Molecular Motors of ESX-Type Secretion Systems

Abstract

Tuberculosis is an enormous global health problem. Despite decades of research, the mechanism(s) by which Mycobacterium tuberculosis (Mtb) mediates virulence remains incompletely understood. The ESX-1 secretion system is critical for Mtb to survive and cause disease in vivo, but its primary function and mechanism of action are unclear. The many inherent challenges of working with this slow-growing pathogen often limit the experimental approaches that can be used to address these questions. Thus, we have developed a model system in the nonpathogenic bacterium Bacillus subtilis to study ESX-type secretion systems. Here, we demonstrate that the B. subtilis yuk operon encodes an ESX-type secretion system responsible for the secretion of YukE. Additionally, we demonstrate that the yuk system is active in B. subtilis during conditions of nutrient deprivation and is required for normal biofilm formation. Interestingly, this is similar to our findings that the Mtb ESX-1 system plays dual roles in protein secretion and modulating cell wall integrity. One defining feature of all ESX loci is the presence of an FtsK/SpoIIIE family ATPase. Interestingly, these ATPases have a domain structure unique to ESX-associated ATPases, where each protein contains multiple (2-3) enzymatic domains. We used our B. subtilis system to dissect the mechanism of action of this unique class of motor proteins. We find that the yuk-encoded ATPase YukBA dimerizes to form a hexamer of enzymatic subunits that are differentially required for secretion. Strikingly, we find a unique requirement for rotational symmetry in the nucleotide binding activity of the subunits. Finally, we compared the energy requirements of the Mtb ESX-1 system and the B. subtilis yuk system. We find that these systems have some overlapping ATPase requirements for protein secretion and cell wall integrity/biofilm formation, suggesting that there is a conservation of function among ESX-type systems. We also find that some ATPase domains are differentially required for function between these two systems, which we postulate is due to the split protein architecture of the ESX-1-encoded ATPases. Together, these findings highlight the power of using a B. subtilis model system to understand the function and mechanism of action of ESX-type secretion systems.