Rieske head domain dynamics and indazole-derivative inhibition of Candida albicans complex III

ABSTRACT During cellular respiration, electron transfer between the integral membrane protein complexes of the electron transport chain is coupled to proton translocation across the inner mitochondrial membrane, which in turn powers synthesis of ATP and transmembrane transport processes. The homodimeric electron transport chain Complex III (CIII 2 ) oxidizes ubiquinol (UQH 2 ) to ubiquinone (UQ), transferring electrons to cytochrome c , and translocating protons through a mechanism known as the Q cycle. The Q cycle involves UQH 2 oxidation and UQ reduction at two different sites within each CIII monomer, as well as movement of the head domain of the Rieske subunit. We used cryoEM to determine the structure of CIII 2 from Candida albicans , revealing density for endogenous UQ in the structure and allowing us to directly visualize the continuum of conformations of the Rieske head domain. Analysis of these conformations does not indicate cooperativity in the position of the Rieske head domains or binding of ligands in the two CIIIs of the CIII 2 dimer. CryoEM with the indazole derivative Inz-5, which inhibits fungal CIII 2 and is fungicidal when administered with fungistatic azole drugs, showed that inhibition by Inz-5 alters the equilibrium of the Rieske head domain positions.