Earlier me detected a novel enzymatic activity in salivary gland secretion of the medicinal leech, splitting isopeptide bonds between the glutamine γ-carboxamide and lysine ε-amino group. This activity is due to destabilase. We described its partial amino acid sequence and sequences of two closely related cDNAs, but none of them perfectly matched the protein isolated. Here we report the isolation and sequence peculiarities of the third cDNA of the family as well as the complete sequence of the destabilase protein. The inferred mature protein product of this cDNA matches the independently determined destabilase protein sequence. It contains 115 amino acid residues including 14 highly conserved Cys residues and is formed from a precursor containing specific leader peptide.