Evidence for the Mycobacterial Mce4 Transporter Being a Multiprotein Complex

Area of Science:

• Microbiology
• Molecular Biology
• Biochemistry

Background:

• Mycobacteria utilize Mce transporters for lipid import, distinct from typical ATP-binding cassette (ABC) transporters.
• Mce transporters involve multiple solute-binding proteins (SBPs), permeases, an ATPase, and associated membrane proteins.
• The complex assembly and function of Mce transporters in lipid transport remain incompletely understood.

Purpose of the Study:

• To investigate the roles of individual Mce, Mam, and Omam proteins in Mce4-mediated cholesterol transport in *Mycobacterium smegmatis*.
• To elucidate the assembly and interaction dynamics of the Mce4 transporter complex.

Main Methods:

• Utilized *Mycobacterium smegmatis* as a model organism.
• Performed protein stability experiments to assess the requirement of individual components.
• Employed immunoprecipitation-mass spectrometry to identify protein-protein interactions within the Mce4 transporter complex.

Main Results:

• All investigated Mce4 and Mam4 proteins were essential for cholesterol uptake.
• OmamA and OmamB were required for cholesterol import, while other Omam proteins were not.
• Absence of specific Mce4, Mam4, or Omam proteins led to decreased Mce4A and Mce4E abundance, suggesting complex assembly and stabilization.
• Immunoprecipitation-mass spectrometry confirmed the association of all functionally relevant Mce4, YrbE, MceG, Mam, and Omam proteins with Mce4A.

Conclusions:

• Mycobacterial Mce transporters function as multiprotein complexes, not just individual components.
• Specific Mce, Mam, and Omam proteins are critical for the assembly, stability, and function of the Mce4 cholesterol transporter.
• This study provides the first direct evidence of protein associations within the Mce4 transporter system.