The crystal structure of MtMce4A39–140 shows four molecules in asymmetric units.
The structure of the substrate-binding mammalian cell entry (Mce)4A protein from Mycobacterium tuberculosis, the causative agent of the disease Tuberculosis, was unknown — until recent research at BioMAX beamline. It showed the unique Mce domain of Mce4A, a part of the cholesterol importing complex of Mycobacterium tuberculosis.
Tuberculosis disease caused the death of an estimated 1,4 million people in 2019 and is still considered an epidemic (WHO, 2020). It mainly affects the lungs (pulmonary TB), but can also affect other organs (extrapulmonary TB). Mycobacterium tuberculosis (Mtb) is one of the most successful pathogens to survive within the host by using lipid as the source of carbon. During the latent stage of infection, Mtb is thought to be dependent on mammalian cell entry (Mce) complexes to transport lipids from the host across the cell wall.
Motivated to understand the structure and organization of the mycobacterial Mce proteins, a team of researchers from Finland, Denmark, and the US, took the matter to BioMAX beamline and solved the structure of a domain of Mce4A by using the Selenomethionine-SAD phasing method.
Lipid transportation using a tunnel-based mechanism
The research is the first to provide the crystal structure of any mycobacterial Mce proteins. Each of the four Mce complexes in Mtb is made up of six substrate-binding proteins that comprise four conserved domains. This suggests that the six substrate-binding proteins may form heterohexamers, a base for a tunnel-based structure for lipid transport.
Rajaram Venkatesan, one of the researchers involved, stated that the domain swapping in the crystal structure of the Mce domain of Mce4A was unexpected. “Also, the fact that these proteins required detergent for solubilizing even after removing the membrane anchoring regions made the whole project challenging as well as very interesting,” Venkatesan added.
However, the research was not without challenges. Venkatesan explained that the analysis of the SAXS data of the Mce1A and Mce4A domain constructs was not standard and therefore required optimizing further the in-house programs developed by Prof. Jan Skov Pedersen.
This research builds the foundation for future high-resolution studies of Mtb Mce substrate-binding proteins and Mce complexes to understand their structural arrangement and lipid-transport mechanisms.
“The team is currently continuing their work in understanding the structure and organization of the Mce complexes as an entirety,” explained Venkatesan.
Pooja Asthana et al. Structural insights into the substrate-binding proteins Mce1A and Mce4A from Mycobacterium tuberculosis. IUCrJ (2021). 8, 757-774. https://doi.org/10.1107/S2052252521006199.