In vitro ubiquitination of Mycobacterium tuberculosis by E3 ubiquitin ligase, MKRN1.

OBJECTIVES: Ubiquitination has a role as a host defense mechanism against pathogens. To channelize autophagic mycobacteria to destruction, ubiquitin ligase like, Makorin Ring Finger Protein 1 (MKRN1) was speculated to play a role in ubiquitinating M. tuberculosis. We have developed a flow cytometry based in vitro ubiquitin ligase assay to understand the role of MKRN1 in ubiquitinating mycobacteria and confirmed the results by western blotting. RESULTS: MKRN1 was cloned and expressed in E. coli BL21 (DE3) strain. The recombinant MKRN1 protein was solubilised, purified and refolded to restore the activity. In addition, through autoubiquitination assay, the activity of protein was confirmed. The corresponding E1 and E2 enzymes for MKRN1, UBE1 and UBE2D3 respectively, were selected using BioGrid tool. Surprisingly, flow cytometric assay revealed that at a concentration of 300 nM of MKRN1, 38% of M. tuberculosis was found to be ubiquitinated in vitro with 3.5% of the cells having bound MKRN1. Immunoblot results also substantiates the ubiquitination of M. tuberculosis. MKRN1 did not ubiquitinate B. Subtilis and therefore, we speculate that the E3 Ub ligase activity might be specific to M. tuberculosis. CONCLUSION: This clearly demonstrates that recombinant MKRN1 ubiquitinates M. tuberculosis which opens up a novel, potential role of MKRN1 against mycobacteria which has to be unfolded.