(cell biology) A nuclear body containing proteasomes and linked to ubiquitin-proteasome system where it serves as a site for protein degradation
In eukaryotic cell, the nucleus is the organelle responsible for maintaining the integrity of DNA and for controlling cellular activities such as metabolism, growth, and reproduction by regulating gene expression. It is a double-membraned organelle containing nuclear structures, e.g. chromatin and nuclear bodies. A nuclear body refers to any of the prominent non-membraned structures in the nucleus of the eukaryotic cell. Nuclear bodies are prominent structures of non-chromatinic fibrillary material and mostly proteinaceous. Examples of nuclear bodies are nucleolus, Cajal bodies and gems (Gemini of Cajal bodies), polymorphic interphase karyosomal association (PIKA) domains, promyelocytic leukaemia protein (PML) bodies, splicing speckles, paraspeckles, perichromatin fibrils, and clastosomes.
Clastosomes are a type of nuclear body whose name is derived from the Greek klastos, meaning broken and soma, meaning body. The diameter of a clastosome ranges from 0.2 µm to 1.2 µm. Due to the peripheral capsule forming around it, it has a characteristic thick ring-shape. Clastosomes contain a high concentration of 1) ubiquitin conjugates, 2) the proteolytically active 20S core and the 19S regulatory complexes of the 26S proteasome, and 3) protein substrates of the proteasome. Although detected in a variety of cell types, clastosomes are scarce under normal conditions; however, they become more abundant when proteasomal activity is stimulated. Osmotic stress has also been suggested to cause the increase of clastosomes.
Clastosomes become ubiquitous during proteolytic conditions. Thus, they are said to have a role in proteolysis (protein degradation) inside the nucleus. Nevertheless, they are not required for proteasome activity as it was observed to proceed despite the scarcity of clastosomes. Proteasomal inhibition, though, causes the disassembling of clastosomes.
Common biological reactions
In eukaryotes, two major systems involved in proteolysis (i.e. a protein degradation process by breaking peptide bonds) are the lysosomes and the proteasomes. Proteasomes are proteolytic complexes that degrade cytosolic and nuclear proteins via the ATP-dependent ubiquitin protein complex degradation. They degrade proteins, particularly those tagged by the protein ubiquitin. Misfolded proteins, short-lived regulatory proteins are examples of types of proteins targeted by ubiquitin-proteasome protein degradation. Polyubiquitination, i.e. tagging of target protein by a chain of ubiquitins, is catalyzed by the enzymes ubiquitin ligases. The protein tagged by ubiquitin is subject for proteasome degradation. Proteasomes are present in eukaryotes, archaea, and certain bacteria. In eukaryotes, proteasomes are also present in the nucleus, e.g. in clastosomes. Since clastosomes contain proteasomes they are therefore implicated in the protein degradation inside the nucleus.
- Carmo-Fonseca, M., Berciano, M. T., & Lafarga, M. (September 2010). "Orphan nuclear bodies". Cold Spring Harbor Perspectives in Biology. 2 (9): a000703. doi:10.1101/cshperspect.a000703
- Lafarga, M., Berciano, M. T., Pena, E., Mayo, I., Castaño, J. G., Bohmann, D., Rodrigues, J. P., Tavanez, J. P., & Carmo-Fonseca, M. (August 2002). "Clastosome: a subtype of nuclear body enriched in 19S and 20S proteasomes, ubiquitin, and protein substrates of proteasome". Molecular Biology of the Cell. 13 (8): 2771–82. doi:10.1091/mbc.e02-03-0122
- Sampuda, K. M., Riley, M., & Boyd, L. (April 2017). "Stress induced nuclear granules form in response to accumulation of misfolded proteins in Caenorhabditis elegans". BMC Cell Biology. 18 (1): 18. doi:10.1186/s12860-017-0136-x
- Carmo-Fonseca, M., Berciano, M. T., & Lafarga, M. (2010). Orphan Nuclear Bodies. Cold Spring Harbor Perspectives in Biology, 2(9), a000703–a000703. https://doi.org/10.1101/cshperspect.a000703
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