An isolated protein having at least 90 % homology with the dimeric proteinhaving the following amino acid sequence (I) with the proviso that at the C-terminal end of the monomeric forms, 2 to 10 amino acid distance from Z2, aCys is present for forming the dimeric or multimeric protein, Z1 is a signalpeptide for protein transport to the ER and/or through the plasma membrane orNH2 or a derivative of an NH2 group such as an alkylated, or acylatedderivative, or polyethylene glycol (PEG) moiety, Z2 is an amino acid residuewith up to 20, in particular 10 or 8 amino acids which are selected ofsubstantially non-hydrophobic naturally occurring amino acids or COOZ3whereinZ3 is hydrogen, a metal ion, a hydrocarbon moiety substituted or non-substituted with heteroatoms.
