Conformational flexibility in the multidrug efflux system protein AcrA

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Multidrug resistance in gram-negative bacteria is often mediated by resistance nodulation cell division (RND) efflux pumps.
• These pumps consist of three components: an inner membrane drug:proton antiporter (e.g., AcrB), an outer membrane channel (e.g., TolC), and a periplasmic adaptor protein (e.g., AcrA).
• The Escherichia coli AcrA-AcrB-TolC system is a well-characterized example of an RND efflux pump.

Purpose of the Study:

• To complete the molecular understanding of the RND efflux system by determining the structure of a stable fragment of the AcrA component.
• To investigate the structural features and potential conformational flexibility of AcrA.

Main Methods:

• Purification and structural determination of a stable AcrA fragment.
• Comparison of the AcrA structure with its homologs, such as Pseudomonas aeruginosa MexA.

Main Results:

• The crystal structure of a stable AcrA fragment was determined.
• The AcrA fragment exhibits an elongated, sickle-like shape, similar to MexA.
• AcrA comprises three domains: a beta-barrel, a lipoyl domain, and an alpha-helical hairpin.
• Significant conformational flexibility was observed in the alpha-helical hairpin domain of AcrA.

Conclusions:

• The determined structure of the AcrA fragment provides a key piece of the molecular architecture of RND efflux pumps.
• The observed conformational flexibility in the AcrA alpha-helical hairpin domain may play a critical role in the functional mechanism of the efflux pump, potentially by coupling AcrA conformation to TolC channel opening.