GABP Transcription Factor (Nuclear Respiratory Factor 2) Is Required for Mitochondrial Biogenesis

Abstract

c Mitochondria are membrane-bound cytoplasmic organelles that serve as the major source of ATP production in eukaryotic cells. GABP (also known as nuclear respiratory factor 2) is a nuclear E26 transformation-specific transcription factor (ETS) that binds and activates mitochondrial genes that are required for electron transport and oxidative phosphorylation. We conditionally deleted Gabpa, the DNA-binding component of this transcription factor complex, from mouse embryonic fibroblasts (MEFs) to examine the role of Gabp in mitochondrial biogenesis, function, and gene expression. Gabp␣ loss modestly reduced mitochondrial mass, ATP production, oxygen consumption, and mitochondrial protein synthesis but did not alter mitochondrial morphology, membrane potential, apoptosis, or the expression of several genes that were previously reported to be GABP targets. However, the expression of Tfb1m, a methyltransferase that modifies ribosomal rRNA and is required for mitochondrial protein translation, was markedly reduced in Gabp␣-null MEFs. We conclude that Gabp regulates Tfb1m expression and plays an essential, nonredundant role in mitochondrial biogenesis. Mitochondria are semiautonomous, membrane-bound cytoplasmic organelles that are the major source of ATP production in the eukaryotic cell. In addition to their roles in generating cellular energy through electron transfer and oxidative phosphorylation, mitochondria are also required for lipid biosynthesis, cell signaling, apoptosis, and other essential cellular functions. Because the 16-kb mitochondrial genome does not encode any transcription factors, it depends on nuclear transcription factors for the expression of mitochondrial DNA (mtDNA) genes. Mitochondrial transcription factor A (TFAM) and TFBM are nuclear transcription factors that exclusively regulate mtDNA genes. Other transcription factors, including NRF1 (nuclear respiratory factor 1) and NRF2, control both mtDNA genes and nuclear genes (1-4).There are two distinct TFBM proteins, TFB1M and TFB2M, both of which have rRNA methyltransferase activity (5, 6). Shadel and colleagues reported that TFB1M and TFB2M play crucial but distinct roles in the control of mitochondrial biogenesis in both human and Drosophila cells (7-9). TFB2M mainly regulates mtDNA replication and mitochondrial gene transcription. Ectopic expression of TFB1M did not significantly affect mitochondrial function, but reduced expression of TFB1M decreased mitochondrial protein translation through impaired methylation of the 12S rRNA, impaired mitochondrial ribosome assembly, and abolished mitochondrial translation. Genetic disruption of Tfb1m caused early (embronic day 8.5 [E8.5]) embryonic lethality but neither activated nor repressed mitochondrial gene transcription (10). Thus, adequate levels of TFB1M are required for normal mitochondrial protein synthesis and biogenesis.Scarpulla and colleagues recognized that the multiprotein NRF2 complex is the human homologue of GABP, or GA-binding protein (11). GABP is the only obligate multimer among th...

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