Imagine that it is 9 a.m. on a workday morning. You have arrived at your workplace and start the workday, calling customers by phone, accomplishing tasks using computers, laptops, and other devices. Suddenly, a power outage occurs. All the machines are fine, but there is no energy to keep them functioning.
Isn’t that stressful? This is a situation everyone wants to avoid.
Now apply this analogy to the cells. In most cases, cells use the chemical energy stored in adenosine triphosphate(ATP), to drive the biological interactions and processes like chemical synthesis, neuro signal transmission, and muscle contraction. Therefore, generating ATP efficiently is a very important process for the cells. In eukaryotic organisms, mitochondria are the organelle that specializes in synthesizing ATP. Based on this function, mitochondria are popularly nicknamed “the powerhouse of the cell.”
The most effective way for mitochondria to generate ATP is via the oxidative phosphorylation (OXPHOS) pathway. Proteins consisting of OXPHOS complexes are encoded by either nuclear DNA or mitochondrial DNA. The information stored in the DNA are translated into proteins by ribosome. In eukaryotic cells, different ribosomes are present in the cytoplasm and mitochondria. Mitochondrial ribosomes, also known as mitoribosomes, are essential for the synthesis of key OXPHOS proteins and eventually the proper function of mitochondria.
Dysfunction of mitoribosome, however, decreases cellular energy production and increases oxidative stress, ultimately leading to multiple diseases such as hearing loss, cardiomyopathy, encephalomyopathy, developmental delays, and neuropathy. Many questions remain regarding the regulation of mitoribosome biogenesis and function, whose full understanding is the long-term goal of our research.
A study by Dr. Alexey Amunts’ group recently published in Nature reported the discovery of iron-sulfur (Fe-S) clusters in the structure of human mitoribosome. Fe-S clusters are one of the most ancient inorganic protein cofactors. Interestingly, human mitoribosome is the only type of ribosome that contains Fe-S clusters that we know of. Fe-S clusters are not reported to present in the bacteria ribosome, cytosolic ribosome, or yeast mitoribosome.
The presence of Fe-S clusters in human mitoribosome raises important questions: Why do human mitoribosome gain Fe-S clusters through evolution? Are the Fe-S clusters essential for the function of mitoribosome? What delivers the Fe-S clusters to mitoribosome? We used human cell models to identify the roles of Fe-S clusters and the delivery to mitoribosome.
We started by testing the function of mitoribosome when the Fe-S clusters’ coordinating proteins are knocked out. In the structure, each Fe-S cluster is coordinated by two different mitoribosomal proteins. When either of the coordinating proteins is lacking, the integrity of mitoribosome is negatively affected and it cannot function properly.
We then put back the missing proteins into the knocked-out cell lines but introduced mutations in the Fe-S cluster coordinating amino acid residues. The mutations abolished the protein’s ability to coordinate Fe-S cluster. Without binding Fe-S cluster, the steady-state levels of coordinating proteins were significantly decreased, the structure of mitoribosome could not properly form, therefore disrupting the function of mitoribosome.
Our data suggests the Fe-S clusters are essential for the stability of coordinating proteins, and these proteins are essential for the integrity and function of mitoribosome.
Fe-S clusters must be carried by Fe-S scaffold proteins to deliver to target proteins. To study the possible scaffold proteins that deliver Fe-S clusters to mitoribosome, we inhibited individual protein expression and analyzed the effects on mitoribosome. Through unbiased mass spectrometry study, we found that BOLA3 is the most possible protein that delivers Fe-S clusters to mitoribosome.
The identification of BOLA3 indicates the possible role of Fe-S clusters in mitoribosome: regulatory targets under stress conditions. Studies show that the expression of BOLA3 was downregulated in hypoxia conditions. When the cells suffer from low oxygen levels, cells decrease the levels of BOLA3, thus decreasing the amounts of mitoribosomal proteins stabilized by binding Fe-S clusters. With less mitoribosomal proteins, less intact mitoribosomes can be formed, and the translation of OXPHOS proteins slow down. Cells then generate less ATP and keep biological activities at low levels until the hypoxia stress disappears, like animals that hibernate in winter when stressed by insufficient food.
Our study uncovers a new regulatory pathway of the essential protein synthesis machinery in mitochondria - mitoribosome. Healthy, functional mitochondria are as importance as the power stations for human society. Elucidating these molecular mechanisms helps us better understand how cells handle stress and avoid damages to mitochondria. This, in turn, will provide theoretical support for future drug discovery and therapies for mitochondrial disorder diseases.
Hui Zhong is a graduate student in the Miller School of Medicine at the University of Miami. Read more about the inaugural “Op-ed Challenge.”
