Protein engineering, production, reconstitution in lipid nanoparticles, and initial characterization of the Mycobacterium tuberculosis EfpA drug exporter

Mycobacterium tuberculosis (Mtb) drug exporters contribute an efficient mechanism for drug resistance. Therefore, understanding the structure/function relationship in these proteins is important. We focused on the Mtb EfpA efflux pump, which belongs to the major facilitator superfamily (MSF) and transports anti-tuberculosis drugs outside the bacterial cell. Here, we report on our advancements in producing and in vitro characterization of this protein. We engineered a construct of apolipoprotein AI (apoAI) fused to the N-terminus of EfpA (apoAI-EfpA) and cloned it in an E. coli expression vector. This fusion construct was found in a membrane-bound form, unlike the deposited in inclusion bodies EfpA without apoAI. We purified the apoAI-EfpA in detergent to a sufficient degree and reconstituted it in DOPC/DOPS lipids. We found that upon reconstitution in lipid, the apoAI-EfpA forms discoidal protein-lipid nanostructures with a diameter of about 20 nm, resembling nanodiscs. We further detected apoAI-EfpA form oligomers in β-DDM and lipid. To the best of our knowledge, this is the first complete protocol on the expression, purification, and lipid reconstitution of the Mtb EfpA reported. Our bioinformatic analysis confirmed the earlier proposed 14-transmembrane helices of the Mtb EfpA. We also found very high identity, >80%, among the EfpAss of diverse mycobacterial species. Outside of mycobacteria, EfpA has no close homologues with only low identity with the QacA family of transporters. These findings possibly indicate high specificity of EfpA mechanisms. Our developments provide a foundation for more comprehensive in vitro studies on the EfpA exporter. ### Competing Interest Statement The authors have declared no competing interest.