Researchers at the University of Miami Miller School of Medicine have used yeast to gain new understanding of the molecular pathways that allow the cells to build the machinery they need to undergo respiration and aerobic energy production. This knowledge will contribute to a better understanding of the development of mitochondrial diseases and cancer in humans.
Their exploration began with ribosomes — the intracellular macromachines where all proteins are made. Ribosomes are formed by ribosomal RNA and many proteins. These components are distributed in two complex subunits named the large subunit and the small subunit.
Mitochondria, the powerhouse of our cells, contain their own mitochondrial DNA and ribosomes. The mitochondrial ribosomes or mitoribosomes are in charge of the synthesis of a few proteins encoded in the mitochondrial DNA, all of which are essential components of the mitochondrial oxidative phosphorylation system responsible for energy production. The composition and the structure of mitochondrial ribosomes are known, but how these components are put together — what we call their assembly or biogenetic pathway — remains largely unknown.
The research team — Antonio Barrientos, Ph.D., professor of neurology, and biochemistry and molecular biology, Rui Zeng, a recently graduated Ph.D. student in the Department of Biochemistry and Molecular Biology, and Erin Smith, an undergraduate student who wrote her honors thesis on her work in Barrientos’ laboratory — used the yeast Saccharomyces cerevisiae to decipher the assembly pathway of the mitoribosome large subunit. They engineered and evaluated a collection of 44 gene-deletion yeast strains for mitoribosome assembly and function. Each of these strains was deleted for a gene encoding a protein component of the mitoribosome large subunit.
This analysis revealed the essentiality of most, but not all, mitoribosome proteins, for building a functional mitoribosome large subunit. The researchers were able to determine an assembly pathway based on hierarchical incorporation of protein clusters and modules, and identify the use of the mitochondrial inner membrane as a platform for the assembly of the mitoribosome large subunit.
“The biomedical relevance of our findings stems from the fact that mutations in mitoribosome proteins, ribosomal RNAs and translation factors are responsible for a heterogeneous group of human multisystemic OXPHOS disorders, such as Leigh’s syndrome, sensorineural hearing loss, encephalomyopathy and hypertrophic cardiomyopathy,” said Barrientos. “Furthermore, several clinically useful antibiotics used as frontline therapy against microbial infectious diseases can unintentionally target mitoribosomes, which can result in mild to severe side effects in physiological and pathological conditions. Moreover, mitochondrial ribosomes and their components are emerging as both cancer biomarkers and targets for cancer therapy. For these reasons, understanding mitoribosome biogenesis is timely.”
The findings were published in an article)30064-0, “Yeast mitoribosome large subunit assembly proceeds by the hierarchical incorporation of protein clusters and modules on the inner membrane,” in the journal Cell Metabolism.
Understanding the biogenesis of yeast mitoribosomes will provide invaluable information to understand how the process occurs in human cells. Furthermore, it is expected to provide clues for the identification of therapeutic targets to combat mitochondrial diseases and cancer. Also, because mitochondrial ribosomes and bacterial ribosomes are inhibited by the same set of antibiotics, the data will help to optimize the design of antibiotics in order to minimize side effects on mitoribosomes and, therefore, mitochondrial energy production.
“Now that we have obtained an initial yeast mitoribosome large subunit assembly pathway, we plan to use novel sophisticated biochemical approaches to refine it,” said Barrientos. “We are also using yeast to describe the assembly pathway of the small subunit. Finally, we have started translating our results to the human mitoribosome context, by creating and characterizing human cell lines defective in mitoribosome assembly. As we continue to learn how mitoribosomes are made, it will be exciting to see how new targets emerge that offer promise for therapeutics to combat not only human disorders associated with mitochondrial translation defects but also metastasis and cancer.”
Miller School of Medicine