Single TolC-AcrA complex formation monitoredby time dependent single-channel electrophysiology

Characterizing protein-protein interaction on a single molecular level is a challenge, experimentally as well as the interpretation of the data. For example, Gram-negative bacteria contain protein complexes spanning the outer and inner cell wall being able to efflux effectively cell toxic substances. Within the search for antibiotics with novel modes of action, inhibition of the efflux pump assembly is an interesting possible target. Recent seminal work revealed the high-resolution structure of the tripartic composition TolC-AcrA-AcrB. Here, we reconstitute a single TolC homotrimer into a planar lipid membrane and follow the assembly of AcrA using the modulation of the ion current through TolC during binding. In particular, the presence of AcrA increases the average ionic current through TolC and, moreover, reduces the ion-current fluctuations caused by flickering of TolC. Here, we demonstrate that statistical properties of ion-current fluctuations (the power spectral density) provide a complementary measure of the interaction of the TolC-AcrA complex with potential inhibitors. Both characteristics, the average current and the current noise, taken into consideration together, provide more robust information.