Tardigrade specific proteins are types of intrinsically disordered proteins specific to tardigrades. These proteins help tardigrades survive desiccation, one of the adaptations which contribute to tardigrade's extremotolerant nature. Tardigrade specific proteins are strongly influenced by their environment, leading to adaptive malleability across a variety of extreme abiotic environments.

History

The mechanisms of tardigrade desiccation protection were originally thought to result from high levels of the sugar trehalose. Trehalose is used by organisms like yeast to avoid desiccation in dry environments by working with heat shock proteins[1] to keep desiccation-sensitive proteins in solution.[2][3] However, while tardigrades can accumulate small levels of trehalose, the levels are insufficient to provide protection from extreme conditions.[4] Other molecules which help certain organisms avoid cellular desiccation include late embryogenesis abundant proteins, which provide protection to embryonic cotton seeds.[5] Certain proteins actually responsible for the tardigrade's hardiness, including the cytoplasmic and secreted abundant heat soluble proteins, were discovered when searching for late embryogenesis abundant proteins in tardigrades.[6]

One strategy used by the tardigrade to survive in dry environments is anhydrobiosis. Anhydrobiosis is a process in which an organism can lose nearly all of its water and enter an ametabolic state.[7]

Function

Tardigrade specific proteins are a type of intrinsically disordered proteins, which have no predetermined shape or task. These proteins use many different conformations, called an ensemble, to move through different structures. Because of this, intrinsically disordered proteins may react strongly to the environment they inhabit.[8] There are three families of tardigrade specific proteins, each named after where the protein is localized within a cell. These proteins are similar to late embryogenesis abundant proteins but are specific to tardigrades. The three families do not resemble each other and are expressed or enriched during desiccation. Unlike traditional proteins, intrinsically disordered proteins do not precipitate out of solution or denature during high heat.[9] Tardigrades rely on these proteins to help them survive extreme environments, where they put their bodies in a dehydrated state called a tun. In most organisms, dehydration causes problems for cells, which need a hydrated environment for their proteins to function. However, tardigrade specific proteins assist in preventing aggregation of cell contents upon dehydration, and maintain the integrity of the cell membrane upon rehydration.

Types

Cytoplasmic

Cytoplasmic abundant heat soluble (CAHS) proteins are highly expressed in response to desiccation. There are two hypotheses for their function in tardigrades. The vitrification hypothesis is the idea that, when a tardigrade becomes desiccated, the viscosity within its cells increases to the point that denaturation and membrane fusion in proteins would stop.[10] A second hypothesis, the water replacement hypothesis, posits that CAHS proteins replace water in other desiccation-sensitive proteins, protecting the hydrogen bonds normally reliant on water.[11] CAHS proteins are dispersed throughout the cell in normal conditions, but form a network of filaments during environmentally stressful conditions. This network transforms the cytoplasm into a gel-like matrix and prevents the cell from collapsing as water leaches out.[12] This state is reversible and the proteins disaggregate when exposed to less stressful conditions.[13]

When forming the filament network, CAHS proteins have long helical domains that interact in a coiled manner with each other. These interactions are possible due to the proteins' partial disorder, with two flexible tails surrounding the helical domains.[14]

CAHS proteins have been studied to observe their interactions with trehalose, a sugar used by other species to prevent desiccation. Trehalose was found to interact at higher levels with CAHS proteins than other sugars such as sucrose.[15] However, the exact functions of trehalose inside tardigrade cells are still unknown.

Secreted

Secreted abundant heat soluble (SAHS) proteins are similar to fatty acid-binding proteins, notably in their structure with an antiparallel beta-barrel and internal fatty acid binding pocket.[16][17] SAHS proteins are often secreted into media and associated with special extracellular structures.[18] Dried tardigrades have an abundance of secretory cells which are not found in hydrated individuals. The mechanism behind SAHS proteins has not yet been determined, but the presence of secretory cells only during desiccation suggests they are used to protect cells during periods of dehydration.

Mitochondrial

Mitochondrial abundant heat soluble (MAHS) proteins are localized in mitochondria and are responsible for protecting mitochondria during desiccation.[19] Because of its role in metabolizing reactive oxygen species, the mitochondrion is an important organelle to protect in extreme environments. During dehydration, the mitochondria of tardigrades grow much smaller and lose their cristae.[5] MAHS proteins may act to replace water in the membrane of the mitochondria, preventing uneven rehydration and membrane rupture.[20]

References

  1. ↑ Kim SX, Çamdere G, Hu X, Koshland D, Tapia H (July 2018). Storz G, Hyman AA (eds.). "Synergy between the small intrinsically disordered protein Hsp12 and trehalose sustain viability after severe desiccation". eLife. 7: e38337. doi:10.7554/eLife.38337. PMC 6054528. PMID 30010539.
  2. ↑ 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. Bibcode:2015PNAS..112.6122T. doi:10.1073/pnas.1506415112. PMC 4434740. PMID 25918381.
  3. ↑ 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.
  4. ↑ Boothby TC, Tapia H, Brozena AH, Piszkiewicz S, Smith AE, Giovannini I, et al. (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. 1 2 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.
  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. ↑ Giovannini I, Boothby TC, Cesari M, Goldstein B, Guidetti R, Rebecchi L (February 2022). "Production of reactive oxygen species and involvement of bioprotectants during anhydrobiosis in the tardigrade Paramacrobiotus spatialis". Scientific Reports. 12 (1): 1938. Bibcode:2022NatSR..12.1938G. doi:10.1038/s41598-022-05734-6. PMC 8816950. PMID 35121798.
  8. ↑ Moses D, Yu F, Ginell GM, Shamoon NM, Koenig PS, Holehouse AS, Sukenik S (December 2020). "Revealing the Hidden Sensitivity of Intrinsically Disordered Proteins to their Chemical Environment". The Journal of Physical Chemistry Letters. 11 (23): 10131–10136. doi:10.1021/acs.jpclett.0c02822. PMC 8092420. PMID 33191750.
  9. ↑ 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): 211–234. doi:10.1080/07391102.2003.10506918. PMID 12956606. S2CID 824815.
  10. ↑ 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): 5093–5098. Bibcode:2008PNAS..105.5093S. doi:10.1073/pnas.0706197105. PMC 2278217. PMID 18362351.
  11. ↑ Crowe LM (March 2002). "Lessons from nature: the role of sugars in anhydrobiosis". Comparative Biochemistry and Physiology. Part A, Molecular & Integrative Physiology. 131 (3): 505–513. doi:10.1016/S1095-6433(01)00503-7. PMID 11867276.
  12. ↑ Tanaka A, Nakano T, Watanabe K, Masuda K, Honda G, Kamata S, et al. (September 2022). "Stress-dependent cell stiffening by tardigrade tolerance proteins that reversibly form a filamentous network and gel". PLOS Biology. Live Science. 20 (9): e3001780. doi:10.1371/journal.pbio.3001780. PMC 9592077. PMID 36067153.
  13. ↑ Tanaka A, Nakano T, Watanabe K, Masuda K, Honda G, Kamata S, et al. (September 2022). "Stress-dependent cell stiffening by tardigrade tolerance proteins that reversibly form a filamentous network and gel". PLOS Biology. 20 (9): e3001780. doi:10.1371/journal.pbio.3001780. PMC 9592077. PMID 36067153.
  14. ↑ Malki A, Teulon JM, Camacho-Zarco AR, Chen SW, Adamski W, Maurin D, et al. (January 2022). "Intrinsically Disordered Tardigrade Proteins Self-Assemble into Fibrous Gels in Response to Environmental Stress". Angewandte Chemie. 61 (1): e202109961. doi:10.1002/anie.202109961. PMC 9299615. PMID 34750927.
  15. ↑ Nguyen K, Kc S, Gonzalez T, Tapia H, Boothby TC (October 2022). "Trehalose and tardigrade CAHS proteins work synergistically to promote desiccation tolerance". Communications Biology. 5 (1): 1046. doi:10.1038/s42003-022-04015-2. PMC 9526748. PMID 36182981.
  16. ↑ 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.
  17. ↑ 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.
  18. ↑ 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.
  19. ↑ 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.
  20. ↑ 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): 1879–1887. doi:10.1016/j.bbamem.2011.03.009. PMID 21443857.
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