Overcoming barriers: Unraveling the functions of membrane proteins in mycobacteria

Tuberculosis remains a significant global health challenge, claiming over a million lives annually, and the emergence of drug-resistant strains of Mycobacterium tuberculosis complicates efforts to eradicate the disease. Researchers are intensifying their search for new drug targets, focusing particularly on the type VII secretion system in pathogenic mycobacteria, using Mycobacterium marinum as a model. Our research aimed to gain more information about the permeability of the mycobacteria cell envelope. We investigated some substrates, believed to be membrane proteins involved in specific functions necessary for the pathogenesis of M. tuberculosis. These proteins belonged to the type VII secretion system subclassified into five types, ESX-1 to ESX-5. With our study, we further characterize the role of the different secretion system subtypes and their importance in the pathogenesis, nutrient uptake and membrane permeability of mycobacteria. In Chapter 2, we characterize CpnT, the major cytotoxic factor of M. tuberculosis and elucidate how it is secreted during infection in macrophages. Chapter 3 provides details on the transport of a big water-soluble molecule, vitamin B12, and how membrane permeability impacts this transport. Chapter 4 investigates further this B12 transport system. A new protein was found to participate in the B12 uptake. Characteristics regarding how this protein is secreted are also studied here. In Chapter 5, we characterize a protein involved in glucose and glycerol uptake. This protein appears to be also linked to outer membrane integrity in mycobacteria. In Chapter 6, we summarize and reflect on the results obtained in this thesis. We also discuss the implications of our work in the mycobacteria research field.