Mitochondrial ribosome

A diagram showing mtDNA (circular) and mitochondrial ribosomes among other mitochondria structures

The mitochondrial ribosome, or mitoribosome, is a protein complex that is active in mitochondria and functions as a riboprotein for translating mitochondrial mRNAs encoded in mtDNA. The mitoribosome is attached to the inner mitochondrial membrane. ^[1] Mitoribosomes, like cytoplasmic ribosomes, consist of two subunits — large (mt-LSU) and small (mt-SSU). ^[2] Mitoribosomes consist of several specific proteins and fewer rRNAs. ^[2] While mitochondrial rRNAs are encoded in the mitochondrial genome, the proteins that make up mitoribosomes are encoded in the nucleus and assembled by cytoplasmic ribosomes before being implanted into the mitochondria. ^[3]

Function

Mitochondria contain around 1000 proteins in yeast and 1500 proteins in humans. However, only 8 and 13 proteins are encoded in mitochondrial DNA in yeast and humans respectively. Most mitochondrial proteins are synthesized via cytoplasmic ribosomes. ^[4] Proteins that are key components in the electron transport chain are translated in mitochondria. ^{[5] [6]}

Structure

Mammalian mitoribosomes have small 28S and large 39S subunits, together forming a 55S mitoribosome. ^{[7] [8]} Plant mitoribosomes have small 33S and large 50S subunits, together forming a 78S mitoribosome. ^{[7] [8]}

Animal mitoribosomes only have two rRNAs, 12S (SSU) and 16S (LSU), both highly minimized compared to their larger homologues. ^[7] Most eukaryotoes use 5S mitoribosomal RNA, animals, fungi, alveolates and euglenozoans being the exceptions. ^[9] A variety of methods have evolved to fill in the gap left by a missing 5S, with animals co-opting a Mt-tRNA (Val in vertebrates). ^{[7] [10]}

Comparison to other ribosomes

Like mitochondria itself is descended from bacteria, mitochondrial ribosomes are descended from bacterial ribosomes. ^[1] As mitochondria evolved however, the mitoribosome has significantly diverged from its bacterial cousins leading to differences in configuration and function. ^[1] In configuration, the mitoribosome includes additional proteins in both its large and small subunits. ^[1] In function, mitoribosomes are much more limited in the proteins they translate, only producing a few proteins, used mostly in the mitochondrial membrane. ^[1] Below is a table showing some properties of different ribosomes:

Properties of mitoribosomes

	Bacteria [1] [11]	Cytosolic (Eukaryote) [11] [1]	Mammalian mitochondria [1] [11]	Yeast Mitochondria [1] [11]	Plant Mitochondria [12]
Sedimentation Coefficient (LSU/SSU)	70S (50S/30S)	80S (60S/40S)	55S (39S/28S)	74S (54S/37S)	~80S
Number of proteins (LSU/SSU)	54 (33/21)	79-80 (46-47/33)	80 (50/30)	84 (46/38)	68-80
Number of rRNAs (LSU/SSU)	3 (2/1)	4 (3/1)	3 (2/1)	2 (1/1)	3 (2/1)

Diseases

As the mitoribosome is responsible for the manufacture of proteins necessary for the electron transport chain, malfunctions in the mitoribosome can result in metabolic disease. ^{[13] [3]} In humans, disease particularly manifests in energy-reliant organs such as the heart, brain, and muscle. ^[3] Disease either originates from mutations in mitochondrial rRNA or genes encoding the mitoribosomal proteins. ^[3] In the case of mitoribosomal protein mutation, heredity of disease follows Mendelian inheritance as these proteins are encoded in the nucleus. ^[13] On the other hand, because mitochondrial rRNA is encoded in the mitochondria, mutations in rRNA are maternally inherited. ^[13] Examples of diseases in humans caused by these mutations include Leigh syndrome, deafness, neurological disorders, and various cardiomyopathies. ^[13] In plants, mutation in mitoribosomal proteins can result in stunted size and distorted leaf growth. ^[14]

Genes

The mitochondrial ribosomal protein nomenclature generally follows that of bacteria, with extra numbers used for mitochondrion-specific proteins. (For more information on the nomenclature, see Ribosomal protein § Table of ribosomal proteins.)

• MRPS1, MRPS2, MRPS3, MRPS4, MRPS5, MRPS6, MRPS7, MRPS8, MRPS9, MRPS10, MRPS11, MRPS12, MRPS13, MRPS14, MRPS15, MRPS16, MRPS17, MRPS18, MRPS19, MRPS20, MRPS21, MRPS22, MRPS23, MRPS24, MRPS25, MRPS26, MRPS27, MRPS28, MRPS29, MRPS30, MRPS31, MRPS32, MRPS33, MRPS34, MRPS35
• MRPL1, MRPL2, MRPL3, MRPL4, MRPL5, MRPL6, MRPL7, MRPL8, MRPL9, MRPL10, MRPL11, MRPL12, MRPL13, MRPL14, MRPL15, MRPL16, MRPL17, MRPL18, MRPL19, MRPL20, MRPL21, MRPL22, MRPL23, MRPL24, MRPL25, MRPL26, MRPL27, MRPL28, MRPL29, MRPL30, MRPL31, MRPL32, MRPL33, MRPL34, MRPL35, MRPL36, MRPL37, MRPL38, MRPL39, MRPL40, MRPL41, MRPL42
• rRNA: MT-RNR1, MT-RNR2, MT-TV (mitochondrial)

References

1. Greber BJ, Ban N (June 2016). "Structure and Function of the Mitochondrial Ribosome". Annual Review of Biochemistry. 85 (1): 103–132. doi: 10.1146/annurev-biochem-060815-014343 . PMID   27023846.
2. Amunts A, Brown A, Toots J, Scheres SH, Ramakrishnan V (April 2015). "Ribosome. The structure of the human mitochondrial ribosome". Science. 348 (6230): 95–98. doi:10.1126/science.aaa1193. PMC   4501431 . PMID   25838379.
3. Sylvester JE, Fischel-Ghodsian N, Mougey EB, O'Brien TW (March 2003). "Mitochondrial ribosomal proteins: candidate genes for mitochondrial disease". Genetics in Medicine. 6 (2): 73–80. doi: 10.1097/01.GIM.0000117333.21213.17 . PMID   15017329. S2CID   22169585.
4. Wenz LS, Opaliński Ł, Wiedemann N, Becker T (May 2015). "Cooperation of protein machineries in mitochondrial protein sorting". Biochimica et Biophysica Acta (BBA) - Molecular Cell Research. 1853 (5): 1119–1129. doi: 10.1016/j.bbamcr.2015.01.012 . PMID   25633533.
5. Johnston IG, Williams BP (February 2016). "Evolutionary Inference across Eukaryotes Identifies Specific Pressures Favoring Mitochondrial Gene Retention". Cell Systems. 2 (2): 101–111. doi: 10.1016/j.cels.2016.01.013 . PMID   27135164.
6. Hamers L (2016). "Why do our cell's power plants have their own DNA?". Science. doi:10.1126/science.aaf4083.
7. Greber BJ, Bieri P, Leibundgut M, Leitner A, Aebersold R, Boehringer D, Ban N (April 2015). "Ribosome. The complete structure of the 55S mammalian mitochondrial ribosome". Science. 348 (6232): 303–308. doi:10.1126/science.aaa3872. hdl: 20.500.11850/100390 . PMID   25837512. S2CID   206634178.
8. Spremulli LL (2016-01-01). "The Protein Biosynthetic Machinery of Mitochondria". In Bradshaw RA, Stahl PD (eds.). Encyclopedia of Cell Biology. Waltham: Academic Press. pp. 545–554. doi:10.1016/b978-0-12-394447-4.10066-5. ISBN   978-0-12-394796-3.
9. Valach M, Burger G, Gray MW, Lang BF (December 2014). "Widespread occurrence of organelle genome-encoded 5S rRNAs including permuted molecules". Nucleic Acids Research. 42 (22): 13764–13777. doi:10.1093/nar/gku1266. PMC   4267664 . PMID   25429974.
10. Brown A, Amunts A, Bai XC, Sugimoto Y, Edwards PC, Murshudov G, et al. (November 2014). "Structure of the large ribosomal subunit from human mitochondria". Science. 346 (6210): 718–722. Bibcode:2014Sci...346..718B. doi:10.1126/science.1258026. PMC   4246062 . PMID   25278503.
11. De Silva D, Tu YT, Amunts A, Fontanesi F, Barrientos A (2015-07-18). "Mitochondrial ribosome assembly in health and disease". Cell Cycle. 14 (14): 2226–2250. doi:10.1080/15384101.2015.1053672. PMC   4615001 . PMID   26030272.
12. Robles P, Quesada V (December 2017). "Emerging Roles of Mitochondrial Ribosomal Proteins in Plant Development". International Journal of Molecular Sciences. 18 (12): 2595. doi: 10.3390/ijms18122595 . PMC   5751198 . PMID   29207474.
13. De Silva D, Tu YT, Amunts A, Fontanesi F, Barrientos A (2015-07-18). "Mitochondrial ribosome assembly in health and disease". Cell Cycle. 14 (14): 2226–2250. doi:10.1080/15384101.2015.1053672. PMC   4615001 . PMID   26030272.
14. Robles P, Quesada V (December 2017). "Emerging Roles of Mitochondrial Ribosomal Proteins in Plant Development". International Journal of Molecular Sciences. 18 (12): 2595. doi: 10.3390/ijms18122595 . PMC   5751198 . PMID   29207474.

Further reading

• Greber BJ, Ban N (June 2016). "Structure and Function of the Mitochondrial Ribosome". Annual Review of Biochemistry. 85: 103–132. doi: 10.1146/annurev-biochem-060815-014343 . PMID   27023846.