Mitochondrial dysfunction is associated with many human diseases, including cancer and neurodegeneration, that are often linked to proteins and pathways that are not well-characterized. To begin defining the functions of such poorly characterized proteins, we used mass spectrometry to map the proteomes, lipidomes and metabolomes of 174 yeast strains, each lacking a single gene related to mitochondrial biology. 144 of these genes have human homologs, 60 of which are associated with disease and 39 of which are uncharacterized. We present a multi-omic data analysis and visualization tool that we use to find covariance networks that can predict molecular functions, correlations between profiles of related gene deletions, gene-specific perturbations that reflect protein functions, and a global respiration deficiency response. Using this multi-omic approach, we link seven proteins including Hfd1p and its human homolog ALDH3A1 to mitochondrial coenzyme Q (CoQ) biosynthesis, an essential pathway disrupted in many human diseases. This Resource should provide broad molecular insights into mitochondrial protein functions.

SUMMARY The UbiB protein kinase-like (PKL) family is widespread—comprising one-quarter of microbial PKLs and five human homologs—yet its biochemical activities remain obscure. COQ8A (ADCK3) is a mammalian UbiB protein associated with ubiquinone (CoQ) biosynthesis and an ataxia (ARCA2) through unclear means. We show that mice lacking COQ8A develop a slowly progressive cerebellar ataxia linked to Purkinje cell dysfunction and mild exercise intolerance, recapitulating ARCA2. Interspecies biochemical analyses show that COQ8A and yeast Coq8p specifically stabilize a CoQ biosynthesis complex through unorthodox PKL functions. While COQ8 was predicted to be a protein kinase, we demonstrate that it lacks canonical protein kinase activity in trans. Instead, COQ8 has ATPase activity and interacts with lipid CoQ intermediates—functions that are likely conserved across all domains of life. Collectively, our results lend insight into the molecular activities of the ancient UbiB family and elucidate the biochemical underpinnings of a human disease.

Protein arginine methyltransferases (PRMTs) introduce arginine methylation, a post-translational modification with the increasingly eminent role in normal physiology and disease. PRMT4 or coactivator-associated arginine methyltransferase 1 (CARM1) is a propitious target for cancer therapy; however, few CARM1 substrates are known, and its mechanism of substrate recognition is poorly understood. Here we employed a quantitative mass spectrometry approach to globally profile CARM1 substrates in breast cancer cell lines. We identified >130 CARM1 protein substrates and validated in vitro >90% of sites they encompass. Bioinformatics analyses reveal enrichment of proline-containing motifs, in which both methylation sites and their proximal sequences are frequently targeted by somatic mutations in cancer. Finally, we demonstrate that the N-terminus of CARM1 is involved in substrate recognition and nearly indispensable for substrate methylation. We propose that development of CARM1-specific inhibitors should focus on its N-terminus and predict that other PRMTs may employ similar mechanism for substrate recognition.