Abstract

Large macromolecular assemblies play a critical role in human metabolism and its control. They act as biological signal integrators to directly link primary catalytic activities to a multitude of regulatory input signals or output pathways. Individual catalytic mechanisms have been comprehensively characterized based on crystallographic and mechanistic studies of isolated domains. However, such divide-and-conquer approaches often failed to provide insights into the allosteric regulation by small molecules across multiple domains as well as the recognition of protein interactors in intact assemblies. Using the examples of the multienzyme acetyl-CoA carboxylase (ACC) and the mammalian target of rapamycin (mTOR) complexes, we demonstrate how visualization of intact assemblies by cryo electron microscopy provides key mechanistic insights into the functional interplay of the individual components of these giant protein systems. Importantly, the analysis of intact assemblies reveals novel paths and unexplored sites for controlling activity that are truly specific and unique to the respective systems and might be exploited for fine-grained intervention in metabolism and cell growth.