Elucidation of separate, but collaborative functions of the rRNA methyltransferase-related human mitochondrial transcription factors B1 and B2 in mitochondrial biogenesis reveals new insight into maternally inherited deafness

Abstract

Mitochondrial biogenesis is controlled by signaling networks that relay information to and from the organelles. However, key mitochondrial factors that mediate such pathways and how they contribute to human disease are not understood fully. Here we demonstrate that the rRNA methyltransferase-related human mitochondrial transcription factors B1 and B2 are key downstream effectors of mitochondrial biogenesis that perform unique, yet cooperative functions. The primary function of h-mtTFB2 is mtDNA transcription and maintenance, which is independent of its rRNA methyltransferase activity, while that of h-mtTFB1 is mitochondrial 12S rRNA methylation needed for normal mitochondrial translation, metabolism and cell growth. Over-expression of h-mtTFB1 causes 12S rRNA hypermethylation, aberrant mitochondrial biogenesis and increased sorbitol-induced cell death. These phenotypes are recapitulated in cells harboring the pathogenic A1555G mtDNA mutation, implicating a deleterious rRNA methylation-dependent retrograde signal in maternally inherited deafness pathology and shedding significant insight into how h-mtTFB1 acts as a nuclear modifier of this disease.

The core human mitochondrial transcription initiation complex

“…12,13 While both human proteins have demonstrable transcription factor activity in vitro and rRNA methyltransferase activity in a bacterial assay system, 14 subsequent studies appeared to arrive at the consensus that h-mtTFB1 primarily, if not exclusively, functions in vivo as the mitochondrial 12S rRNA methyltransferase, while h-mtTFB2 functions as the mitochondrial transcription factor of this type. 15,16 Interestingly, despite this division of labor, these factors cooperate to promote normal mitochondrial biogenesis and, when this coordination is disrupted, cells are more prone to apoptosis in culture. Previous mechanistic studies using purified recombinant mitochondrial transcription components led to the conclusion that the core human transcription machinery is an obligate threecomponent system comprising POLRMT, have been shown to change their composition as a way to adjust mtDNA expression in response to metabolic signals.…”

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

“…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)(8)(9). TFB2M mainly regulates mtDNA replication and mitochondrial gene transcription.…”

Mitochondrial Ribosomal Protein L12 selectively associates with human mitochondrial RNA polymerase to activate transcription

“…The role of h-mtTFA as a transcriptional activator is well documented, but it also has proposed roles in packaging (21)(22)(23) and repair (24) of mtDNA. A paralog of h-mtTFB2, h-mtTFB1 (or TFB1M), that was originally characterized as a mitochondrial transcription factor in vitro (11,25), is the functional ortholog of the bacterial KsgA rRNA methyltransferase that dimethylates a conserved stem-loop in the mitochondrial 12S rRNA (26) and, hence, has a conserved function in mitochondrial ribosome biogenesis (27) and translation (8,12,28) in vivo. That h-mtTFB1 and h-mtTFB2 are derived from an rRNA methyltransferase, suggests a long evolutionary linkage between mitochondrial proteins involved in transcription, translation, and ribosome biogenesis.…”