MAMMALIAN MITOCHONDRIAL RIBOSOMAL PROTEINS : A MATTER OF LIFE AND DEATH

Open Access
Author:
Han, Min-Joon
Graduate Program:
Biochemistry, Microbiology, and Molecular Biology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 08, 2010
Committee Members:
  • Dr Emine Koc, Dissertation Advisor
  • Emine C Koc, Committee Chair
  • Hasan Koc, Committee Chair
  • Craig Eugene Cameron, Committee Member
  • Wendy Hanna Rose, Committee Member
  • Teh Hui Kao, Committee Member
  • Seogchan Kang, Committee Member
Keywords:
  • translation
  • mitochondria
  • apoptosis
Abstract:
Human mitochondria are essential for cell survival while playing key roles in programmed cell death also called apoptosis. First of all, mitochondria are the main source of energy for the eukaryotic cell. Mitochondria produce more than 90% of the energy used by mammalian cells in a process referred to as oxidative phosphorylation. This demanding mechanism requires a lot of proteins, which are nucleus-encoded, synthesized in cytoplasm, and imported into mitochondria. However, it also requires 13 mitochondrial-encoded proteins. Mitochondria have their own 16.5 kb circular genome (mtDNA) and ribosome to translate these 13 proteins. Even though mammalian mitochondrial ribosomes (55S) differ from bacterial (70S) and mammalian cytoplasmic ribosomes (80S), many of the mitochondrial ribosomal proteins have bacterial homologs and similar functions. Some of the mitochondrial ribosomal proteins, which have no homologs in bacterial ribosomes, are involved in the process of apoptosis. Therefore, mitochondrial ribosome is essential not only for the translation of 13 proteins for energy production but also the regulation of apoptosis. Here, two regulatory mechanisms involved in mitochondrial functions by mitochondrial ribosomal proteins were investigated; energy production by OXPHOS and apoptosis. First, we reported that the presence of different splice variants of MRPS29, which contained an uORF in the 5'-UTR in humans and the expression of MRPS29 could be translationally controlled by the uORF found in the 5'-UTR. Data presented in this thesis also suggest that reduction in MRPS29 expression by uORF may inhibit MRPS29-induced apoptosis. Therefore, the presence of uORF-MRPS29 mRNAs in human potentially indicates a new mechanism for regulation of apoptosis. Another regulatory mechanism via post-translational modification has been revealed by studying mitochondrial ribosomal L7/L12 stalk region. The mitochondrial ribosomal protein L10 (MRPL10) was identified as a major acetylated protein in the mitochondrial ribosome. We also found that ribosome associated SIRT3 was responsible for deacetylation of MRPL10 in a NAD+-dependent manner. A mechanism by which the mitochondrial translation is regulated by reversible acetylation is proposed. Increased acetylation status of mitochondrial ribosome in SIRT3 knock-out (Sirt3-/-) mice enhanced MRPL12 binding to the ribosome and recruitment of elongation factors to increase translation. On the other hand, over-expression of SIRT3 reduced the translation and MRPL12 binding to ribosomes by deacetylation of the mitochondrial ribosome. Therefore, acetylation of MRPL10 due to inhibition of SIRT3 could enhance its interaction with MRPL12, resulting in increased MRPL12 binding to ribosomes and the recruitment of elongation factors during translation. These findings constitute the first evidence for the regulation of mitochondrial protein synthesis through the reversible acetylation of the mitochondrial ribosome and identified MRPL10 as a novel substrate of NAD+-dependent deacetylase, SIRT3.