. A recombinant protein comprising an A chain of a ricin-like toxin, a B chain of a ricin- like toxin and a heterologous linker amino acid sequence, linking the A and B chains, wherein the linker sequence contains a cleavage recognition site for a disease-specific protease. 15. The recombinant protein of claim 14 wherein the linker sequence contains a cleavage recognition site which is recognized by a protease selected from the group consisting of: a cancer, viral, fungal, and a parasitic protease. 16. A recombinant protein of claim 14 wherein the A chain is ricin A chain, abrin toxin B chain, diphtheria toxin A chain, or Domain I of Pseudomonas endotoxin. 17. A recombinant protein of claim 14 wherein the A chain is volkensin toxin A chain, cholera toxin A chain, modeccin toxin A chain or shiga toxin A chain. 18. A recombinant protein of claim 14 wherein the B chain is ricin B chain, abrin toxin B chain, diphtheria toxin B chain, or Domain II of Pseudomonas endotoxin. 19. A recombinant protein of claim 14 wherein the B chain is volkensin toxin B chain, cholera toxin B chain, modeccin toxin B chain or shiga toxin B chain. 20. A recombinant protein of claim 14 wherein the cleavage recognition site is recognized by a cancer-associated protease selected from the group consisting of: cathepsin B, an Epstein-Barr virus-specific protease, a matrix metalloproteinase, cathespin L, cathespin D, urokinase-type plasminogen activator, tissue-type plasminogen activator, human prostate-specific antigen, kallikrein, neutrophil elastase, and calpain. 21. A recombinant protein of claim 14 wherein the cleavage recognition site is recognized by a parasitic protease which is a Plasmodium falciparum protease. 22. A recombinant protein of claim 14 wherein the cleavage recognition site is recognized by a viral protease which is selected from the group consisting of: human cytomegalovirus, human herpes virus, varicella zoster virus, hepatitis A virus, hepatitis C virus and infectious laryngotracheitis virus. 23. A recombinant protein of claim 14 wherein the cleavage recognition site is recognized by a fungal protease which is a Candida acid protease. 24. A recombinant protein of claim 14 having the linker amino acid sequence according to SEQ ID No. 40; SEQ ID No. 41; SEQ ID No. 42; SEQ ID No. 43; SEQ ID No. 44; SEQ ID No. 45; SEQ ID No. 46; SEQ ID No. 55; SEQ ID No. 56; SEQ ID No. 57; SEQ ID No. 58; SEQ ID No. 59; SEQ ID No. 60; SEQ ID No. 61; SEQ ID No. 62; SEQ ID No. 63; SEQ ID No. 64; SEQ ID No. 65; SEQ ID No. 66; SEQ ID No. 67; SEQ ID No. 68; SEQ ID No. 69; SEQ ID No. 70; SEQ ID No. 71; SEQ ID No. 72; SEQ ID No. 75; SEQ ID No. 78; SEQ ID No. 81; SEQ ID No. 84; SEQ ID No. 87; SEQ ID No. 90; SEQ ID No. 93; SEQ ID No. 96; SEQ ID No. 99; SEQ ID No. 102; SEQ ID No. 105; SEQ ID No. 108; SEQ ID No. Ill; SEQ ID No. 114; SEQ ID No. 117; SEQ ID No. 120; SEQ ID No. 123; or SEQ ID No. 126. 25. A method of inhibiting or destroying cells affected by a disease, which cells are associated with a protease specific to the disease comprising the steps of: (a) preparing a purified and isolated nucleic acid having a nucleotide sequence encoding an A chain of a ricin-like toxin, a B chain of a ricin-like toxin, and a heterologous linker amino acid sequence, linking the A and B chains, wherein the linker sequence contains a cleavage recognition site for the protease; (b) introducing the nucleic acid into a host cell and expressing the nucleic acid in the host cell to obtain a recombinant protein comprising an A chain of a ricin-like toxin, a B chain of a ricin-like toxin and a linker amino acid sequence; (c) suspending the protein in a pharmaceutically acceptable carrier, diluent or excipient, and (d) contacting the cells with the recombinant protein. 26. The method of claim 25 where the disease is one of cancer or cells infected with a fungus, virus or parasite. 27. A method of inhibiting or destroying cells affected by a disease, which cells are associated with a protease specific to the disease comprising the step of contacting the cells with a recombinant protein according to anu one of claims 14 to 24. 28. A method of treating a disease comprising administering a recombinant protein according to any one of claims 14 to 24 to an animal in need thereof. 29. A method of treating a disease comprising administering a nucleic acid molecule according to any one of claims 2 to 11 to an animal in need thereof. 30. A method of treating a mammal with cancer or infected with a fungus, virus or parasite, comprising the steps of preparing a recombinant protein of claim 14 wherein the linker sequence contains a cleavage recognition site for a cancer, fungal, viral or parasitic protease and administering the protein to the mammal. 31. A process for preparing a pharmaceutical for treating a mammal with cancer, fungal infection, viral infection or parasitic infection, comprising the steps of : (a) preparing a purified and isolated nucleic acid having a nucleotide sequence encoding an A chain of a ricin-like toxin, a B chain of a ricin-like toxin, and a heterologous linker amino acid sequence, linking the A and B chains, wherein the linker sequence contains a cleavage recognition site for a cancer, viral or parasitic protease; (b) introducing the nucleic acid into a host cell and expressing the nucleic acid in the host cell to obtain a recombinant protein comprising an A chain of a ricin-like toxin, a B chain of a ricin-like toxin and a linker amino acid sequence; (c) suspending the protein in a pharmaceutically acceptable carrier, diluent or excipient. 32. A use of a recombinant protein according to any one of claims 14 to 24 to treat a disease. 33 A use of a nucleic acid molecule according to any one of claims 1 to 11 to treat a disease. 34. A pharmaceutical composition for treating cancer or a fungal, or viral, or parasitic infection in an animal comprising the recombinant protein of claim 14 and a pharmaceutically acceptable carrier, diluent or excipient. 35. A pharmaceutical composition for treating cancer or a fungal, or viral, or parasitic infection in an animal comprising the nucleic acid molecule of claim 2 and a pharmaceutically acceptable carrier, diluent or excipient.

Inventors: BORGFORD THOR
Applicants: BORGFORD THOR, NOVO ENZYME CORP DE
Application year: 1998
Main classification: C12N15
IPC codes: A61K35, A61K36, A61K38, A61K48, A61P31, A61P33, A61P35, C07K14, C12N1, C12N15