Identification of Targeting Factors and Substrates of the Mycobacterial SecA2 Protein Export System.

摘要

Tuberculosis disease is a major global health crisis, as nearly one-third of the world's population is infected with Mycobacterium tuberculosis, resulting in 1.4 million deaths annually. The essential general Sec export pathway is the most widely conserved system for exporting proteins in bacteria. Central to Sec export is the SecA ATPase, which powers translocation of unfolded preproteins containing Sec signal peptides through the SecYEG membrane channel. Mycobacteria have two non-redundant SecA homologs: SecA1 and SecA2. While the essential SecA1 handles housekeeping export, the nonessential SecA2 exports a subset of proteins and is required for M. tuberculosis virulence. SecA2 is thought to function in concert with the SecA1/SecYEG machinery, but this relationship is poorly understood. Using two SecA2 substrates of the model organism M. smegmatis, we demonstrated that there does not appear to be SecA2-specific signal peptides, and instead it is the mature portion of the preprotein that dictates SecA2-dependent export. We also demonstrated that export of a SecA2 substrate is influenced by presence of both SecA2 and a functional twin-arginine translocation (Tat) pathway. Because the Tat system only accommodates proteins that fold in the cytoplasm, this result suggests that mycobacterial SecA2 substrates might be amenable to cytoplasmic folding. Furthermore, there may be a preprotein pool shared between the mycobacterial SecA2 and Tat export systems, suggesting that some mycobacterial Sec signal peptides might be compatible for export by the Tat pathway. We also performed a proteomic analysis of the M. tuberculosis DeltasecA2 mutant cell wall, which led to the identification of several candidate SecA2-dependent exported effectors that could explain the attenuation of the DeltasecA2 mutant. Additionally, our proteomic analysis revealed that export of several predicted Tat substrates is reduced in the M. tuberculosis DeltasecA2 mutant. We propose that SecA2 functions in export by adapting proteins with cytoplasmic folding tendencies, a property shared with Tat substrates, for export via SecYEG. Thus, our work has led to a model for mycobacterial SecA2 export where SecA2 would serve an integral role in connecting Sec- and Tat-mediated export.