Mycobacterial type VII secretion system unraveled!

Mycobacterial type VII secretion system unraveled!

Tuberculosis is a highly contagious infectious disease that is typically spread through aerosols and mainly affects the lungs. According to the World Health Organization (WHO), an estimated 1.7 million people die from such an infection worldwide every year. In addition, a quarter of the world's population carries a form of tuberculosis that lies dormant without symptoms for a long time, but can break out eventually.

During infection Mycobacterium tuberculosis, the main causative agent of tuberculosis, secretes a large number of effector proteins through type VII secretion systems - small nanomachines which are composed of proteins that reside in the cell envelope. The effector proteins are specialized in fighting the immune defense or enable the uptake of nutrients to ensure the bacterial survival in the host. How these central secretion systems work, is still poorly understood.

Scientists have now succeeded in deciphering the molecular architecture of these nanomachines. The scientists have published their work in the journal Nature.

Over the past five years, the research group has worked intensively on the stable reconstitution of one of these secretion machines and the preparation of the sensitive sample for measurements on the cryo electron microscope, which requires the protein complexes to be shock frozen under defined conditions.  The researchers were able to create a model of its molecular structure at 3.7 Å resolution and identify important elements of the nanomachine that form the transport pore as well as to locate elements that convert chemical energy into motion and thus drive the transport of effector proteins through the pore.

The core of the ESX-3 secretion machine consists of four protein components, EccB3:EccC3:EccD3:EccE3 in a 1:1:2:1 stoichiometry, building two identical protomers. The EccC3 coupling protein comprises a flexible array of four ATPase domains, which are linked to the membrane through a stalk domain. The ‘domain of unknown function’ (DUF) adjacent to the stalk is identified as an ATPase domain essential for secretion.

EccB3 is predominantly periplasmatic but a small segment crosses the membrane and contacts the stalk domain, suggesting that conformational changes in the stalk domain triggered by substrate binding at the distal end of EccC3 and subsequent ATP hydrolysis in the DUF could be coupled to substrate secretion to the periplasm.

The findings of the researchers lead to a deeper functional understanding of Type VII secretion systems. In times of rising resistance of mycobacteria to the antibiotics in use and no effective vaccination against tuberculosis in place, the researcher provide an important basis for the development of novel antibiotics that target the assembly or function of the type VII secretion systems.