Abstract:
The present application concerns mutant proteins of the fusion protein (F protein) of the parainfluenza virus (PIV) which are currently indexed as type 5 PIV (PIV-5 or PIV5) and type 2 PIV (PIV-2 or PIV2). The present application concerns products deriving therefrom, such as: nucleic acids, vectors, cells, fusion inhibitors of the antibody, aptamer, interfering RNA type; myelomas, hybridomas; stem and progenitor cells. The present application also concerns mutant proteins and products derived therefrom for use in medical and biotechnological applications.

Description:
This Application is in the National Phase Under 35 U.S.C. §371 of PCT International Application No. PCT/FR2009/001317 filed on Nov. 17, 2009, which claims priority on French application Ser. No. 08/06548 filed on Nov. 21, 2008. The entire contents of which are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present application relates to mutant proteins of the fusion protein (F protein) of parainfluenza virus (PIV) which are currently indexed as type 5 PIV (PIV-5 or PIV5) and type 2 IV (PIV-2 or PIV2). 
     The present application relates to products derived therefrom, such as:
         nucleic acids, vectors, cells;   fusion inhibitors of the antibody, aptamer, interfering RNA type;   myelomas, hybridomas;   stem and progenitor cells.       

     The present application also relates to these mutant proteins and products derived therefrom for their use in medical and biotechnological applications. 
     PRIOR ART 
     The parainfluenza virus (PIV) which is currently indexed as type 5 PIV (PIV-5 or PIV5) is an enveloped virus of the genus rubulavirus in the paramyxoviridae family. PIV-5 was previously known by the name SV5 (simian virus 5), since it was initially isolated from primary monkey cell cultures. However, the natural host of PIV-5 appears to be the dog, in which it causes a cough known as kennel cough. 
     Subsequently, several PIV-5 isolates were obtained from samples collected from humans, but which were cultured on animal cells. Further, no symptoms or disease are associated with PIV-5 in the human being. The question of whether the human being is actually a host for PIV-5 is thus still being hotly debated. 
     In any event, the PIV-5 virus is currently considered to be an animal virus. 
     The parainfluenza virus (PIV) which is currently indexed as type 2 PIV (PIV-2 or PIV2) is also an enveloped virus of the genus rubulavirus in the paramyxoviridae family. 
     Currently, only human isolates of PIV-2 have been identified (hPIV-2), and so PIV-2 is considered to be a human virus. 
     PIV-5 and PIV-2 viruses are very similar to each other in terms of nucleic acid sequences, protein sequences, organisation, structure and morphology. 
     Selected from human parainfluenza viruses, PIV-2 is the closest virus to PIV-5 which has been found in the human being. 
     PIV-2 may be considered, and is at least considered to be so by the inventors, to be the human equivalent of PIV-5. 
     Infection by PIV-5 or PIV-2 leads to the formation of plurinuclear structures known as syncytia, which result from the fusion of cells from the infected host. 
     PIV-5 and PIV-2 viruses enter the host cell by fusion of the viral envelope with the cell membrane. 
     This fusion involves two viral glycoproteins: the haemagglutinin-neuraminidase attachment protein (FIN) and the fusion protein (F). 
     The fusion protein F of PIV-5 and PIV-2 is synthetized in the form of a simple precursor (F0) and is in the form of a glycosylated homotrimer. The fusion protein F of PIV-5 and PIV-2 requires proteolytic cleavage by host furines of the host to generate a “pre-activated” form consisting of two subunits linked via disulphide bridges: a large carboxy-terminal subunit F1 and a small amino-terminal subunit F2. 
     The subunit F1 is composed of a hydrophobic fusion peptide (FP) as well as two heptade repeat domains (HR-1 and HR-2) having a coiled-coil type conformation. After activation by HN, the fusion protein undergoes a series of conformational changes resulting in insertion of the fusion peptide into the target cell membrane. Next, interaction occurs between the HR-1 and HR-2 domains which bring the viral envelope close to the cell membrane (Russell et al, 2006). These domains are known to form a very stable bundle of six helices constituted by a trimeric coiled-coil structure in which three HR-I domains are linked with three HR-2 domains in an anti-parallel orientation. This conformation represents the post-fusion form of the F protein (Baker et al, 1999, Sergel-Germano et al, 2000, West et al, 2005). 
     The fusion protein F of PIV-5 and PIV-2 requires a HN protein deriving from the same viral type for there to be promotion of fusion (Yao et al, 1997). However, the precise nature of the interactions which exist between F and HN is still not properly known. 
     Nevertheless, several studies have shown that certain strains of PIV-5 and PIV-2 differ in their requirement for HN to trigger fusion. 
     As an example, Ito et al, 1997 describe two strains of “SV5” (PIV-5), namely W3A and WR, the F proteins of which differ by only three amino acids (residues 22, 443 and 516). The F protein of the W3A strain is capable of inducing fusion in an autonomous manner, i.e. in the absence of the HN protein, while the F protein of the WR strain is not capable thereof. 
     Ito et al, 1997 indicate that the fusogenic activity of the F protein of the WR strain can be re-established by replacing the amino acid in position 22 of this protein by the amino acid proline. 
     Ito et al, 2000 suggest that the amino acid E in position 132 and the amino acid A in position 290 of the F protein of the W3A and WR strains could be involved in the capacity of the F protein of these strains to be autonomous of the HN protein. 
     Paterson et al, 2000 indicate that the presence of the amino acid proline in position 22 of the F protein of the W3A strain, and the presence of the amino acid 443 of the F protein of the WR strain could increase the fusogenic capacity of these viral strains. 
     Russell et al, 2003 indicate that replacing the residues L447 and 1449 of the F protein of the strain W3A by aromatic amino acids could increase the fusogenic activity of the F protein of this viral strain. 
     Gardner and Dutch, 2007, indicate that the mutation 149A of the F protein of a wild type “SV5” (PIV-5) virus should have a pro-fusogenic effect. 
     Gardner et al, 2007, indicate that the mutation V402A of the F protein of a wild type “SV5” (PIV-5) virus should have a pro-fusogenic effect. 
     Concerning the protein PIV-2, there does not appear to be a prior art document which describes the introduction of mutation(s) into the sequence for the F protein of this virus, of the type to attempt to thereby increase the autonomy and fusogenic capacity. 
     Furthermore, there are many other viral proteins capable of fusion, such as the influenza proteins HA1/HA2, the rhabdovirus G protein, or the gp41/gp120 proteins of HIV. 
     Current knowledge concerning the fusogenicity capacities and autonomy of these various viral proteins is still very limited. 
     In any event, current knowledge concerning viral fusion proteins does not provide sufficient technical know-how to envisage effective medical and/or biotechnological applications. 
     SUMMARY OF THE INVENTION 
     The inventors have assumed that having available a fusion protein which was hyperfusogenic and which also exhibited substantial autonomy in its capacity for fusion could provide a solution to a certain number of medical and biotechnological situations. 
     The inventors have thus selectively selected the F protein of PIV-5 and/or PIV-2 from a series of other viral fusion proteins, such as the HA1/HA2 proteins from influenza, the G protein from rhabdovirus, or the gp41/gp120 proteins from HIV, for example. 
     They then constructed and produced mutant F proteins which are capable of fusogenicity in the absence of the HN protein. 
     The mutant proteins of the invention proved to have a high capacity for fusogenicity and high autonomy. They did not require the presence of the HN protein to induce cell fusion and the formation of syncytia. 
     The inventors also showed that it is possible to introduce into these mutant proteins a cleavage site which is different from that which the F protein presents in the natural state, and more particularly a tissue-specific cleavage site. 
     The inventors also propose medical (therapeutic, preventative, palliative, but also diagnostic) and/or biotechnological applications for the mutant proteins of the invention and/or products derived or following therefrom. 
     More particularly, the inventors propose the use of the mutant proteins of the invention or nucleic acids encoding them, to treat, prevent or mitigate, in vivo or ex vivo, diseases or dysfunctions for which it is desirable to induce or increase the formation of syncytia, such as cancers (more particularly metastatic cancers, preferably metastatic melanomas) or deficiencies in placental development. 
     The inventors further propose agents blocking the F protein of PIV-5 and/or PIV-2, such as antibodies, recombinant dendritic cells, antisense cells, siRNAs, nucleic acid aptamers, to treat, prevent or mitigate, in vivo or ex vivo, diseases or dysfunctions for which it is desired to inhibit or block the formation of syncytia, such as enveloped virus infections, allergies, auto-immune diseases or graft rejections. 
     The inventors further propose diagnostic means for detecting the excessive or, in contrast, insufficient formation of syncytia. 
     The inventors further propose different biotechnological means, especially for screening active principles capable of reducing the formation of syncytia. 
     The inventors further propose cancer cells, more particularly myelomas, which comprise a mutant protein of the invention. The inventors further propose hybridomas comprising a mutant protein of the invention. The cancer cells, more particularly myelomas, of the invention are capable of fusing with another cell, and more particularly with a B lymphocyte, without the use of polyethylene glycol (PEG), nor of electroporation, nor any other fusion-inducing means. The cancer cells, more particularly myelomas, of the invention are capable of autonomous fusion. 
     The hybridomas of the invention may be produced by fusion (of B lymphocytes to myelomas) which does not require the use of polyethylene glycol (PEG), nor of electroporation, nor of any other fusion-inducing means. The hybridomas of the invention may in fact be produced by employing at least one cancer cell, more particularly at least one myeloma of the invention. 
     The inventors further propose recombinant stem or progenitor cells which express a mutant protein of the invention, and their applications in the production of muscle fibres. 
     Other aspects of the invention will be described in the “detailed description” section below. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The set of figures for the application as filed was filed in colour. It can be accessed by consulting the file at the Office. 
         FIG. 1A : reference sequence for the F protein of PIV-5 (SEQ ID NO: 31; sequence for the F protein of the WR isolate) and corresponding CDS sequence (SEQ ID NO: 30; nucleic acid sequence encoding the protein of SEQ ID NO: 31). The bold and underlined characters in the sequence of SEQ ID NO: 31 indicate the amino acids of positions:
         22 (L),   49 (I),   132 (E, pre-existing mutation in this isolate),   147 (T),   158 (T),   290 (A, pre-existing mutation in this isolate),   402 (V),   443 (P, mutation theoretically pre-existing in this isolate),   447 (L),   449 (I), and   463 (A),       
         FIG. 1B : reference sequence for the F protein of PIV-2 (SEQ ID NO: 33; sequence for the F protein of the Greer isolate) and corresponding CDS sequence (SEQ ID NO: 32; nucleic acid sequence encoding the protein of SEQ ID NO: 33). 
       The bold and underlined characters in the sequence of SEQ ID NO: 33, indicate the amino acids of positions:
         24 (I),   53 (I),   133 (K),   151 (T),   162 (S),   294 (I),   406 (A),   428 (S),   439 (S),   445 (I),   474 (S).       

         FIG. 2  A: alignment of the F protein of PIV-5 (amino acids 4 to 529 of SEQ ID NO: 31) on that of PIV-2 (amino acids 8 to 533 of SEQ ID NO: 33): the protein identity is 47.7% between these two proteins. The consensus sequence resulting from this alignment is referenced as SEQ ID NO: 34. 
         FIG. 2B : amino acid(s) and mutation(s) in the PIV-2 F protein which correspond to those of the F protein of PIV-5. 
         FIG. 3 : illustration of the substitution of the natural cleavage site (SEQ ID NO: 23) of the F protein of PIV-5 by a tissue-specific cleavage site, in this case the site of an enzyme specifically expressed by metastatic tumour tissue, namely matrix metallo-protease 9 (MMP-9). 
       Consensus site of a MMP-9 site: sequence of SEQ ID NO: 27 (PXXhyS/T site where X=any amino acid and Hy=any hydrophobic amino acid, i.e. any amino acid selected from F, M, V, L, I). 
       Illustrative sequence for a MMP-9 site: sequence of SEQ ID NO: 28, sequence of SEQ ID NO: 29. 
         FIG. 4 : structure of plasmid pcDNA3.1 onto which the sequence encoding the F protein of PIV-5 has been cloned.
         Amp: ampicillin resistance gene   pCMV: cytomegalovirus promoter   MCS: multiple cloning site   F PIV5: F protein of PIV-5   BGHpolyA: bovine growth hormone polyA.       
         FIGS. 5A ,  5 B,  5 C,  5 D: visualisation of mutations produced by the inventors in the F protein of PIV-5. 
         FIG. 6A : illustration of microscope observations carried out during semi-quantitative fusion tests (large panel of mutants produced by the inventors). 
         FIG. 6B : diagram presenting the fusion scores obtained after semi-quantitative fusion tests (large panel of mutants produced by the inventors). 
         FIG. 7A : illustration of microscope observations carried out during semi-quantitative fusion tests (selection of mutants produced by the inventors). 
         FIG. 7B : diagram presenting the fusion scores obtained after semi-quantitative fusion tests (selection of mutants produced by the inventors). 
     
    
    
     DETAILED DESCRIPTION 
     In the present application, the term “protein” includes the term “glycoprotein” in its scope. This is especially the case for the F and HN proteins which are in fact glycoproteins. 
     F Protein of PIV-5 and PIV-2 (Non-Mutant Protein): 
     A sequence for the F protein of PIV-5 is presented in  FIG. 1A  (protein sequence of SEQ ID NO: 31; coding nucleic acid sequence of SEQ ID NO: 30). It is the sequence for the WR isolate, which is a simian isolate. These sequences are those available from the Genbank database with accession number AB021962. 
     The sample of the WR isolate which the inventors received from the ATCC and which they used for the construction and the production of the mutant proteins described in the examples below do not, however, have the amino acid P in position 443 of the F protein (in contrast to that which was expected in view of the sequence available from Genbank), but rather the amino acid S. This alternative sequence for the F protein of the WR isolate is thus identical to the sequence of SEQ ID NO: 31, with the exception of the amino acid in position 443 which is S and not P. For the purposes of brevity, this alternative sequence will herein be denoted “SEQ ID NO: 31 with S at 443”. 
     The sequence of SEQ ID NO: 31 and the alternative sequence “SEQ ID NO: 31 with S at 443”, preferably the alternative sequence “SEQ ID NO: 31 with S at 443”, act as reference sequence(s) for the F protein of PIV-5 in the context of the present patent application. 
     However, clearly, isolates other than the WR isolate exist, in particular:
         other simian isolates, such as the W3A isolate, for example;   isolates from other non-human animals, such as:
           canine isolates, for example the canine isolates CPI+, CPI−, H221, 78524, T1;   porcine isolates, for example the porcine isolate SER;   
           isolates termed “human” isolates which are derived from samples taken from human beings but which have been cultured on animal cells (see introduction section above), such as the MIL isolate, the DEN isolate, the LN isolate, the MEL isolate and the isolate which, in WO 02 077211, is described as being a “cryptovirus”.       

     The variations in the sequences for the F proteins of these various PIV-5 isolates are very slight. 
     A description of these variations is given by Chatziandreou et al, 2004, the contents of which, and more particularly Table 3 and the comments associated therewith in that article, are herewith incorporated into the present patent application by reference. 
     Table 3 of that article is reproduced here: 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 (reproduced from the article by Chatziandreou et al, 2004): 
               
             
          
           
               
                 Strain 
                   
               
             
          
           
               
                   
                 WR 
                 MIL 
                 DEN 
                 LN 
                 MEL 
                   
                   
                 CPI− 
                 H221 
                   
                 T1 
                 SER 
                   
               
               
                 W3A 
                 (SEQ 
                 (SEQ 
                 (SEQ 
                 (SEQ 
                 (SEQ ID 
                 cryptovirus 
                 CPI+ 
                 (SEQ 
                 (SEQ ID 
                 78524 
                 (SEQ 
                 (SEQ 
               
               
                 (SEQ 
                 ID NO: 
                 ID NO: 
                 ID NO: 
                 ID NO: 
                 NO: 
                 (SEQ ID 
                 (SEQ ID 
                 ID NO: 
                 NO: 
                 (SEQ ID 
                 ID NO: 
                 ID NO: 
               
               
                 ID NO: 35) 
                 31) 
                 36) 
                 37) 
                 38) 
                 39) 
                 NO: 40) 
                 NO: 41) 
                 42) 
                 43) 
                 NO: 44) 
                 45) 
                 46) 
                 aa 
               
               
                   
               
             
          
           
               
                 G 
                   
                   
                   
                   
                   
                 S 
                   
                   
                   
                   
                   
                   
                 2 
               
               
                 T 
                   
                 I 
                 I 
                 I 
                 I 
                   
                   
                   
                   
                   
                   
                   
                 3 
               
               
                 I 
                   
                   
                   
                   
                   
                   
                   
                   
                 R 
                 R 
                 R 
                   
                 4 
               
               
                 F 
                   
                   
                   
                   
                   
                 S 
                   
                   
                   
                   
                   
                   
                 7 
               
               
                 A 
                   
                   
                   
                   
                   
                   
                   
                   
                 S 
                 S 
                 T 
                   
                 17 
               
               
                 S 
                   
                 G 
                 G 
                 G 
                 G 
                   
                   
                   
                   
                   
                   
                   
                 19 
               
               
                 P 
                 L 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 22 
               
               
                 S 
                   
                   
                   
                   
                   
                   
                 P 
                 P 
                   
                   
                   
                   
                 71 
               
               
                 V 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 M 
                   
                 76 
               
               
                 N 
                   
                   
                   
                   
                   
                   
                   
                   
                 Y 
                 Y 
                 Y 
                   
                 92 
               
               
                 E 
                   
                   
                   
                   
                   
                   
                 K 
                 K 
                 K 
                 K 
                 K 
                 K 
                 132 
               
               
                 A 
                   
                   
                   
                   
                   
                 T 
                   
                   
                   
                   
                   
                   
                 134 
               
               
                 A 
                   
                   
                   
                   
                   
                   
                   
                   
                 V 
                 V 
                 V 
                   
                 135 
               
               
                 A 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 T 
                 149 
               
               
                 V 
                   
                   
                   
                   
                   
                 I 
                   
                   
                   
                   
                   
                   
                 176 
               
               
                 I 
                   
                   
                   
                 M 
                   
                   
                   
                   
                   
                   
                   
                   
                 271 
               
               
                 T 
                   
                   
                   
                   
                   
                   
                   
                 A 
                   
                   
                   
                   
                 279 
               
               
                 A 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 V 
                   
                 290 
               
               
                 T 
                   
                   
                   
                   
                   
                   
                 K 
                 K 
                   
                   
                   
                   
                 307 
               
               
                 M 
                   
                   
                   
                   
                   
                 I 
                 I 
                 I 
                 I 
                 I 
                 I 
                 I 
                 310 
               
               
                 M 
                   
                   
                   
                   
                 R 
                   
                   
                   
                   
                   
                   
                   
                 346 
               
               
                 L 
                   
                   
                   
                 F 
                   
                   
                   
                   
                   
                   
                   
                   
                 366 
               
               
                 V 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 M 
                 370 
               
               
                 Y 
                   
                   
                   
                   
                   
                   
                   
                   
                 F 
                 F 
                   
                   
                 377 
               
               
                 M 
                   
                   
                   
                   
                   
                   
                 I 
                 I 
                   
                   
                   
                   
                 407 
               
               
                 Y 
                   
                   
                   
                   
                   
                 H 
                 H 
                 H 
                   
                   
                   
                   
                 408 
               
               
                 N 
                   
                   
                   
                   
                   
                 D 
                   
                   
                   
                   
                   
                   
                 417 
               
               
                 F 
                   
                   
                   
                   
                   
                   
                 L 
                 L 
                   
                   
                   
                   
                 420 
               
               
                 V 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 I 
                   
                 428 
               
               
                 S 
                   
                   
                   
                   
                   
                 T 
                 T 
                 T 
                 T 
                 T 
                 T 
                 T 
                 438 
               
               
                 S 
                 P 
                 P 
                 P 
                 P 
                 P 
                 P 
                 P 
                 P 
                 P 
                 P 
                 P 
                 P 
                 443 
               
               
                 H 
                   
                   
                   
                   
                   
                   
                 N 
                 N 
                   
                   
                   
                   
                 451 
               
               
                 I 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 M 
                 489 
               
               
                 L 
                   
                 F 
                 F 
                 F 
                 F 
                   
                   
                   
                   
                   
                   
                   
                 498 
               
               
                 L 
                   
                   
                   
                   
                   
                   
                 S 
                 S 
                   
                   
                   
                   
                 500 
               
               
                 V 
                   
                   
                   
                   
                   
                   
                 A 
                 A 
                   
                   
                   
                   
                 507 
               
               
                 K 
                   
                   
                   
                   
                   
                   
                 R 
                 R 
                   
                   
                   
                   
                 510 
               
               
                 V 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
                 T 
                 516 
               
               
                 K 
                   
                 N 
                 N 
                 N 
                 N 
                 N 
                 N 
                 N 
                 N 
                 N 
                 N 
                 N 
                 529 
               
               
                 Stop 
                 Stop 
                 Q 
                   
                   
                   
                 S 
                 S 
                 S 
                 S 
                 S 
                 S 
                 S 
                 530 
               
               
                   
                   
                 H 
                   
                   
                   
                   
                 Y 
                 Y 
                   
                   
                   
                   
                 533 
               
               
                   
                   
                 S 
                   
                   
                 Stop 
                 P 
                   
                   
                   
                   
                   
                   
                 535 
               
               
                   
                   
                 Q 
                   
                   
                   
                 R 
                 R 
                 R 
                 R 
                 R 
                 R 
                 R 
                 536 
               
               
                   
               
               
                 aa: position of the amino acid 
               
             
          
         
       
     
     In Table 1 above, an empty box indicates that the F protein concerned has the same amino acid as the F protein of the W3A strain indicated in the left hand column. The amino acids for which their positions are not expressly listed in this table are of course identical to those which correspond to them in the sequence for the F protein of W3A. These amino acids are themselves identical to those which correspond to them in the sequence for the F protein of the WR isolate (see sequence of SEQ ID NO: 31). 
     Hence, the sequences for SEQ ID NO: 35 to 46 are the sequences which result from replacement in the sequence of SEQ ID NO: 31 of the amino acids indicated in Table 1 for each of these sequences (and, if appropriate, addition at the C-terminal portion to the sequence of SEQ ID NO: 31 of the amino acids indicated). 
     The F protein of the W3A isolate, as well as that of the other isolates mentioned above, has:
         in position 147, the amino acid T;   in position 158, the amino acid T;   in position 447, the amino acid L; and   in position 449, the amino acid I.       

     Thus, it will be seen that the W3A and WR isolates do not have the cytoplasmic extension which, in the other isolates, extends beyond position 529. Depending on the isolate concerned, this cytoplasmic extension contains two to seven amino acids. 
     It will also be seen that the sequences for the F proteins of these isolates vary by less than 5% (more particularly, a maximum of 3%) compared with the sequence for the F protein of the WR strain (without taking into account the cytoplasmic extension, i.e. by calculating this percentage from the length of the F protein of WR); see end of page 85 of the article by Chatziandreou et al, 2004. 
     A F protein of PIV-5 may thus consist of:
         the sequence of SEQ ID NO: 31, or said alternative sequence “SEQ ID NO: 31 with S at 443”; or of   a variant sequence for this sequence of SEQ ID NO: 31 or of this alternative sequence “SEQ ID NO: 31 with S at 443”; this variant sequence may be defined as:
           being identical in size to that of SEQ ID NO: 31 or smaller by a maximum of 7 amino acids than that of SEQ ID NO: 31 or larger by a maximum of 7 amino acids than that of SEQ ID NO: 31 [said alternative sequence “SEQ ID NO: 31 with S at 443” is the same size as the sequence of SEQ ID NO: 31], preferably with a size identical to that of SEQ ID NO: 31 or larger by a maximum of 7 amino acids than that of SEQ ID NO: 31; and   having a sequence identity of more than 95%, preferably at least 96%, more preferably at least 97%, with respect to the sequence of SEQ ID NO: 31 or to said alternative sequence “SEQ ID NO: 31 with S at 443” (this identity being calculated using the length of the sequence of SEQ ID NO: 31 or, if appropriate, of said alternative sequence “SEQ ID NO: 31 with S at 443”).   
               

     The variant sequences for the F protein of PIV-5 of SEQ ID NO: 31 in particular comprise the sequences for the F proteins of the isolates W3A, MIL, DEN, LN, MEL, cryptovirus, CPI+, CPI−, H221, 78524, T1 and SER mentioned above (see Table 1 above and article by Chatziandreou et al, 2004). 
     In similar manner, the sequence which in the present application acts as a reference for the PIV-2 F protein is the sequence for the Greer strain which is presented in  FIG. 1B  (protein sequence of SEQ ID NO: 33, coding nucleic acid sequence of SEQ ID NO: 32). 
     Clearly, there are PIV-2 isolates other than Greer isolates, such as the V98, V94 Toshiba isolates, for example. 
     The sequence for the F protein of these other PIV-2 isolates is very close to that of the Greer isolate, but has several small variations which are inter-isolate variations. 
     Hence, a PIV-2 F protein may thus consist of:
         the sequence of SEQ ID NO: 33; or of   a variant sequence for this sequence of SEQ ID NO: 33; this variant sequence may be defined as:
           being identical in size to that of SEQ ID NO: 33 or smaller by a maximum of two amino acids than that of SEQ ID NO: 33 or larger by a maximum of two amino acids than that of SEQ ID NO: 33, preferably with a size identical to that of SEQ ID NO: 33, preferably being identical in size to that of SEQ ID NO: 33; and   having a sequence identity of more than 95%, preferably at least 96%, more preferably at least 97%, with respect to the sequence of SEQ ID NO: 33 (this identity being calculated using the length of the sequence of SEQ ID NO: 33).   
               

     The consensus sequence (SEQ ID NO: 34) resulting from alignment of the sequence for the F protein of PIV-5 (SEQ ID NO: 31) on that of PIV-2 (SEQ ID NO: 33) is as follows and may be read in  FIG. 2A . This consensus sequence may be re-written as follows: 
                     (SEQ ID NO: 34)       I----V------G--------L--IGVI----R-LMYYT-----FIVVKL-               P--------CNITS---YN-T--KLL-P—-ENL--I---------R-RF               AGVV-GLAALGVATAAQ-TAAVA-VKAN-NAAAI-NL---IQ-TN-AV-D               V-A------TAVQA-QD-IN------IT-A-C-A-DA-IGSILNLYLTEL               TTIFHNQITNPAL-P--IQALRILLGSTLP-V-E---NT----AELLSSG               LLTGQI------YMQM-I-I-PT----QP----IDL—-ISA----QEV--               Q-P-R--------Q-YPA—C--TPN-V-CRYN-----------CL-GNL--               CTF-P--G-FL-RF----G--YANC-S-LC-C—P-—V--Q--------               ID---C----LD---F-IT---N-TY----------I----PLD-               S------NKSL--A----A-S---------A-T---LS-1A-L-----               L----LL-----KL-------R--------H-,            
the symbol “-” indicating that the F proteins of PIV-5 and of PIV-2 have different amino acids in this position.
 
     This consensus sequence may also be formalised as follows: 
                     (SEQ ID NO: 34)       IXXXXVXXXXXXGXXXXXXXXLXXIGVIXXXXRXLMYYTXXXXXFIVVKL               XPXXXXXXXXCNITSXXXYNXTXXKLLXPXXENLXXIXXXXXXXXXRXRF               AGVVXGLAALGVATAAQXTAAVAXVKANXNAAAIXNLXXXIQXTNXAVXD               VXAXXXXXXTAVQAXQDXINXXXXXXITXAXCXAXDAXIGSILNLYLTEL               TTIFHNQITNPALXPVXIQALRILLGSTLPXVXEXXXNTXXXXAELLSSG               LLTGQIXXXXXXYMQMXIXIXPTXXXXQPXXXXIDLXXISAXXXXQEVXX               QXPXRXXXXXXXXQXYPAXXCXXTPNXVXCRYNXXXXXXXXXXXCLXGNL               XXCTFXPXXGXFLXRFXXXXGXXYANCXSXLCXCXXPXXVXXQXXXXXXX               XIDXXXCXXXXLDXXXFXITXXXNXTYXXXXXXXXXXIXXXXPLDXSXXX               XXXNKSLXXAXXXXAXSXXXXXXXXXAXTXXXLSXIAXLXXXXXLXXXXL               LXXXXXKLXXXXXXXRXXXXXXXXHX,            
where X=any amino acid.
 
     The sequence for the F protein of the WR isolate of PIV-5 is the sequence of SEQ ID NO: 34 preceded by the amino acids MGT at the N-terminal end (see  FIGS. 1A and 2A ). 
     The sequence for the F protein of the Greer isolate of PIV-2 is the sequence of SEQ ID NO: 34, preceded by the amino acids MHHLHPM (SEQ ID NO: 86) at the N-terminal end and followed by the amino acids ENPAFFSKNNHGNIYGIS (SEQ ID NO: 87) at the C-terminal end (see  FIGS. 1B and 2A ). 
     The sequence for the F proteins of PIV-5 and PIV-2 may be considered to be a sequence comprising the sequence of SEQ ID NO: 34, preferably to be the sequence for a F protein of the PIV virus which comprises the sequence of SEQ ID NO: 34. More particularly, the sequence for the F proteins of PIV-5 and PIV-2 may be considered to be: 
     a) the sequence of SEQ ID NO: 34: 
     
         
         
           
             preceded by 3 to 7 amino acids at the N-terminal end, more particularly by 3 amino acids (such as MGT) or by 7 amino acids (such as MHHLHPM) at the N-terminal end; and 
             optionally followed by 18 amino acids at the C-terminal end, more particularly the amino acids ENPAFFSKNNHGNIYGIS at the C-terminal end; or
 
b) a variant sequence for the sequence described in a) above, said variant sequence being:
 
             either:
           i. with a size identical to that of SEQ ID NO: 31 or smaller by a maximum of 7 amino acids than that of SEQ ID NO: 31 or larger by a maximum of 7 amino acids than that of SEQ ID NO: 31, preferably with a size identical to that of SEQ ID NO: 31 or larger by a maximum of 7 amino acids than that of SEQ ID NO: 31; and   ii. having a sequence identity of more than 95%, preferably at least 96%, more preferably at least 97%, with respect to the sequence of SEQ ID NO: 31 or to said alternative sequence “SEQ ID NO: 31 with S at 443” (this identity being calculated using the length of the sequence of SEQ ID NO: 31 or, if appropriate, of said alternative sequence “SEQ ID NO: 31 with S at 443”);   
         
             or:
           i. with a size identical to that of SEQ ID NO: 33 or smaller by a maximum of two amino acids than that of SEQ ID NO: 33 or larger by a maximum of two amino acids than that of SEQ ID NO: 33, preferably with a size identical to that of SEQ ID NO: 33; and   ii. having a sequence identity of more than 95%, preferably at least 96%, more preferably at least 97%, with respect to the sequence of SEQ ID NO: 33 (this identity being calculated using the length of the sequence of SEQ ID NO: 33).
 
Mutant Proteins of the Invention:
   
         
           
         
       
    
     The present application relates to a mutant protein, the amino acid sequence for which comprises a sequence which is derivable from that of the F protein of a PIV-5 or PIV-2 virus:
         by replacement:
           of the amino acid which, in the sequence of said PIV-5 F protein, is in position 22, or which, in the sequence of said PIV-2 F protein, is in position 24, by the amino acid P (mutation 22P in the F of PIV-5; mutation 24P in the F of PIV-2); and   of the amino acid which, in the sequence of said PIV-5 F protein, is in position 132, or which, in the sequence of said PIV-2 F protein, is in position 133 (mutation 132E in the F of PIV-5; mutation 133E in the F of PIV-2), by the amino acid E; and   of the amino acid which, in the sequence of said PIV-5 F protein, is in position 290, or which, in the sequence of said PIV-2 F protein, is in position 294, by the amino acid A (mutation 290A in the F of PIV-5; mutation 294A in the F of PIV-2); and   of the amino acid which, in the sequence of said PIV-5 F protein, is in position 447, or which, in the sequence of said PIV-2 F protein, is in position 445, by the amino acid P (mutation 447P in the F of PIV-5; mutation 445P in the F of PIV-2);   
           and by replacement:
           of the amino acid which, in the sequence of said PIV-5 F protein, is in position 147, or which, in the sequence of said PIV-2 F protein, is in position 151, by a hydrophobic amino acid selected from V, I, L, preferably V (mutation 147Hy in the F of PIV-5; mutation 151Hy in the F of PIV-2); and/or   of the amino acid which, in the sequence of said PIV-5 F protein, is in position 158, or which, in the sequence of said PIV-2 F protein, is in position 162, by a hydrophobic amino acid selected from V, I, L, preferably V (mutation 158Hy in the F of PIV-5; mutation 162Hy in the F of PIV-2);   
           and optionally:
           by substitution of the native (or natural) cleavage site of said F protein by another enzymatic cleavage site, and/or by insertion into said F protein of an enzymatic cleavage site other than the native (or natural) cleavage site of said F protein; and/or   by deletion of a C-terminal portion of said F protein, said C-terminal portion extending in the N-terminal direction from the last amino acid at the C-terminal end of the protein, but without extending beyond the HR2 domain of said F protein.   
               

     Said amino acid positions are calculated with respect to the sequence for the precursor form (F0) of said F protein (i.e. the sequence for the F protein before cleaving), counting the positions from the N-terminal end to the C-terminal end. 
     The positions indicated in the PIV-2 F protein are the positions which correspond to those indicated in the F protein of PIV-5: see  FIG. 2B , giving the table for correspondence of positions. 
     The sequence of said F protein of the PIV-5 or PIV-2 virus is as defined above. Thus, it may in particular be defined as comprising the sequence of SEQ ID NO: 34 (consensus sequence for the F proteins of PIV-5 and PIV-2). 
     Mutant Protein of PIV-5F Protein: 
     In accordance with one aspect of the invention, a mutant protein of the invention comprises a sequence which is derivable from that of the F protein of a PIV-5 virus:
         the sequence of SEQ ID NO: 31 (sequence for the F protein of the WR isolate of PIV-5 presented in  FIG. 1A ), or of said alternative sequence “SEQ ID NO: 31 with S at 443”; or of   a variant sequence for this sequence of SEQ ID NO: 31 or of said alternative sequence “SEQ ID NO: 31 with S at 443”, this variant sequence:
           being identical in size to that of SEQ ID NO: 31 (i.e. consisting of 529 amino acids), or being of a size larger by a maximum of 7 amino acids than that of SEQ ID NO: 31 (i.e. consisting of 530, 531, 532, 533, 534, 535 or 536 amino acids), or being of a size smaller by a maximum of 7 amino acids than that of SEQ ID NO: 31 (i.e. consisting of 522, 523, 524, 525, 526, 527 or 528 amino acids); and   having a sequence identity of more than 95%, preferably at least 96%, more preferably at least 97%, with respect to the sequence of SEQ ID NO: 31 or to said alternative sequence “SEQ ID NO: 31 with S at 443”, this identity being calculated using the length of the sequence of SEQ ID NO: 31 or (if appropriate) of said alternative sequence “SEQ ID NO: 31 with S at 443”.   
               

     Preferably, the sequence of said F protein of PIV-5 consists of:
         the sequence of SEQ ID NO: 31 (sequence for the F protein of the WR isolate of PIV-5 presented in  FIG. 1A ), or said alternative sequence “SEQ ID NO: 31 with S at 443”; or of   a variant sequence for this sequence of SEQ ID NO: 31 or of said alternative sequence “SEQ ID NO: 31 with S at 443”, this variant sequence:
           being identical in size to that of SEQ ID NO: 31 (i.e. consisting of 529 amino acids), or being of a size larger by a maximum of 7 amino acids than that of SEQ ID NO: 31 (i.e. consisting of 530, 531, 532, 533, 534, 535 or 536 amino acids); and   having a sequence identity of more than 95%, preferably at least 96%, more preferably at least 97%, with respect to the sequence of SEQ ID NO: 31 or to said alternative sequence “SEQ ID NO: 31 with S at 443”, this identity being calculated using the length of the sequence of SEQ ID NO: 31, or, if appropriate, of said alternative sequence “SEQ ID NO: 31 with S at 443”.   
               

     Particular examples of such variant sequences comprise the sequence for the F protein of one of the W3A, MIL, DEN, LN, MEL, cryptovirus, CPI+, CPI−, H221, 78524, T1 and SER isolates presented in Table 1 in the present application (see above), i.e. one of the sequences of SEQ ID NO: 35 to 46. 
     Preferably, the sequence of said F protein of PIV-5 consists of the sequence of SEQ ID NO: 31 (sequence for the F protein of the WR isolate of PIV-5 presented in  FIG. 1A ), or of said alternative sequence “SEQ ID NO: 31 with S at 443”, highly preferably in said alternative sequence “SEQ ID NO: 31 with S at 443”. 
     Preferably, said sequence which is derivable from that of said F protein of PIV-5 is derivable from this F protein sequence by at least said mutations 22P, 132E, 290A, 447P and 158Hy mentioned above. 
     Preferably, said sequence which is derivable from that of said F protein of PIV-5 is derivable from this F protein sequence by at least said mutations 22P, 132E, 290A, 447P and 147Hy mentioned above. 
     Preferably, said sequence which is derivable from that of said F protein of NV-5 is derivable from this F protein sequence by at least said mutations 22P, 132E, 290A, 447P, 147Hy and 158Hy mentioned above. 
     Said sequence derivable from that of the F protein of PIV-5 does not have to comprise a mutation other than the mutations 22P, 132E, 290A, 447P and 147Hy/158Hy mentioned above, with respect to said F protein sequence for PIV-5. 
     Alternatively, said sequence derivable from that of the F protein of PIV-5 may be derivable from this F protein sequence by said mutations 22P, 132E, 290A, 447P and 147Hy1158FHy mentioned above and by at least one mutation other than these mutations 22P, 132E, 290A, 447P mentioned above, preferably by:
         at least one pre-fusion mutation selected from:
           replacement of the amino acid in position 49 by the amino acid A;   replacement of the amino acid in position 402 by the amino acid A;   replacement of the amino acid in position 443 by the amino acid P;   replacement of the amino acid in position 449 by the amino acid P;
 
and/or by
   
           at least one post-fusion mutation selected from:
           replacement of the amino acid in position 463 by a hydrophobic amino acid.   
               

     Preferably, said sequence which is derivable from that of said F protein of PIV-5 is derivable from this F protein sequence by said mutations 22P, 132E, 290A, 447P and 147Hy/158Hy mentioned above, and by:
         at least one pre-fusion mutation selected from:
           replacement of the amino acid in position 49 by the amino acid A;   replacement of the amino acid in position 402 by the amino acid A;
 
and/or by
   
           at least one post-fusion mutation selected from:
           replacement of the amino acid in position 463 by a hydrophobic amino acid.   
               

     Preferably, said sequence which is derivable from that of said F protein of PIV-5 is derivable from this F protein sequence by said mutations 22P, 132E, 290A, 447P and 147Hy/158Hy mentioned above, and by at least one post-fusion mutation selected from:
         replacement of the amino acid in position 463 by a hydrophobic amino acid.       

     Said hydrophobic amino acid selected to replace the amino acid in position 463 is advantageously selected from V, I, L, preferably V. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 selection of mutant proteins of the invention 
               
               
                 (mutant proteins of F protein of PIV-5) 
               
             
          
           
               
                 Fus No 
                   
                   
               
               
                 In parentheses: nature of the 
               
               
                 post-fusion mutation(s) over the 
               
               
                 three mutations for autonomy 
               
               
                 (22P, 132E, 290A) and the 
                 Fusion score 
               
               
                 pre-fusion mutation 447P 
                 (see FIG. 6B) 
                 SEQ ID NO: 
               
               
                   
               
               
                 7.1 
                 approx 5 
                 62 
               
               
                 (147V) 
               
               
                 7.2 
                 approx 6.5 
                 63 
               
               
                 (158V) 
               
               
                 7.3 
                 approx 7 
                 64 
               
               
                 (147V et 158V) 
               
               
                   
               
             
          
         
       
     
     The mutations indicated in Table 4 may be introduced into the F protein of any PIV-5 isolate, i.e. the WR isolate or a variant isolate. More particularly, they may thus be introduced into the F protein sequence of SEQ ID NO: 31 presented in  FIG. 1A  (F protein of WR available from Genbank database with accession number AB021962). 
     As indicated above, the sample of the WR isolate which the inventors received from the ATCC and which they used to construct and produce the mutant proteins described in the examples below did not, however, have the amino acid P in position 443 of the F protein (in contrast to that which was expected in view of the sequence available from Genbank), but rather the amino acid S. The mutations indicated in Table 4 may thus be introduced into said alternative sequence “SEQ ID NO: 31 with S at 443” (SEQ ID NO: 62, 63, 64). 
     Mutant Protein of PIV-2 F Protein; 
     In accordance with another aspect of the invention, a mutant protein of the invention comprises a sequence which is derivable from that of the F protein of a PIV-2 virus. 
     The sequence for said PIV-2 F protein is as defined above. In particular, it may consist of
         the sequence of SEQ ID NO: 33 (sequence for the F protein of the Greer isolate of PIV-2 presented in  FIG. 1B ); or of   a variant sequence for this sequence of SEQ ID NO: 33, this variant sequence:
           being identical in size to that of SEQ ID NO: 33 (i.e. consisting of 551 amino acids), or being of a size larger by a maximum of 2 amino acids than that of SEQ ID NO: 33 (i.e. consisting of 552 or 553 amino acids), or being of a size smaller by a maximum of 2 amino acids than that of SEQ ID NO: 33 (i.e. consisting of 549 or 550 amino acids); and   having a sequence identity of more than 95%, preferably at least 96%, more preferably at least 97%, with respect to the sequence of SEQ ID NO: 33, this identity being calculated using the length of the sequence of SEQ ID NO: 33.   
               

     Preferably, the sequence of said F protein of PIV-2 consists of:
         the sequence of SEQ ID NO: 33 (sequence for the F protein of the Greer isolate of PIV-2 presented in  FIG. 1B ); or of   a variant sequence for this sequence of SEQ ID NO: 33, this variant sequence:
           being identical in size to that of SEQ ID NO: 33 (i.e. consisting of 551 amino acids), or being of a size larger by a maximum of 2 amino acids than that of SEQ ID NO: 33 (i.e. consisting of 552 or 553 amino acids); and   having a sequence identity of more than 95%, preferably at least 96%, more preferably at least 97%, with respect to the sequence of SEQ ID NO: 33, this identity being calculated using the length of the sequence of SEQ ID NO: 33.   
               

     More preferably, the sequence of said F protein of PIV-2 consists of:
         the sequence of SEQ ID NO: 33 (sequence for the F protein of the Greer isolate de PIV-2 presented in  FIG. 1B ); or of   a variant sequence for this sequence of SEQ ID NO: 33, this variant sequence:
           being identical in size to that of SEQ ID NO: 33 (i.e. consisting of 551 amino acids); and   having a sequence identity of more than 95%, preferably at least 96%, more preferably at least 97%, with respect to the sequence of SEQ ID NO: 33, this identity being calculated using the length of the sequence of SEQ ID NO: 33.   
               

     Highly preferably, the sequence of said F protein consists of the sequence of SEQ ID NO: 33 (sequence for the F protein of the Greer isolate of PIV-2 presented in  FIG. 1B ). 
     Preferably, said sequence which is derivable from that of said PIV-2 F protein is derivable from this F protein sequence by at least said mutations 24P, 133E, 294A, 445P and 162Hy mentioned above. 
     Preferably, said sequence which is derivable from that of said F protein of PIV-2 is derivable from this F protein sequence by at least said mutations 24P, 133E, 294A, 445P and 151Hy mentioned above. 
     Preferably, said sequence which is derivable from that of said F protein of PIV-2 is derivable from this F protein sequence by at least said mutations 24P, 133E, 294A, 445P, 162Hy and 151Hy mentioned above. 
     Said sequence which is derivable from that of the F protein of PIV-5 does not have to comprise a mutation other than the mutations 24P, 133E, 294A, 445P and 151Hy/162Hy mentioned above, with respect to said F protein sequence for PIV-2. 
     Alternatively, said sequence derivable from that of the PIV-2 F protein may be derivable from this F protein sequence by said mutations 24P, 133E, 294A, 445P and 151Hy/162Hy mentioned above and by at least one mutation other than these mutations 24P, 133E, 294 A, 445P mentioned above, preferably by:
         at least one pre-fusion mutation selected from:
           replacement of the amino acid in position 53 by the amino acid A;   replacement of the amino acid in position 406 by the amino acid A;   replacement of the amino acid in position 428 by the amino acid P;   replacement of the amino acid in position 439 by the amino acid P;
 
and/or by
   
           at least one post-fusion mutation selected from:
           replacement of the amino acid in position 474 by a hydrophobic amino acid.   
               

     Preferably, said sequence which is derivable from that of said F protein of the PIV-2 is derivable from this F protein sequence by said mutations 24P, 133E, 294A, 445P and 151Hy/162Hy mentioned above, and by:
         at least one pre-fusion mutation selected from:
           replacement of the amino acid in position 53 by the amino acid A;   replacement of the amino acid in position 406 by the amino acid A;
 
and/or by
   
           at least one post-fusion mutation selected from:
           replacement of the amino acid in position 474 by a hydrophobic   
               

     Preferably, said sequence which is derivable from that of said F protein of the PIV-2 is derivable from this F protein sequence by said mutations 24P, 133E, 294A, 445P and 151Hy/162Hy mentioned above, and by at least one post-fusion mutation selected from:
         replacement of the amino acid in position 474 by a hydrophobic amino acid.       

     Said hydrophobic amino acid selected in replacement of the amino acid in position 474 is advantageously selected from V, I, L, preferably V. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 selection of mutant proteins of the invention 
               
               
                 (mutant proteins of PIV-2 F protein) 
               
             
          
           
               
                   
                 Nature of post-fusion mutation(s) over 
                   
               
               
                   
                 the three autonomy mutations (24P, 133E, 
               
               
                   
                 294A) and the pre-fusion mutation 445P 
                 SEQ ID NO: 
               
               
                   
                   
               
               
                   
                 151V 
                 88 
               
               
                   
                 162V 
                 89 
               
               
                   
                 151V and 162V 
                 90 
               
               
                   
                   
               
             
          
         
       
     
     The mutations indicated in Table 5 may be introduced into the F protein of any PIV-2 isolate, i.e. the Greer isolate or a variant isolate. Thus, more particularly they may be introduced into the F protein sequence of SEQ ID NO: 33 presented in  FIG. 1B  (F protein of Greer; SEQ ID NO: 88 to 90). 
                                                                                       SEQ ID NO: 88                MHHLHPMIVC IFVMYTGIVG SDAPAGDQLL NIGVIQSKIR SLMYYTDGGA SFIVVKLLPN    60                   LPPSNGTCNI TSLDAYNVTL FKLLTPLIEN LSKISTVTDT KTRQKRFAGV VVGLAALGVA   120               TAAQITAAVA IVEANANAAA INNLASSIQS VNKAVSDVID ASRTIATAVQ AIQDRINGAI   180               VNGITSASCR AHDALIGSIL NLYLTELTTI FHNQITNPAL TPLSIQALRI LLGSTLPIVI   240               ESKLNTNFNT AELLSSGLLT GQIISISPMY MQMLIQINVP TFIMQPGAKV IDLAAISANH   300               KLQEVVVQVP NRILEYANEL QNYPANDCVV TPNSVFCRYN EGSPIPESQY QCLRGNLNSC   360               TFTPIIGNFL KRFAFANGVL YANCKSLLCR CADPPHVVSQ DDTQGISIID IKRCSEMMLD   420               TFSFRITSTF NATYVTDFSM INANPVHLSP LDLSNQINSI NKSLKSAEDW IADSNFFANQ   480               ARTAKTLYSL SAIALILSVI TLVVVGLLIA YIIKLVSQIH QFRSLAATTM FHRENPAFFS   540               KNNHGNIYGI S   551                    SEQ ID NO: 89                MHHLHPMIVC IFVMYTGIVG SDAPAGDQLL NIGVIQSKIR SLMYYTDGGA SFIVVKLLPN    60                   LPPSNGTCNI TSLDAYNVTL FKLLTPLIEN LSKISTVTDT KTRQKRFAGV VVGLAALGVA   120               TAAQITAAVA IVEANANAAA INNLASSIQS TNKAVSDVID AVRTIATAVQ AIQDRINGAI   180               VNGITSASCR AHDALIGSIL NLYLTELTTI FHNQITNPAL TPLSIQALRI LLGSTLPIVI   240               ESKLNTNFNT AELLSSGLLT GQIISISPMY MQMLIQINVP TFIMQPGAKV IDLAAISANH   300               KLQEVVVQVP NRILEYANEL QNYPANDCVV TPNSVFCRYN EGSPIPESQY QCLRGNLNSC   360               TFTPIIGNFL KRFAFANGVL YANCKSLLCR CADPPHVVSQ DDTQGISIID IKRCSEMMLD   420               TFSFRITSTF NATYVTDFSM INANPVHLSP LDLSNQINSI NKSLKSAEDW IADSNFFANQ   480               ARTAKTLYSL SAIALILSVI TLVVVGLLIA YIIKLVSQIH QFRSLAATTM FHRENPAFFS   540               KNNHGNIYGI S   551                    SEQ ID NO: 90                MHHLHPMIVC IFVMYTGIVG SDAPAGDQLL NIGVIQSKIR SLMYYTDGGA SFIVVKLLPN    60                   LPPSNOTCNI TSLDAYNVTL FKLLTPLIEN ISKISTVTDT KTRQKRFAGV VVGLAALGVA   120               TAAQITAAVA IVEANANAAA INNLASSIQS VNKAVSDVID AVRTIATAVQ AIQDRINGAI   180               VNGITSASCR AHDALIGSIL NLYLTELTTI FHNQITNPAL TPLSIQALRI LIGSTLPIVI   240               ESKLNTNFNT AELLSSGLLT GQIISISPMY MQMLIQINVP TFIMQPGAKV IDLAAISANH   300               KLQEVVVQVP NRILEYANEL QNYPANDCVV TPNSVFCRYN EGSPIPESQY QCLRGNLNSC   360               TFTPIIGNFL KRFAFANGVL YANCKSLLCR CADPPHVVSQ DDTQGISIID IKRCSEMMLD   420               TFSFRITSTF NATYVTDFSM INANPVHLSP LDLSNQINSI NKSLKSAEDW IADSNFFANQ   480               ARTAKTLYSL SAIALILSVI TLVVVGLLIA YIIKLVSQIH QFRSLAATTM FHRENPAFFS   540               KNNHGNIYGI S   551            
Cleavage Site:
 
     In accordance with the present invention, a mutant protein of the invention may comprise a sequence which is derivable from that of the F protein of a PIV-5 or PIV-2 virus by:
         the mutations mentioned above; and further by   substitution of the native cleavage site of said F protein by another enzymatic cleavage site, and/or by insertion into said F protein of an enzymatic cleavage site other than the native cleavage site of this F protein, preferably by substitution of the native cleavage site of said F protein by another enzymatic cleavage site.       

     The cleavage site of a PIV-5 or PIV-2 F protein is the cleavage site of two subunits (F1 and F2) of this F protein. 
     In the native form of the F protein of PIV-5 and PIV-2, this cleavage site is a site cleaved by furines. 
     In the native form of the F protein of PIV-5, this cleavage site consists of the sequence RRRRR (SEQ ID NO: 23). It is in positions 98 to 102 of the native form of the F protein of PIV-5 (see  FIG. 1A ). An example of a fragment of a PIV-5 F protein sequence comprising the native (or natural) cleavage site of the F protein of PIV-5 is: 
     
       
         
               
               
             
           
               
                 IGENLETIRNQLIPT RRRRR FAGVVIGL. 
                 (SEQ ID NO: 24) 
               
             
          
         
       
     
     In the native form of the PIV-2 F protein, the cleavage site consists of the sequence KTRQKR (SEQ ID NO; 25). It is in positions 101 to 106 of the native form of the PIV-2 F protein (see  FIG. 1B ). An example of a fragment of a PIV-2 F protein sequence comprising the native (or natural) cleavage site of the F protein of PIV-2 is 
     
       
         
               
               
             
           
               
                   
                 (SEQ ID NO: 26) 
               
               
                   
                 LTPLIENLSKISTVTDT KTRQKR FAGVVVGLAALGVA. 
               
             
          
         
       
     
     Preferably, said cleavage site other than the native cleavage site is a tissue-specific cleavage site. 
     Preferably, said cleavage site other than the native cleavage site is a cleavage site for an enzyme specifically expressed by tumour tissue or tissues, highly preferably a cleavage site for an enzyme specifically expressed by metastatic tissue or tissues. 
     As an example, said cleavage site other than the native cleavage site may be a cleavage site for a metallo-protease, such as the cleavage site for matrix metallo-protease 9 (MMP-9); see Example 2 below. 
     A cleavage site for matrix metallo-protease 9 (MMP-9) may comprise or consist of the sequence PXXHy SIT (SEQ ID NO: 27) where X=any amino acid, and where Hy=any hydrophobic amino acid (i.e. any amino acid selected from F, M, V, L, I). 
     As an example, a cleavage site for matrix metallo-protease 9 (MMP-9) may comprise or consist of the sequence PRRIT (SEQ ID NO: 28) and/or the sequence 
                         IGENLETIRNQLIPT PRRIT FAGVVIGL.   (SEQ ID NO: 29)            
Nucleic Acids of the Invention:
 
     The present application also relates to a nucleic acid, DNA or RNA, which encodes a mutant protein in accordance with the invention (in accordance with the universal genetic code and allowing for degeneracy of that code), and to a complementary nucleic acid of such a nucleic acid (perfectly complementary nucleic acid of the same length). 
     Such nucleic acids derive from the sequence of SEQ ID NO: 30 (sequence encoding the native PIV-5 F protein), or of an alternative sequence coding for said alternative sequence “SEQ ID NO: 31 with S at 443”, or of a variant sequence encoding a variant F protein, or even of the sequence of SEQ ID NO: 32 (sequence encoding the native PIV-2 F protein) or of a variant sequence encoding a variant F protein. 
     Vectors of the Invention: 
     The present application also relates to a nucleic acid vector, more particularly to a transfection, transduction or transformation vector, comprising at least one nucleic acid in accordance with the invention. 
     Advantageously, such a vector may be an expression vector. 
     Preferably, it is a vector allowing expression of said at least one nucleic acid in an animal cell (non-human animal cell and/or human cell), more preferably:
         in a human cell, advantageously in a pathological human cell, more particularly a human tumour cell, more preferably a metastatic melanoma cell; or   in a placental cell.       

     Such an expression vector may advantageously be an adenoviral vector. 
     Said adenoviral vector may comprise elements for regulating the expression of said nucleic acid, preferably a promoter, allowing expression of said nucleic acid in tumour cells, preferably in metastatic cells, more preferably in metastatic melanoma cells. 
     Preferably, this expression is specific. Advantageously, this expression is sufficiently specific to allow the expression of said nucleic acid in said tumour or metastatic cells, without there being significant expression in non-tumour (or non-metastatic) cells. 
     Advantageously, such an adenoviral vector is an oncolytic adenoviral vector. 
     Alternatively, an expression vector of the invention may be an adenoviral vector which comprises elements for regulating the expression of said nucleic acid, preferably a promoter, allowing expression of said nucleic acid in placental cells, preferably in pathological placental cells which have insufficient fusogenicity. 
     Preferably, this expression is specific. Advantageously, this expression is sufficiently specific to allow the expression of said nucleic acid in said placental cells, without there being significant expression in non-placental cells. 
     A vector of the invention may alternatively or complementarily be a vector allowing the insertion of said at least one nucleic acid into the genome of an animal cell (non-human animal cell and/or human cell), more preferably a human cell, advantageously a pathological human cell, more particularly a human tumour cell, preferably a metastatic human cell, more preferably in metastatic melanoma cells. Such a vector is more particularly intended for the gene therapy of tumours, particularly metastatic tumours, more particularly metastatic melanomas. 
     The present application also relates to a vector which comprises at least one nucleic acid of the invention, and which allows the insertion of said at least one nucleic acid into the genome of an animal cell (non-human animal cell and/or human cell), more preferably a human cell, advantageously a placental cell, preferably a human placental cell. Such a vector is more particularly intended for the gene therapy of diseases or conditions involving deficient placental development. 
     Cells of the Invention: 
     The present application also relates to a cell which comprises at least one mutant protein in accordance with the invention, and/or at least one nucleic acid, DNA or RNA, in accordance with the invention, and/or at least one vector in accordance with the invention. 
     Such a cell may be a human cell or a non-human animal cell. 
     Preferably, such a cell is a tumour cell, preferably a metastatic cell, more preferably a metastatic melanoma cell. 
     Such a cell finds applications as a cell with a fusogenic capacity capable of inducing the formation of syncytia, as described below. 
     Alternatively, a cell of the invention may be a non-tumoral cell of the human or non-human animal immune system, preferably a non-tumoral human or non-human animal dendritic cell, said cell expressing at least one mutant protein in accordance with the invention at its surface. Such a cell finds applications as an agent capable of inducing the production of cell fusion inhibitor, for example by active immunisation, as described below. 
     Medical Applications (Pro-Fusion): 
     A mutant protein of the invention, and/or a nucleic acid, DNA or RNA, of the invention, and/or a vector of the invention, and/or a cell of the invention may be used in the treatment and/or prevention and/or mitigation of a disease or a condition which involves the presence and/or proliferation of cells which are pathological and/or not favourable to the health of the organism, more particularly in the treatment and/or prevention and/or mitigation of a disease or a condition which involves an insufficiency of cellular fusogenicity, preferably in the treatment and/or prevention and/or mitigation of a disease or a neoplasic condition, such as a tumour, a metastatic tumour, advantageously a metastatic melanoma. 
     Such diseases or conditions may be treated and/or prevented and/or mitigated by reduction or removal of these pathological and/or non-favourable cells. 
     A mutant protein of the invention expressed at the surface of such cells will induce fusion of these cells, and consequently the formation of syncytia, leading to the destruction (or at least to a reduction in number) of these cells. 
     A mutant protein of the invention, and/or a nucleic acid, DNA or RNA, of the invention, and/or a vector of the invention, and/or a cell of the invention, may be used in the treatment and/or prevention and/or mitigation of a disease or a condition which involves a deficiency in placental development. 
     Such diseases or conditions may be treated and/or prevented and/or mitigated by induction or stimulation of placental cell fusion. 
     The present application thus also relates to a pharmaceutical composition or a drug which comprises at least one mutant protein of the invention and/or at least one nucleic acid, DNA, RNA, of the invention, and/or at least one vector of the invention and/or a cell of the invention. 
     Such a pharmaceutical composition or such a drug may in particular be intended for the treatment and/or prevention and/or mitigation of a disease or a condition which involves the presence and/or proliferation of cells which are pathological and/or not favourable to the health of the organism, as indicated above (as an example, tumour, metastatic tumour, metastatic melanoma), or to the treatment and/or prevention and/or mitigation of a disease or a condition which involves an insufficiency of cellular fusogenicity (as an example, deficiency of placental development). 
     Such a pharmaceutical composition or such a drug may further comprise at least one pharmaceutically and/or physiologically acceptable vehicle. 
     A mutant protein of the invention (or a nucleic acid, DNA, RNA, or an expression vector of the invention), may be employed to be expressed by a human cell or a non-human animal cell, preferably to be expressed at the surface of such a cell. 
     This cell may be a pathological cell, preferably a tumour cell, more preferably a metastatic cell, more preferably a metastatic melanoma cell, or it may be a non-tumoral cell, for example a healthy cell. 
     More particularly, pathological cells which have been removed from a human patient or a sick non-human animal subject, may be treated ex vivo (or in vitro) by contact with at least one mutant protein of the invention and/or at least one nucleic acid, DNA or RNA, of the invention and/or at least one expression vector of the invention so as to cause them to express a mutant protein of the invention. 
     Alternatively or complementarily, non-pathological cells which are however localized close to pathological cells of the patient or subject may be removed to undergo that treatment. 
     The cells thus treated ex vivo (or in vitro) may then be intended to be re-administered to said patient or subject. 
     Such cells are useful for the treatment and/or prevention and/or mitigation of the pathology with which said patient or subject is affected, for example a tumour, a metastatic tumour, a metastatic melanoma. 
     Alternatively, this cell may be a placental cell, more particularly a placental cell suffering from an insufficiency of fusogenicity. Once treated by expression of a mutant protein of the invention at its surface, such a cell may be intended for the treatment and/or prevention and/or mitigation of a deficiency of placental development. 
     The present application is thus more particularly relative to a mutant protein in accordance with the invention, a nucleic acid, DNA or RNA, in accordance with the invention, a vector in accordance with the invention, a cell in accordance with the invention, for use in the treatment and/or prevention and/or mitigation of a disease or a neoplasic condition, preferably a tumour, more preferably a metastatic tumour, highly preferably a metastatic melanoma. 
     The present application is also more particularly relative to a mutant protein in accordance with the invention, a nucleic acid, DNA or RNA, in accordance with the invention, a vector in accordance with the invention, a cell in accordance with the invention, for use in the treatment and/or prevention and/or mitigation of a deficiency of placental development. 
     Fusion Inhibitors in Accordance with the Invention: 
     The present application also relates to products which have the capacity to reduce or block cell fusion. These products are inhibitors of one or more mutant proteins of the invention. 
     Such inhibitors may be used in the treatment and/or prevention and/or mitigation of a disease or a condition which involves an excess of cellular fusogenicity, preferably in the treatment and/or prevention and/or mitigation of an enveloped virus infection (such as an HIV, influenza, parainfluenza or rhabdovirus infection), an allergy, an auto-immune disease or a graft rejection. 
     Preferably, an inhibitor in accordance with the invention is:
         an antibody directed against a mutant protein in accordance with the invention, or a Fab or F(ab′)2 fragment of such an antibody; or   a nucleic acid aptamer or a peptide aptamer which binds specifically to at least one mutant protein in accordance with the invention, or to a nucleic acid, DNA, RNA, in accordance with the invention; or   a recombinant immune system cell, preferably a recombinant dendritic cell, which expresses at least one mutant protein in accordance with the invention at its surface; or   an antisense nucleic acid of a nucleic acid in accordance with the invention; or   a small interfering RNA, siRNA, comprising a double strand RNA containing 19 to 22 nucleotides, capable of binding (hybridizing) to a nucleic acid in accordance with the invention.       

     The present application thus also pertains to a non-tumoral cell of the human or non-human animal immune system, preferably a human or non-human animal dendritic cell, said cell expressing at least one mutant protein in accordance with the invention at its surface, as well as to the use of this cell in the treatment and/or prevention and/or mitigation of a disease or a condition which involves an excess of cellular fusogenicity, preferably in the treatment and/or prevention and/or mitigation of an enveloped virus infection (such as a HIV, influenza, parainfluenza or rhabdovirus infection), an allergy, an auto-immune disease or a graft rejection. 
     The present application also relates to an antibody directed against a mutant protein in accordance with the invention, or against several mutant protein or proteins of the invention. Preferably, this antibody is a specific antibody for said mutant proteins of the invention. Advantageously, this antibody is a monoclonal antibody. An inhibitor of the invention may be a conserved fragment of such an antibody, such as a Fab or F(ab′)2 fragment. 
     Such an antibody or antibody fragment may be intended to block or inhibit a cell fusion mechanism, for example by administration of said antibody or antibody fragment to a patient or subject in need thereof. 
     Alternatively or complementarily, a mutant protein of the invention may itself be intended to be administered to said patient or subject so as to induce active immunisation against this protein, i.e. so as to induce the production by said patient or subject of anti-mutant protein antibody. If necessary or desired, one or more vaccine adjuvants may be administered jointly with or at a different time to said mutant protein or proteins. 
     The present application thus also pertains to a therapeutic and/or preventative and/or mitigating vaccine, which comprises at least one mutant protein of the invention as an immunogenic agent, and advantageously at least one immunisation adjuvant. Such a vaccine may be intended for the treatment and/or prevention and/or mitigation of a disease or a condition which involves an excess of cellular fusogenicity, preferably in the treatment and/or prevention and/or mitigation of an enveloped virus infection (such as a HIV, influenza, parainfluenza or rhabdovirus infection), an allergy, an auto-immune disease or a graft rejection. 
     The present application also relates to:
         a nucleic acid aptamer or a peptide aptamer which binds specifically to at least one mutant protein of the invention, or to a nucleic acid, DNA, RNA, of the invention;   a recombinant immune system cell, preferably a recombinant dendritic cell, which expresses at least one mutant protein of the invention at its surface;   an antisense nucleic acid of a nucleic acid of the invention;   a small interfering RNA, siRNA, comprising a double strand RNA containing 19 to 22 nucleotides, capable of binding (hybridizing) to a nucleic acid of the invention, and advantageously of blocking or inhibiting transcription of said nucleic acid.       

     Such products are also inhibitors of the invention. They may thus be intended for the treatment and/or prevention and/or mitigation of a disease or a condition which involves an excess of cellular fusogenicity, as indicated above. More particularly, they are intended for the treatment and/or prevention and/or mitigation of a disease or a condition which involves at least one gene for expression or hyper-expression of the F protein. 
     The present application thus also pertains to a pharmaceutical composition or a drug which comprises at least one inhibitor of the invention. 
     Such a pharmaceutical composition or such a drug may in particular be intended for the treatment and/or prevention and/or mitigation of a disease or a condition which involves an excess of cellular fusogenicity, as indicated above. 
     Such a pharmaceutical composition or such a drug may further comprise at least one pharmaceutically and/or physiologically acceptable vehicle. 
     The present application more particularly pertains to an inhibitor in accordance with the invention, for use in the treatment and/or prevention and/or mitigation of a disease or a condition involving an excess of cellular fusogenicity, said disease or condition being an enveloped virus infection (preferably a HIV and/or influenza and/or parainfluenza and/or rhabdovirus infection), an allergy, an auto-immune disease or a graft rejection. 
     The present application more particularly pertains to a mutant protein in accordance with the invention, for use as an immunogenic agent in the treatment and/or prevention and/or mitigation of a disease or a condition involving an excess of cellular fusogenicity, said disease or condition being an enveloped virus infection (preferably a HIV and/or influenza and/or parainfluenza and/or rhabdovirus infection), an allergy, an auto-immune disease or a graft rejection. 
     The present application more particularly pertains to a vaccine or vaccine composition, more particularly a vaccine or vaccine composition intended for the treatment and/or prevention and/or mitigation of a disease or a condition involving an excess of cellular fusogenicity, said disease or condition being an enveloped virus infection (preferably a HIV and/or influenza and/or parainfluenza and/or rhabdovirus infection), an allergy, an auto-immune disease or a graft rejection. Such a vaccine or vaccine composition comprises at least the mutant protein in accordance with the invention, and optionally at least one physiologically acceptable adjuvant. 
     Diagnostic and Prognostic Applications: 
     The present application also relates to a method, more particularly an in vitro method, for the diagnosis or prognosis of a disease or a condition involving:
         insufficient formation of syncytia, such as a tumour, a metastatic tumour, a metastatic melanoma or a deficiency in placental development; or in contrast   an excessive formation of syncytia, such as an enveloped virus infection (preferably a HIV and/or influenza and/or parainfluenza and/or rhabdovirus infection), an allergy, an auto-immune disease or a graft rejection.       

     The diagnostic or prognostic method of the invention comprises detection of at least one mutant protein in accordance with the invention or at least one nucleic acid in accordance with the invention, for example in a biological sample such as a biological sample which has been taken from the patient or subject undergoing said diagnosis or prognosis. 
     This detection may, for example, be carried out by sequencing proteins or nucleic acids contained in said sample. 
     This detection may, for example, be carried out by detection of said at least one mutant protein of the invention using an antibody, a peptide aptamer or an oligonucleotide aptamer binding to said at least one mutant protein, more particularly using an antibody, peptide aptamer or oligonucleotide aptamer of the invention. 
     This detection may, for example, be carried out by detection of said at least one nucleic acid of the invention using a nucleic acid, a peptide aptamer or an oligonucleotide aptamer binding to said at least one nucleic acid, more particularly using a nucleic acid complementary to a nucleic acid of the invention, a peptide aptamer or an oligonucleotide aptamer of the invention. 
     The present application also relates to said antibody, peptide aptamer, oligonucleotide aptamer, complementary nucleic acid, for their use in a method for the diagnosis or prognosis of insufficient formation, or in contrast excessive formation, of syncytia. 
     Biotechnological Applications (Screening): 
     The present application also relates to a method, more particularly an in vitro method, for screening a compound capable of reducing or blocking the formation of syncytia. The method of the invention comprises bringing a candidate compound into contact with cells expressing at least one mutant protein of the invention, so as to determine whether said candidate compound reduces or blocks fusion of said cells (for example by comparing the degree of fusion achieved in the presence of said candidate compound with that achieved in its absence). 
     Such compounds are candidate active principles for the treatment and/or prevention and/or mitigation of a disease or a condition involving an excess of cellular fusogenicity, such as enveloped virus infections, allergies, auto-immune diseases or graft rejections. 
     Biotechnological Applications (Myeloma, Hybridoma): 
     The present application also relates to a tumour cell, more particularly myeloma, comprising at least one mutant protein in accordance with the invention, preferably comprising at least one such mutant protein on its surface, and/or comprising at least one nucleic acid in accordance with the invention, and/or comprising at least one vector in accordance with the invention, more particularly an expression vector in accordance with the invention. 
     Such a tumour cell, more particularly such a myeloma, may in particular be used in the production of a hybridoma (by fusion of this tumour cell with a B lymphocyte), more particularly in the production of an antibody-producing hybridoma. 
     The present application also relates to a hybridoma, more particularly an antibody-producing hybridoma, which comprises at least one mutant protein in accordance with the invention, and/or at least one nucleic acid in accordance with the invention, and/or at least one vector in accordance with the invention. Such a hybridoma may in particular be produced by bringing at least one B lymphocyte into contact with at least one tumour cell, more particularly myeloma, comprising at least one mutant protein in accordance with the invention, preferably comprising at least one such mutant protein on its surface, and/or comprising at least one nucleic acid in accordance with the invention, and/or comprising at least one vector in accordance with the invention. Such a tumour cell has an intrinsic fusogenic capacity: it is thus capable of fusing with said at least one B lymphocyte, without employing polyethylene glycol (PEG) or electroporation means or any other means which, in the prior art, are conventionally used to induce fusion of a tumour cell to a B lymphocyte with the aim of producing a hybridoma. 
     Biotechnological Applications (Stem or Progenitor Cells): 
     The present application also relates to a stem or progenitor cell comprising at least one mutant protein in accordance with the invention, preferably comprising at least one such mutant protein on its surface, and/or comprising at least one nucleic acid in accordance with the invention, and/or comprising at least one vector in accordance with the invention, more particularly an expression vector in accordance with the invention. 
     Such a stem or progenitor cell has an intrinsic fusogenic capacity: it is thus capable of forming syncytia by fusion. 
     If this stem or progenitor cell also has a capacity for differentiation into muscle cell, it is then capable of forming a muscle fibre (by cell fusion and formation of a syncytium). 
     The present application thus also pertains to such a stem or progenitor cell for its use in the production, for example in the in vitro production, of a muscle fibre. 
     This production may, for example, be carried out by placing a plurality of said stem or progenitor cells in mutual contact on or in a culture medium allowing the proliferation of stem cells, or if appropriate progenitor cells, such that the fusogenic capacity of said stem or progenitor cells can be exercised, thereby inducing the formation of a syncytium, more particularly a muscle fibre. Examples of culture media allowing the proliferation of stem cells, or if appropriate progenitor cells, and which are also appropriate to the expression of their possible capacity to differentiate into muscle cell, more particular muscle fibre, are known to the skilled person; an example is MCDB medium. 
     Examples of cell markers that allow the differentiation of a stem or progenitor cell into muscle cell, more particularly into muscle fibre, to be observed are also known to the skilled person, for example CD56. 
     In the present application, the term “comprising”, which is synonymous with “including” or “containing”, is an open term that does not exclude the presence of one or more additional elements, ingredients or steps which will not be explicitly indicated, while the term “consisting” or “constituted” is a closed term which excludes the presence of any other additional element, step or ingredient which is not explicitly disclosed. The term “essentially consisting of” or “essentially constituted by” is a partially open term which does not exclude the presence done or more additional elements, ingredients or steps, provided that those additional elements, ingredients or steps do not materially affect the properties at the basis of the invention. 
     As a consequence, the term “comprising” (or “comprise(s)”) includes the terms “consisting of”, “constituted by” as well as terms “essentially consisting of” and “essentially constituted by”. 
     The contents of the documents and the bibliographic references which are cited in the present application are incorporated by reference. 
     The following examples are given purely by way of illustration and do not in any way limit the invention. 
     EXAMPLES 
     Example 1 
     Construction of Mutants and Measurement of their Fusogenicity 
     Methods and Apparatus: 
     Cells and Viruses 
     The cell line LLC-MK2 (Macaca mulatta kidney cell line) is available from the American Type Culture Collection (ATCC) with accession number CCL-7. 
     The cell line A549 (human pulmonary carcinoma cell line) is available from the ATCC with accession number CCL-185. 
     The recombinant line HuH7-Tat (human hepatoma cell line) is available by transduction of cells of the HuH-7 line by HIV-1 Tat. 
     The HuH-7 line is available from the Japanese Collection of Research Bioresources, reference number JCRB0403. 
     Transduction of the HuH-7 line by HIV-Tat was carried out with the aid of the retroviral vector LXSN-tat transducing the Tat plasmid. 
     The cells LLC-MK2, A549 and HuH7-Tat were cultivated in EMEM (Eagle&#39;s Minimum Essential Medium) or DMEM (Dulbecco/Vogt Modified Eagles&#39; Essential Minimal Medium) with 5% foetal calf serum. 
     The PIV-5 WR strain was obtained from the ATCC (number ATCC VR-288), and was cultivated on LLC-MK2 cells as described by Terrier et al, 2008. 
     Extraction of RNA, RT-PCR and Cloning 
     Viral RNA was extracted from the supernatant obtained from infection of LLC-MK2 cells by PIV-5, with the aid of the Absolutely RNA® Microprep Kit (Stratagene, USA), following the instructions provided by the supplier. The reverse transcription was carried out with the aid of pd(N)6 random hexamers (Amersham Biosciences, GB) and a reverse transcriptase (Reverse Transcriptase; RT) of the avian myeloblastosis virus, AMV (AMV-RT reverse transcriptase available from Promega). 
     Amplification of the complete sequence for PIV-5 F was carried out with a primer pair designed from the nucleotide sequence for PIV-5 available from the databases (GenBank accession number AB021962). 
     The primer pair employed was as follows: 
     
       
         
               
               
             
           
               
                   
                 Sense primer (SEQ ID NO: 1): 
               
               
                   
                 5′ TTGCGGCCGCATGGGTACTATAA 3′ 
               
               
                   
                   
               
               
                   
                 Antisense primer (SEQ ID NO: 2): 
               
               
                   
                 5′ CCGCTCGAGTTATGATAAACAAAATTCTCC 3′ 
               
             
          
         
       
     
     Amplification was carried out in accordance with the following protocol: 95° C. for 2 min, then 39 cycles (95° C./30 s, 55° C./1 min, 72° C./3 min) and a final elongation of 10 min at 72° C. 
     The complementary DNA of PIV-5 F was cloned into the expression plasmid pcDNA3.1(+) at the NotI and XhoI sites at the multiple cloning site (see  FIG. 4 ). 
     The PCR products and the plasmids were respectively purified using the Nueleospin® ExtractII and Nucleospin® plasmid kits (Macherey Nagel, Germany), following the instructions provided by the supplier. 
     The sequencing series in this study was executed by MWG Biotech (Ebersberg, Germany). 
     Directed Mutagenesis Using the Polymerase Chain Reaction (PCR) 
     The mutant proteins of the F protein of PIV-5 were produced by directed mutation in the plasmid pcDNA3.1 encoding the PIV-5 F fusion protein. The mutation(s) were generated by PCR using complementary primers, following the protocol provided by the supplier (QuickChange® Site-Directed Mutagenesis System available from Stratagene). The list of primers used is given in Table 2 below. Assembly of the plasmids was checked by sequencing. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 list of primers 
               
             
          
           
               
                   
                 Region 
                   
                   
                   
               
               
                   
                 targeted 
                   
                   
                 SEQ 
               
               
                   
                 in 
                   
                   
                 ID 
               
             
          
           
               
                 Mutation 
                 PIV-5 
                 Sequences for primers used for mutagenesis by PCR 
                 NO: 
               
               
                   
               
             
          
           
               
                 L22P 
                 F2 
                 Sense 
                 5′ GGAGCAGGCAGCCTTGATC   C   AGC   T   GCTCTCATGCAAATCGG 3′ 
                  3 
               
               
                   
                   
                 Antisense 
                 5′ CCGATTTGCATGAGAGC   A   GCT   G   GATCAAGGCTGCCTGCTCC 3′ 
                  4 
               
               
                   
               
               
                 K132E 
                 HR1 
                 — 
                 Pre-existing mutation 
                 — 
               
               
                   
               
               
                 V290A 
                 HR3 
                 — 
                 Pre-existing mutation 
                 — 
               
               
                   
               
               
                 I49A 
                 F2 
                 Sense 
                 5′ GGCCTCATCAGCATTC   GC   TGTTGTGAAGTTAATGCC 3′ 
                  5 
               
               
                   
                   
                 Antisense 
                 5′ GGCATTAACTTCACAACA   GC   GAATGCTGATGAGGCC 3′ 
                  6 
               
               
                   
               
               
                 V402A 
                 between 
                 Sense 
                 5′ CAGCCAAGTTCATCTCCTG   C   AACTGTCATTGACATGTAC 3′ 
                  7 
               
               
                   
                 HR3 and 
                 Antisense 
                 5′ GTACATGTCAATGACAGTT   G   CAGGAGAGTGAACTTGGCTG 3′ 
                  8 
               
               
                   
                 HR2 
                   
                   
                   
               
               
                   
               
               
                 S443P 
                 upstream 
                 Sense 
                 5′ GCTTGAATCATCTCAGATCTTG   T   CCATTGATCCGTTGGATATATCCC 3′ 
                  9 
               
               
                   
                 of HR2 
                 Antisense 
                 5′ GGGATATATCCAACGGATCAATGG   A   CAAGATCTGAGATGATTCAAGC 3′ 
                 10 
               
               
                   
               
               
                 L447p 
                 upstream 
                 Sense 
                 5′ CTCAGATCTTGTCCATTGATCCG   CC   GGATATATCCCAGAATCTAGCTGCG 3′ 
                 11 
               
               
                   
                 of HR2 
                 Antisense 
                 5′ CGCAGCTAGATTCTGGGATATATCC   GG   CGGATCAATGGACAAGATCTGAG 3′ 
                 12 
               
               
                   
               
               
                 I449P 
                 upstream 
                 Sense 
                 5′ CTTGTCCATTGATCCGTTGGAT   CC   ATCCCAGAATCTAGCTGCGGTG 3′ 
                 13 
               
               
                   
                 of HR2 
                 Antisense 
                 5′ CACCGCAGCTAGATTCGTT   GG   ATTTCTCCCAGAATCTAGCTGCGG 3′ 
                 14 
               
               
                   
               
               
                 I449F 
                 upstream 
                 Sense 
                 5′ GCCCATTGATCCGTTGGAT   T   T   C   TCCCAGAATCTAGCTGCGG 3′ 
                 15 
               
               
                   
                 of HR2 
                 Antisense 
                 5′ CCGCAGCTAGATTCTGGGA   G   A   A   ATCCAACGGATCAATGGGC 3′ 
                 16 
               
               
                   
               
               
                 T147V 
                 HR1 
                 Sense 
                 5′ CTCAAAAATGCAATCCAAAAA   GT   AAATGCAGCAGTTGCAGATG 3′ 
                 17 
               
               
                   
                   
                 Antisense 
                 5′ CATCTGCAACTGCTGCATTT   AC   TTTTTGGATTGCATTTTTGAG 3′ 
                 18 
               
               
                   
               
               
                 T158V 
                 HR1 
                 Sense 
                 5′ GCAGATGTGGTCCAGGCC   GT   ACAATCACTAGGAACGGC 3′ 
                 19 
               
               
                   
                   
                 Antisense 
                 5′ GCCGTTCCTAGTGATTGT   AC   GGCCTGGACCACATCTGC 3′ 
                 20 
               
               
                   
               
               
                 A463V 
                 HR2 
                 Sense 
                 5′ GTGAATAAGAGTCTAAGTGATG   T   ACTACAACACTTAGCACAAAGTG 3′ 
                 21 
               
               
                   
                   
                 Antisense 
                 5′ CACTTTGTGCTAAGTGTTGTAGT   A   CATCACTTAGACTCTTATTCAC 3′ 
                 22 
               
               
                   
               
             
          
         
       
     
     The mutation 443P is theoretically pre-existent in the F protein of the WR isolate. However, in the sample of this isolate that the inventors received from the ATCC, this mutation was in fact not present. It thus had to be introduced by the inventors. 
     Transfection of Cells 
     The cells were transfected by the plasmids with the aid of the reagent ExGen500 (Fermentas), following the instructions provided by the supplier. One to three micrograms of plasmidic DNA was added to the cells (at 70 to 80% confluence) for 48 h. The efficiency of transfection was estimated using a plasmid encoding the green fluorescence protein, GFP. 
     Immunofluorescence by Confocal Microscopy 
     The transfected cells were fixed using paraformaldehyde (1% v/v) in phosphate buffer saline, PBS, then washed twice. The cell mats were incubated in the presence of a monoclonal antibody directed against the PIV-5 F protein, in this case the monoclonal antibody F1a described by Randall et al, 1987, diluted to 1/10 in PBS for 3 h. The monoclonal antibody F1a had been obtained by immunisation of mice against an isolate of PIV-5 (in this case the LN isolate), preparation of hybridomas and selection of specific anti-F antibodies. 
     The cell mats were then washed and incubated with a secondary anti-mouse IgG-Alexa Fluor® 633 antibody (Invitrogen) diluted to 1/200 in PBS for 30 minutes. After rinsing, the cells were incubated for 10 minutes with Dapi (4′,6′-diamidino-2-phenyl indole) at 1/1000 mixed or not mixed with wheatgerm agglutinin (WGA) coupled to Alexa Fluor® 488 (WGA-Alexa Fluor® available from Invitrogen) at 1/200 in phosphate buffer saline, PBS. The images were acquired using a TCS SP2 confocal microscope (Leica). 
     Flow Cytometry 
     Flow cytometry was carried out as described in the literature (Horvat and Lamb 1992). A549 cells were transfected by the plasmids encoding the various Fus and were deposited onto ice. The cell mats were rinsed with phosphate buffer saline, PBS, comprising 1% of sodium azide. A monoclonal anti-F protein of PIV-5 antibody (in this case the monoclonal antibody Fla) was then added to the mat (1/500 PBS phosphate buffer with 1% foetal calf serum), and incubated for 30 minutes at 4° C. The mats were then rinsed and incubated in the presence of a secondary anti-mouse antibody coupled to Alexa Fluor® 488 at 1/1000 (Invitrogen). After rinsing, the cells were gently detached using 500 μL of PBS phosphate buffer, 0.5 mM in EDTA (ethylene-diamine-tetraacetic acid). The cells were transferred into dedicated flow cytometry tubes containing 500 μL of a 1% paraformaldehyde solution. The intensity of fluorescence of 5000 cells was measured using fluorescence-activated cell sorting, FACS, in this case using the FACSVantage™ SE flow from Becton Dickinson. 
     Semi-Quantitative Fusion Test (Fusion Scores Established as a Function of Syncytium Size and Number of Nuclei) 
     Mats transfected by the various Fus expression plasmids and observed in immunofluorescence allowed a semi-quantitative analysis to be carried out. This analysis consisted of determining a fusion score for each of the mutants using the following criteria:
         fusion or otherwise (simples agglomerates), denoted −/+;   the size of the syncytium, on a scale of 1 to 5;   the number of nuclei, on a scale of 1 to 5.       

     The score calculated thereby was obtained by adding two marks: the maximum theoretical mark was thus 10 and corresponded to a maximum size of syncytium with a maximum number of nuclei. 
     Quantitative Fusion Test (Measurement of Luciferase Activity) 
     In order to quantify cell-cell fusion, “donor” A549 cells (2.5 million cells per well of a E-well plate) were co-transfected with 2 μg of plasmid pcDNA3.1 encoding the various Fus mutant proteins as well as 50 ng of a plasmid expressing luciferase under the dependency of the long terminal repeat, LTR (Lavillette et al, 2007). 
     The negative control was provided by cells co-transfected with 2 μg of the empty plasmid pcDNA3.1. 
     Twelve hours post-transfection, the “donor” cells were detached using phosphate buffer (PBS), 0.5 mM in EDTA, and were counted then replaced in fresh 6-well plates (10 5  cells/well). “Indicator” HuH7-Tat cells (4×10 5  cells/well) were detached using PBS-EDTA buffer, then rinsed and added to the “donor” cells. 
     The luciferase activity was measured after 72 h of co-culture using a luciferase activity measuring kit, in this case the Luciferase Assay System (E1500) kit from Promega, following the indications provided by the supplier. 
     Results: 
     Mutant proteins constructed and produced by the inventors are shown in Table 3 above. 
     In this Table 3, the inventors have organized the various mutations as a function of the function attributed to them, namely:
         involvement in the function of autonomy as regards HN: positions 22, 132 and 290 of the F protein of PIV-5;   involvement in the function of pre-fusion: positions 49, 402, 443, 447 and 449 of the F protein of PIV-5;   involvement in the function of post-fusion: positions 147, 158 and 463 of the F protein of PIV-5.       

       FIGS. 5  A,  5 B,  5 C and  5 D illustrate the positions of the mutations of Table 3. 
     Mutant proteins have thus been constructed, produced and tested by the inventors. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 list of proteins produced 
               
             
          
           
               
                   
                 Autonomy 
                 Pre-fusion 
                 Post-fusion 
               
             
          
           
               
                   
                 L22P 
                 K132E 
                 V290A 
                 I49A 
                 V402A 
                 S443P 
                 L447P 
                 I449P 
                 I449F 
                 T147V 
                 T158V 
                 A463V 
               
               
                   
               
               
                 Fus1 
                   
                 + 
                 + 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (SEQ ID 
               
               
                 NO: 47) 
               
               
                 Fus1.1 
                   
                 + 
                 + 
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 48) 
               
               
                 Fus 1.2 
                   
                 + 
                 + 
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 49) 
               
               
                 Fus2 
                   
                 + 
                 + 
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 50) 
               
               
                 Fus3 
                 + 
                 + 
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 51) 
               
               
                 Fus3.1 
                 + 
                 + 
                 + 
                   
                   
                   
                   
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 52) 
               
               
                 Fus3.2 
                 + 
                 + 
                 + 
                   
                   
                   
                   
                   
                   
                 + 
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 53) 
               
               
                 Fus 3.3 
                 + 
                 + 
                 + 
                   
                   
                   
                   
                   
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 54) 
               
               
                 Fus4 
                 + 
                 + 
                 + 
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 55) 
               
               
                 Fus5 
                 + 
                 + 
                 + 
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 56) 
               
               
                 Fus6 
                 + 
                 + 
                 + 
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 57) 
               
               
                 Fus 6.1 
                 + 
                 + 
                 + 
                   
                   
                 + 
                   
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 58) 
               
               
                 Fus 6.2 
                 + 
                 + 
                 + 
                   
                   
                 + 
                   
                   
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 59) 
               
               
                 Fus 6.3 
                 + 
                 + 
                 + 
                   
                   
                 + 
                   
                   
                   
                 + 
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 60) 
               
               
                 Fus7 
                 + 
                 + 
                 + 
                   
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 61) 
               
               
                 Fus 7.1 
                 + 
                 + 
                 + 
                   
                   
                   
                 + 
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 62) 
               
               
                 Fus 7.2 
                 + 
                 + 
                 + 
                   
                   
                   
                 + 
                   
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 63) 
               
               
                 Fus 7.3 
                 + 
                 + 
                 + 
                   
                   
                   
                 + 
                   
                   
                 + 
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 64) 
               
               
                 Fus8 
                 + 
                 + 
                 + 
                   
                   
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 65) 
               
               
                 Fus9 
                 + 
                 + 
                 + 
                   
                   
                   
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 66) 
               
               
                 Fus10 
                 + 
                 + 
                 + 
                 + 
                   
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 67) 
               
               
                 Fus 10.4 
                 + 
                 + 
                 + 
                   
                 + 
                   
                   
                 + 
                   
                 + 
                 + 
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 68) 
               
               
                 Fus 10.5 
                 + 
                 + 
                 + 
                 + 
                 + 
                   
                   
                 + 
                   
                 + 
                 + 
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 69) 
               
               
                 Fus11 
                 + 
                 + 
                 + 
                   
                 + 
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 70) 
               
               
                 Fus8.1 
                 + 
                 + 
                 + 
                   
                   
                   
                   
                 + 
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 71) 
               
               
                 Fu8.2 
                 + 
                 + 
                 + 
                   
                   
                   
                   
                 + 
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 72) 
               
               
                 Fus8.4 
                 + 
                 + 
                 + 
                   
                   
                   
                   
                 + 
                   
                 + 
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 73) 
               
               
                 Fus8.5 
                 + 
                 + 
                 + 
                   
                   
                   
                   
                 + 
                   
                 + 
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 74) 
               
               
                 Fus8.6 
                 + 
                 + 
                 + 
                   
                   
                   
                   
                 + 
                   
                   
                 + 
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 75) 
               
               
                 Fus8.7 
                 + 
                 + 
                 + 
                   
                   
                   
                   
                 + 
                   
                 + 
                 + 
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 76) 
               
               
                 Fus10.1 
                 + 
                 + 
                 + 
                 + 
                   
                   
                   
                 + 
                   
                   
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 77) 
               
               
                 Fus10.2 
                 + 
                 + 
                 + 
                 + 
                   
                   
                   
                 + 
                   
                 + 
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 78) 
               
               
                 Fus10.3 
                 + 
                 + 
                 + 
                 + 
                   
                   
                   
                 + 
                   
                 + 
                 + 
                 + 
               
               
                 (SEQ ID 
               
               
                 NO: 79) 
               
               
                   
               
             
          
         
       
     
     The F protein sequence for PIV-5 which was used during the construction and production of these mutant proteins was an alternative F protein sequence for the WR isolate. This alternative sequence was identical to the sequence of SEQ ID NO: 31 (Genbank sequence), with the exception of the amino acid in position 443 which was S and not P (alternative sequence “SEQ ID NO: 31 with S at 443”). 
     The sequences for SEQ ID NO: 47 to 79 were thus the sequences which result from replacement within said alternative sequence “SEQ ID NO: 31 with S at 443”, of the amino acids indicated for each of these sequences in Table 3 above. 
     An illustration of the observations made under the microscope during the semi-quantitative fusion tests is presented in  FIG. 6A . 
     The scores obtained at the end of the semi-quantitative fusion tests are presented in  FIG. 6B . 
     The mutant proteins Fus6, Fus 6.1, Fus 6.2 and Fus 6.3 resulted in agglutination of many cells, but not in cell fusion. 
     The mutant proteins Fus 3.3, Fus 3.1, Fus2, Fus 1.1, Fus 1.2 and Fus 1 produced a zero fusion score. 
     The mutant proteins Fus 9, Fus7, Fus 3, Fus 5 and Fus 4 produced low fusion scores. 
     Beyond the fusion score of Fus 4, a series of mutant proteins with a significant fusion score separated out, namely:
         the group of mutant proteins which in common comprise the three mutations for autonomy and the mutation for pre-fusion 449P, such as the mutant proteins Fus8, Fus10, Fus10.4, Fus10.5, Fus11, Fus8.1, Fus8.2, Fus8.4, Fus8.5, Fus8.6, Fus8.7, Fus10.1, Fus10.2, Fus10.3; and   the group of mutant proteins which in common comprise the three mutations for autonomy, the mutation for pre-fusion 447P and at least one post-fusion mutation (147V or 158V), such as Fus7.1, Fus7.2 or Fus7.3.       

       FIGS. 7A and 7B  present an illustration of the microscope observations and present the fusion scores for a selection of the tested mutant proteins, namely the group of mutant proteins which in common comprise:
         the three mutations for autonomy;   the mutation for pre-fusion 447P; and   at least one post-fusion mutation in position 147 and/or 158 to present at this/these positions a hydrophobic amino acid such as V, I or L, for example the post-fusion mutation 147V and/or the post-fusion mutation 158V; such as the mutant proteins Fus7.1, Fus7.2 and Fus7.3.       

     Example 2 
     Substitution of Natural Cleavage Site by the Site for an Enzyme Specifically Expressed by Metastatic Tumour Tissue 
     The mutant proteins of the invention, and more particularly those described in Example 1 above, had previously been modified by substitution of the natural cleavage site of the native F protein, for example to replace it with a tissue-specific cleavage site. 
     By way of illustration, the natural cleavage site of the F protein of PIV-5 has been substituted by the site for an enzyme specifically expressed by metastatic tumour tissue, namely matrix metallo-protease 9 (MMP-9). 
     The natural cleavage site of the F protein of PIV-5 is: 
     
       
         
               
               
               
             
           
               
                   
                   RRRRR . 
                 (SEQ ID NO: 23) 
               
             
          
         
       
     
     An example of a fragment of the F protein sequence comprising the natural cleavage site of the F protein of PIV-5 is: 
     
       
         
               
               
             
           
               
                 IGENLETIRNQLIPT RRRRR FAGVVIGL. 
                 (SEQ ID NO: 24) 
               
             
          
         
       
     
     The consensus sequence for a MMP-9 cleavage site is: 
                                   PXXHyS / T     (SEQ ID NO: 27)            
where X=any amino acid, and
 
where Hy=any hydrophobic amino acid (i.e. any amino acid selected from F, M, V, L, I).
 
     An example of a MMP-9 cleavage site is: 
     
       
         
               
               
               
             
           
               
                   
                   PRRIT . 
                 (SEQ ID NO: 28) 
               
             
          
         
       
     
     An example of a fragment of a mutant F protein sequence of the invention comprising a MMP-9 cleavage site is: 
                         IGENLETIRNQLIPT PRRIT FAGVVIGL.   (SEQ ID NO: 29)            
Methods and Apparatus:
 
     The mutant proteins of the F protein of PIV-5 were produced as described in Example 1 above. 
     Replacement of the natural cleavage site by the selected cleavage site, in this case the MMP-9 cleavage site of SEQ ID NO: 28, was carried out as follows: 
     Replacement of the cleavage site was carried out by 3 successive directed mutageneses in the plasmid pcDNA3.1 encoding the fusion protein F PIV-5. The mutations were generated by PCR using complementary primers, following the protocol indicated by the supplier (QuickChange® Site-Directed Mutagenesis System available from Stratagene). Assembly of the plasmids was checked by sequencing. 
                     1 st  MUTATION R98P       SENSE       (SEQ ID NO: 80)       5′ CCAGTTGATTCCAACTCCGAGGAGACGCCGGTTTGC 3′               ANTISENSE       (SEQ ID NO: 81)       5′ GCAAACCGGCGTCTCCTCGGAGTTGGAATCAACTGG 3′               2 nd  MUTATION R101I       SENSE       (SEQ ID NO: 82)       5′ GATTCCAACTCCGAGGAGAATCCGGTTTGCAGGAGTGGTG 3′               ANTISENSE       (SEQ ID NO: 83)       5′ CACCACTCCTGCAAACCGGATTCTCCTCGGAGTTGGAATC 3′               3 rd  MUTATIQN R102T       SENSE       (SEQ ID NO: 84)       5′ GATTCCAACTCCGAGGAGAATCACGTTTGCAGGAGTGGTGATTGG 3′               ANTISENSE       (SEQ ID NO: 85)       5′ CCAATCACCACTCCTGCAAACGTGATTCTCCTCGGAGTTGGAATC 3′            
Transfection of Cells
 
     The cells were transfected by the plasmids with the aid of the reagent ExGen500 (Fermentas), following the instructions provided by the supplier. One to three micrograms of plasmidie DNA were added to the cells (70 to 80% confluence) for 48 h. The efficiency of transfection was estimated with the aid of a plasmid encoding green fluorescence protein, GFP. 
     Immunofluorescence by Confocal Microscopy 
     The transfected cells were fixed with the aid of paraformaldehyde (1% v/v) in phosphate buffer saline, PBS, then washed twice. The cell mats were incubated in the presence of a monoclonal antibody directed against the fusion protein NV-5 F, in this case the monoclonal antibody F1a described by Randall et al, 1987, diluted to 1/10 in PBS for 3 h. The monoclonal antibody F1a had been obtained by immunisation of mice against un isolate of PIV-5 (in this case the LN isolate), preparation of hybridomas and selection of specific anti-F antibodies. 
     The cell mats were then washed and incubated with a secondary anti-mouse IgG-Alexa Fluor® 633 antibody (Invitrogen) diluted to 1/200 in PBS for 30 minutes. After rinsing, the cells were incubated for 10 minutes with Dapi (4′, 6′-diamidino-2-phenyl indole) at 1/1000 mixed or not mixed with wheatgerm agglutinin, WGA, coupled to Alexa Fluor® 488 (WGA-Alexa Fluor® available from Invitrogen) at 1/200 in phosphate buffer, PBS. The images were acquired using a TCS SP2 confocal microscope (Leica). 
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