Identification and biochemical characterisation of a novel methionine aminopeptidase from the taiga tick Ixodes persulcatus.

Methionine aminopeptidases (MetAPs), which remove the initiator methionine from nascent peptides, are essential in all organisms and considered to be a valuable targets for the treatment of various diseases, including cancer, malaria, and bacterial infections. However, MetAPs have not been reported in hard ticks (family Ixodidae), and their bioinformatics characterisation in tick's genome sequences is limited. In this study, we cloned, identified, and characterised a novel MetAP from Ixodes persulcatus, a vector for pathogens causing Lyme borreliosis and tick-borne encephalitis. The sequence analysis showed that I. persulcatus MetAP was a type 1 enzyme carrying C-terminal motifs conserved in the M24A family of metallopeptidases. Protein-protein docking simulations using human MetAP revealed conservation of substrate and metal-binding residues in the catalytic site cleft of the novel enzyme, which was designated IpMetAP. Recombinant IpMetAP expressed in Escherichia coli revealed its significant enzymatic activity with the synthetic substrate H-Met-4-methyl-coumaryl-7-amide at pH 7.5 with K of 0.014 mM, k of 0.25 s, and overall catalytic efficiency (k/K) of 18.36 mM s. The activity of IpMetAP was enhanced by the addition of divalent cations Mn and Co and significantly inhibited by EDTA and bestatin. Site-directed mutagenesis of conserved amino acids indicated that the substitution of metal-binding residues D226 and H288 completely abolished the IpMetAP enzymatic activity, whereas that of the other sites had only moderate effects on substrate hydrolysis. The catalytic properties of IpMetAP suggest that the enzyme behaves similar to other MetAPs and such characterization expands our knowledge of aminopeptidases and protein metabolism of tick.