IcmR-regulated membrane insertion and efflux by the Legionella pneumophila IcmQ protein

Area of Science:

• Microbiology
• Cell Biology
• Protein Biochemistry

Background:

• Legionella pneumophila causes Legionnaires' disease by replicating within alveolar macrophages.
• Intracellular growth necessitates the Dot/Icm secretion system for translocating bacterial proteins across the host cell membrane.
• IcmR and IcmQ are proposed components of the Dot/Icm system with a potential chaperone-substrate interaction.

Purpose of the Study:

• To investigate the functional relationship between IcmR and IcmQ.
• To characterize the membrane interaction and pore-forming activity of IcmQ.
• To elucidate the role of IcmQ in the Dot/Icm secretion pathway.

Main Methods:

• Lipid membrane insertion and pore formation assays using calcein and Dextran 3000.
• Trypsin digestion mapping to delineate functional domains of IcmQ.
• Protein binding assays to confirm interaction between IcmR and IcmQ.

Main Results:

• IcmQ inserts into lipid membranes and forms pores permeable to small molecules like calcein, but not larger molecules like Dextran 3000.
• IcmR inhibits both membrane insertion and pore formation by IcmQ.
• IcmQ possesses distinct N-terminal (membrane insertion/pore formation) and C-terminal (bilayer association) domains.
• IcmR binds to the N-terminal domain of IcmQ, explaining its inhibitory effect.

Conclusions:

• IcmQ functions as a pore-forming protein, likely facilitating effector translocation out of Legionella pneumophila.
• The chaperone IcmR regulates IcmQ's pore activity by binding to its N-terminal domain.
• These findings provide mechanistic insights into the Dot/Icm secretion system essential for Legionnaires' disease pathogenesis.