TolC-AcrA complex formation monitored by time dependent single-channel electrophysiology

Understanding how proteins interact at the molecular level remains a challenge in biophysics. Gram-negative bacteria use tripartite efflux systems to expel toxic substances, including antibiotics. High-resolution structural studies have revealed the TolC-AcrA-AcrB complex, but functional validation is still needed. Prior research has shown that TolC forms a channel in the outer membrane, but how AcrA binds to TolC is unclear. This gap motivated researchers to explore electrophysiological methods to monitor TolC-AcrA interactions. No prior work had resolved how AcrA affects TolC's ion current or how inhibitors might interfere. Existing knowledge on TolC function is limited to its role in efflux, but the dynamics of complex formation remain uncertain. This uncertainty drives the need for new experimental approaches. Researchers aim to bridge the structural and functional understanding of TolC-AcrA interactions.

The goal of this study was to investigate the TolC-AcrA interaction using electrophysiological measurements. Researchers aimed to determine if TolC's ion current could be used as a reporter for complex formation. The specific problem addressed is how AcrA affects TolC's conductance and whether this can be modulated by efflux inhibitors. The motivation stems from the need to validate structural findings with functional data. Electrophysiology offers a direct way to monitor protein binding in real time. The study focused on TolC's ion current modulation as a proxy for complex assembly. Researchers also wanted to assess if current fluctuations could reveal structural changes. This approach could help identify compounds that disrupt TolC-AcrA interactions.

Scientists reconstituted TolC homotrimers into planar lipid membranes to form ion channels. A transmembrane voltage was applied to measure ion current through TolC. AcrA was introduced into the solution to observe its effect on TolC's conductance. The study tracked changes in average ion current and current fluctuations over time. Researchers used power spectral density to analyze current noise patterns. They tested the impact of potential efflux inhibitors on TolC-AcrA interactions. The experimental setup allowed for real-time monitoring of complex formation. Data collection focused on statistical properties of current modulation.

The presence of AcrA increased the average ion current through TolC channels. AcrA also reduced current fluctuations caused by TolC flickering. These findings suggest that AcrA stabilizes TolC's open state. Current noise analysis showed a decrease in power spectral density. This indicates a reduction in TolC's dynamic flexibility. Inhibitors altered both current amplitude and noise characteristics. The combination of current and noise data pointed to TolC stiffening at the tip. This stiffening may hinder TolC-AcrA complex formation. The results support the idea that electrophysiology can detect protein interactions.

The authors propose that electrophysiology can detect TolC-AcrA complex formation. They suggest that ion current and noise together reveal structural changes. The findings imply that AcrA binding stabilizes TolC's conformation. Inhibitors may disrupt complex formation by altering TolC dynamics. The study supports using electrophysiology to monitor protein interactions. Researchers propose that current modulation reflects complex assembly. The results suggest that TolC stiffening correlates with reduced complex formation. These conclusions align with the observed effects of AcrA and inhibitors.