Tardigrade specific proteins

Tardigrade specific proteins are specific types of intrinsically disordered proteins that are seen in tardigrades. They are most notably used to help them survive desiccation which makes them very extremotolerant. Likely because of their flexibility, tardigrade specific proteins are strongly influenced by their environment, leading to strong changes during extreme abiotic environments.

History

The mechanism of protection by tardigrades was originally thought to be as a result of high levels of the sugar trehalose. Trehalose has been shown to help other organisms like yeast through desiccation by helping proteins that are sensitive to desiccation and keeping them in solution.[1][2] However, when tested in tardigrades, low or even no levels of trehalose is found. Research into other species that survived prolonged periods without water led to the discovery of Late Embryogenesis Abundant proteins, which provide protection to organisms like cotton seeds, which become desiccation tolerant as an embryo.[3] Trehalose is seen to accumulate in tardigrades and is not sufficient to provide protection and tardigrade specific proteins are needed for them to survive their tun state[4]

Function

Tardigrade specific proteins are a type of intrinsically disordered protein. This means that they have no specific shape unlike traditional proteins which rely on their folding to perform a specific task. There are three families of tardigrade specific proteins. They are each named after where the protein is localized within a cell. These proteins are similar to late embryogenesis abundant proteins, except for their specificity to tardigrades. The three families do not resemble each other and are found to be expressed or enriched during desiccation. Unlike traditional proteins, intrinsically disordered proteins are not found to precipitate out of solution or denature during high heat.[5] Tardigrades rely on these proteins to help them survive extreme environments, where they put their bodies in a dehydrated state called a tun. This state allows them to survive many abiotic factors like freezing and heat. The dehydration causes problems for cells, which typically rely on a hydrated environment for their proteins to perform many functions. Tardigrade specific proteins help the contents to not aggregate when first dehydrated and then maintain membrane integrity upon rehydration.

Discovery of the Cytoplasmic and Secreted Abundant Heat Soluble proteins were found when searching for late embryogenesis abundant proteins in tardigrades.[6]

Types

Cytoplasmic

Cytoplasmic abundant heat soluble proteins have been seen to be highly expressed in response to desiccation. The oldest theory in the mechanism of cytoplasmic abundant heat soluble proteins is the vitrification hypothesis in which when the organism dries, the viscosity within the cell would increase so much that denaturation and membrane fusion in proteins would stop.[7] A second theory is the water replacement theory in which the cytoplasmic abundant heat soluble proteins replaces water in the proteins, protecting the bonds that would normally be affected by the hydrogen in water.[8]

Secreted

Secreted abundant heat soluble proteins have been noted to be similar to fatty acid binding proteins, notably in their structure with an antiparallel beta-barrel and internal fatty acid binding pocket.[9][10] Denoted by their name, they are often secreted into media and often associated with special extracellular structures.[11] Dried tardigrades have been seen to have an abundance of secretory cells which when rehydrated, are not seen. The mechanism behind secreted abundant heat soluble proteins has not been determined yet but the presence of secretory cells only during desiccation leads to an understanding that there is some damage protection by the membrane.

Mitochondrial

Mitochondrial abundant heat soluble proteins are localized in the mitochondria and are responsible for protecting the mitochondria during desiccation.[12] Because of its work with reactive oxygen species, the mitochondria is an important organelle to protect in extreme environments. It has been seen that the mitochondria of desiccated tardigrades is much smaller than their rehydrated counterparts with a loss of cristae.[3] It is thought that the mitochondrial abundant heat soluble proteins act to replace water in the membrane of the mitochondria, preventing uneven rehydration and breaking of the membrane.[13]

References
  1. Tapia H, Young L, Fox D, Bertozzi CR, Koshland D (May 2015). "Increasing intracellular trehalose is sufficient to confer desiccation tolerance to Saccharomyces cerevisiae". Proceedings of the National Academy of Sciences of the United States of America. 112 (19): 6122–6127. doi:10.1073/pnas.1506415112. PMC 4434740. PMID 25918381.
  2. Bellavia G, Giuffrida S, Cottone G, Cupane A, Cordone L (May 2011). "Protein thermal denaturation and matrix glass transition in different protein-trehalose-water systems". The Journal of Physical Chemistry B. 115 (19): 6340–6346. doi:10.1021/jp201378y. PMID 21488647.
  3. Hesgrove C, Boothby TC (November 2020). "The biology of tardigrade disordered proteins in extreme stress tolerance". Cell Communication and Signaling. 18 (1): 178. doi:10.1186/s12964-020-00670-2. PMC 7640644. PMID 33148259.
  4. Boothby TC, Tapia H, Brozena AH, Piszkiewicz S, Smith AE, Giovannini I, Rebecchi L, Pielak GJ, Koshland D, Goldstein B (March 2017). "Tardigrades Use Intrinsically Disordered Proteins to Survive Desiccation". Molecular Cell. 65 (6): 975–984.e5. doi:10.1016/j.molcel.2017.02.018. PMC 5987194. PMID 28306513.
  5. Uversky VN (October 2003). "A protein-chameleon: conformational plasticity of alpha-synuclein, a disordered protein involved in neurodegenerative disorders". Journal of Biomolecular Structure & Dynamics. 21 (2): 211–34. doi:10.1080/07391102.2003.10506918. PMID 12956606. S2CID 824815.
  6. Yamaguchi A, Tanaka S, Yamaguchi S, Kuwahara H, Takamura C, Imajoh-Ohmi S, et al. (2012-08-28). "Two novel heat-soluble protein families abundantly expressed in an anhydrobiotic tardigrade". PLOS ONE. 7 (8): e44209. Bibcode:2012PLoSO...744209Y. doi:10.1371/journal.pone.0044209. PMC 3429414. PMID 22937162.
  7. Sakurai M, Furuki T, Akao K, Tanaka D, Nakahara Y, Kikawada T, et al. (April 2008). "Vitrification is essential for anhydrobiosis in an African chironomid, Polypedilum vanderplanki". Proceedings of the National Academy of Sciences of the United States of America. 105 (13): 5093–8. Bibcode:2008PNAS..105.5093S. doi:10.1073/pnas.0706197105. PMC 2278217. PMID 18362351.
  8. Crowe LM (March 2002). "Lessons from nature: the role of sugars in anhydrobiosis". Comparative Biochemistry and Physiology. Part A, Molecular & Integrative Physiology. 131 (3): 505–13. doi:10.1016/S1095-6433(01)00503-7. PMID 11867276.
  9. Fukuda Y, Miura Y, Mizohata E, Inoue T (August 2017). "Structural insights into a secretory abundant heat-soluble protein from an anhydrobiotic tardigrade, Ramazzottius varieornatus". FEBS Letters. 591 (16): 2458–2469. doi:10.1002/1873-3468.12752. PMID 28703282. S2CID 3434502.
  10. Fukuda Y, Inoue T (May 2018). "Crystal structure of secretory abundant heat soluble protein 4 from one of the toughest "water bears" micro-animals Ramazzottius Varieornatus". Protein Science. 27 (5): 993–999. doi:10.1002/pro.3393. PMC 5916119. PMID 29493034.
  11. Richaud M, Le Goff E, Cazevielle C, Ono F, Mori Y, Saini NL, et al. (March 2020). "Ultrastructural analysis of the dehydrated tardigrade Hypsibius exemplaris unveils an anhydrobiotic-specific architecture". Scientific Reports. 10 (1): 4324. Bibcode:2020NatSR..10.4324R. doi:10.1038/s41598-020-61165-1. PMC 7062702. PMID 32152342.
  12. Tanaka S, Tanaka J, Miwa Y, Horikawa DD, Katayama T, Arakawa K, et al. (2015-02-12). "Novel mitochondria-targeted heat-soluble proteins identified in the anhydrobiotic Tardigrade improve osmotic tolerance of human cells". PLOS ONE. 10 (2): e0118272. Bibcode:2015PLoSO..1018272T. doi:10.1371/journal.pone.0118272. PMC 4326354. PMID 25675104.
  13. Popova AV, Hundertmark M, Seckler R, Hincha DK (July 2011). "Structural transitions in the intrinsically disordered plant dehydration stress protein LEA7 upon drying are modulated by the presence of membranes". Biochimica et Biophysica Acta (BBA) - Biomembranes. 1808 (7): 1879–87. doi:10.1016/j.bbamem.2011.03.009. PMID 21443857.
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