ribosomes

Ribosomes are cytoplasmic organelles found in prokaryotes and eukaryotes. They are large complexes of proteins and three (prokaryotes) or four (eukaryotes) rRNA (ribosomal ribonucleic acid) molecules called subunits made in the nucleolus.

The main function of ribosomes is to serve as the site of mRNA translation ( = protein synthesis, the assembly of amino acids into proteins); once the two (large and small) subunits are joined by the mRNA from the nucleus, the ribosome translates the mRNA into a specific sequence of amino acids, or a polypeptide chain.

The mammalian ribosome is composed of 4 RNA species and approximately 80 different ribsomal proteins (RPs).

Eukaryotic ribosomes are assembled in the nucleolus before export to the cytoplasm. Ribosome formation is a highly dynamic and coordinated multistep process, which requires synthesis, processing and modification of pre-rRNAs, assembly with ribosomal proteins and transient interaction of numerous non-ribosomal factors with the evolving pre-ribosomal particles.

Ribosome release from the endoplasmic reticulum (ER) is coupled to the termination of protein translation. Thus, coincident with termination, membrane-bound ribosomes dissociate into their component subunits and are released into the cytosol.

Free ribosomes are in the cytosol. They may occur as a single ribosome or in groups known as polyribosomes or polysomes. They are in greater number than bound ribosomes in cells that retain most of their manufactured protein. Free ribosomes are responsible for proteins that go into solution in the cytoplasm or form important cytoplasmic structural or motile elements

Bound ribosomes are bound to the exterior of the endoplasmic reticulum (ER) constituting rough ER. They are in greater number than free ribosomes in cells that secrete their manufactured proteins (e.g., pancreatic cells, producers of digestive enzymes). They ar responsible for proteins that become a part of membranes or packaged into vesicles for storage in the cytoplasm or export to the cell exterior.

Ribosomes are also located in mitochondria and chloroplasts of eukaryotic cells; they are always smaller than cytoplasmic ribosomes and are comparable to prokaryotic ribosomes in both size and sensitivity to antibiotics; however, the sedimentation values s (s = Svedberg units: a measure of the rate of sedimentation of a component in a centrifuge, related both to the molecular weight and the 3-D shape of the component) vary somewhat in different phyla.

Prokaryotic and eukaryotic ribosomes perform the same functions by the same set of chemical reactions; however, eukaryotic ribosomes are much larger than prokaryotic ones and most of their proteins are different. Mitochondrial and chloroplast ribosomes resemble bacterial ribosomes.

Cells put forth considerable effort to the production of these essential organelles; for example, one E. coli cell contains about 15,000 ribosomes, each one with a molecular weight of approximately 3 x 106 daltons constituting 25% of the total mass of these bacterial cells.

Ribosme structure

Prokaryotic and eukaryotic ribosomes are very similar in their form. Small prokaryotic and eukaryotic ribosomal subunits have a head and a base with an armlike platform extending from one side as seen below left; however, additional features of the small eukaryotic ribosome subunit include a bill that extends from the head of the small subunit on the side opposite the cleft and a set of lobes at the end of the subunit opposite the head (below right); the lobes are believed to contain the additional sequences that make 18s rRNA larger that its 16s bacterial counterpart.

 Ribosomes, the large RNPs that translate mRNA into protein in the cytoplasm of eukaryotic cells, are synthesized in a subcompartment of the nucleus, the nucleolus. There, transcription by Pol I yields a pre-rRNA which is modified, cleaved and assembled with ribosomal proteins to make functional ribosomes.

Ribosomal proteins

 RPS19

Pathology (ribosomopathies)

 RPS19 (Blackfan-Diamond anemia - MIM.205900)

 Inactivation in human cells of several components of the Pes1-Bop1 complex (BOP1, GRWD1, PES1, ORC6L, and RPL3), involved in ribosomal biogenesis, altered chromosome segregation. (16804918)

Video

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See also

 ribosomopathies

References

 Steitz TA. A structural understanding of the dynamic ribosome machine. Nat Rev Mol Cell Biol. 2008 Mar;9(3):242-53. PMID: 18292779

 Granneman S, Baserga SJ. Crosstalk in gene expression: coupling and co-regulation of rDNA transcription, pre-ribosome assembly and pre-rRNA processing. Curr Opin Cell Biol. 2005 Jun;17(3):281-6. PMID: 15901498

 Tschochner H, Hurt E. Pre-ribosomes on the road from the nucleolus to the cytoplasm. Trends Cell Biol. 2003 May;13(5):255-63. PMID: 12742169

 Ruggero D, Pandolfi PP. Does the ribosome translate cancer?
Nat Rev Cancer. 2003 Mar;3(3):179-92. PMID: 12612653

 Potter MD, Seiser RM, Nicchitta CV. Ribosome exchange revisited: a mechanism for translation-coupled ribosome detachment from the ER membrane. Trends Cell Biol. 2001 Mar;11(3):112-5. PMID: 11306271