A new area of research in the lab involves the analysis mitochondrial disease genes. Our long-term goal is to employ quantitative proteomi approaches to understand how mitochondrial disease genes alter protein assemblies within the mitochondria that are required for energy production, metabolism, and mitochondrial quality control. In our initial foray into this area, we employed interaction proteomics to partners of 15 Complex I components previously found to be mutated in humans with mitochondrial disease. Complex I is the largest (44 subunits) and arguably the least understood complex in the electron transport chain (ETC). It is known to be assembled through a very complex process involving multiple intermediate complexes, and a number of assembly factors. Using our interaction proteomics platform, we identified a previously unrecognized member of the ESCIT-ACAD9-TMEM126B-NDUFAF1 (refered to as MCIA) assembly factor complex, which is known to play a role in integrating Complex I components within the mitochondrial inner membrane. This new protein - TIMMDC1/C3orf1 - is a multipass transmembrane protein which we demonstrated to reside in the mitochondrial inner membrane and to be required for assembly of Complex I. Our studies demonstrated that TIMMDC1 is required for Complex I activity. To explore the biological functions of TIMMDC1, we developed a quantitative proteomic approach that allows us to identify Complex I intermediates that accumulate when TIMMDC1 is depleted by RNAi. This method allows the identification of individual sub-complexes that accumulate in response to loss of a particualr assembly factor. These studies allowed us to put the function of TIMMDC1 into the context of existing models for Complex I assembly. Additional ongooing studies seek to employ analogous approaches to dissect the functions of other mitochondrial proteins that have been linked to human disease.