pH-responsive substrate switching in mycobacterial type VII ESX secretion

Area of Science:

• Microbiology
• Cell Biology
• Pathogen-Host Interactions

Background:

• Pathogenic mycobacteria, including those causing tuberculosis, reside in host macrophages within phagosomes.
• The ESX-1 secretion system (early secreted antigen 6 kilodaltons [ESAT-6] system 1) is crucial for mycobacterial virulence, enabling phagosome lysis and bacterial escape.
• The mechanisms by which ESX-1 functions in diverse phagosomal environments are not fully understood.

Purpose of the Study:

• To investigate how the ESX-1 secretion system functions in varying pH conditions relevant to macrophage infection.
• To determine if mycobacteria switch the proteins they secrete via ESX-1 in response to environmental cues like acidic pH.
• To elucidate the molecular changes associated with ESX-1 substrate switching and its relevance during infection.

Main Methods:

• Culturing *Mycobacterium marinum* at different pH levels *in vitro* to observe substrate secretion.
• Analyzing changes in ESX-1 substrate and chaperone transcript and protein levels.
• Utilizing *in vivo* infection models to assess the role of ESX-1 substrate switching during infection.

Main Results:

• *Mycobacterium marinum* exhibits ESX-1 substrate switching when grown at acidic pH *in vitro*.
• Substrate switching correlates with significant alterations in ESX-1 substrate and chaperone transcript and protein expression.
• *In vivo* studies revealed upregulation of specific ESX-1 transcripts and demonstrated the requirement for distinct substrate sets in an acidic infection model.

Conclusions:

• The ESX-1 secretion system dynamically orchestrates substrate switching in response to environmental pH.
• Mycobacterial pathogens likely employ distinct ESX-1 protein sets to lyse macrophage phagosomes of differing acidity during infection.
• Understanding ESX-1 substrate switching provides insights into mycobacterial pathogenesis and potential therapeutic targets.