Universal Stress Protein Regulates Electron Transfer and Superoxide Generation Activities of the Cytochrome bc1 Complex from Rhodobacter sphaeroides

Interactions between Rhodobacter sphaeroides cytochrome bc(1) complex (Rsbc(1)) and soluble cytosolic proteins were studied by a precipitation pull-down technique. After being purified, detergent-dispersed Rsbc(1) complex was incubated with soluble cytosolic fraction and then dialyzed in the absence of detergent; the interacting proteins were coprecipitated with Rsbc(1) complex upon centrifugation. One of the cytosolic proteins pulled down by Rsbc(1) complex was identified by liquid chromatography-coupled tandem mass spectrometry (LC/MS/MS) to be the reported R sphaeroides universal stress protein (UspA). Incubating purified UspA with the detergent dispersed bc(1) complex resulted in an increase in the Rsbc(1) complex activity by 60% and a decrease in superoxide generation activity by the complex by more than 70%. These UspA effects were only observed with Rsbc(1) complexes containing subunit IV and assayed under aerobic conditions. These results suggest that the interaction between UspA and Rsbc(1) complex may play an important role in R sphaeroides cells during oxidative stress. Using a biotin label transfer technique, cytochrome c(1) of the Rsbc(1) complex was identified as the interacting site for UspA.