Mycobacteria have a type VII secretion system (SST7), called ESX. M. abscessus (Mabs), an opportunistic pathogen, has two ESX loci (ESX-3 and 4), while M. tuberculosis (Mtb), the causative agent of TB and strict pathogen, has five loci (ESX-1 to 5). The ESX-4 locus is a key player in the virulence of Mabs and its intracellular survival, as is ESX-1 in Mtb. Analysis of the Mabs secretome showed that the MAB_3753c and MAB_3754c genes, encoding the EsxT and EsxU proteins respectively, show WXG-100 motifs, characteristic of proteins secreted by SST7. This allows us to consider EsxU and EsxT as the homologues, secreted by ESX-4, of EsxA (ESAT-6) and EsxB (CFP-10) respectively, and secreted by the ESX-1 locus of Mtb. We have focused our efforts on building tools to characterize the secretion, subcellular localization and role of these proteins.
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