Amino-terminal extension present in the methionine aminopeptidase type 1c of Mycobacterium tuberculosis is indispensible for its activity

摘要

Methionine aminopeptidase (MetAP) is a ubiquitous enzyme in both prokaryotes and eukaryotes, which catalyzes co-translational removal of N-terminal methionine from elongating polypeptide chains during protein synthesis. It specifically removes the terminal methionine in all organisms, if the penultimate residue is non-bulky and uncharged. The MetAP action for exclusion of N-terminal methionine is mandatory in 50-70% of nascent proteins. Such an activity is required for proper sub cellular localization, additional processing and eventually for the degradation of proteins. We cloned genes encoding two such metalloproteases (Mt MetAP1a and Mt MetAP1c) present in Mycobacterium tuberculosis and expressed them as histidine-tagged proteins in Escherichia coli. Although they have different substrate preferences, for Met-Ala-Ser, we found, Mt MetAP1c had significantly high enzyme turnover rate as opposed to Mt MetAP1a. Circular dichroism spectroscopic studies as well as monitoring of enzyme activity indicated high temperature stability (up to 50°C) of Mt MetAP1a compared to that of the Mt MetAP1c. Modelling of Mt MetAP1a based on Mt MetAP1c crystal structure revealed the distinct spatial arrangements of identical active site amino acid residues and their mutations affected the enzymatic activities of both the proteins. Strikingly, we observed that 40 amino acid long N-terminal extension of Mt MetAP1c, compared to its other family members, contributes towards the activity and stability of this enzyme, which has never been reported for any methionine aminopeptidase. Furthermore, mutational analysis revealed that Val-18 and Pro-19 of Mt MetAP1c are crucial for its enzymatic activity. Consistent with this observation, molecular dynamic simulation studies of wild-type and these variants strongly suggest their involvement in maintaining active site conformation of Mt MetAP1c. Our findings unequivocally emphasized that N-terminal extension of Mt MetAP1c contributes towards the functionality of the enzyme presumably by regulating active site residues through "action-at-a-distance" mechanism and we for the first time are reporting this unique function of the enzyme.