Team:SIAT-SCIE/Mechanism

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CAHS (Cytosolic Abundant Heat Soluble Protein), is a group of heat soluble protein under the category “Tardigrade Specific Intrinsically Disordered Protein”.

Tardigrade is well known for its ability in survival under extreme conditions, one of Tardigrade’s extremotolerance is desiccation tolerance, anhyrdrobiosis, it can enter an ametabolic for a decade after it is dehydrated to only 3% of water content. People thought their ability was given by trehalose, a candidate for desiccation protecting material.

However, the amount of trehalose was low and sometimes unseen in different species, so people starting to find more possible related molecules.

Late Embryogenesis Abundant protein was seen as a candidate for desiccation tolerance. Late Embryogenesis Abundant protein was first found in cotton, the protein was found to protect cotton seeds from dehydration, low temperature and high concentration solution. Moreover, Late Embryogenesis Abundant proteins were also found in various organisms, like D.radiodurans, C.elegans, Artemia and rotifers. In tardigrades, however, the relevance was still unclear between Late Embryogenesis Abundant protein and anhydrobiosis, report of the appearance of Late Embryogenesis Abundant protein was limited as well, so scientist started to dig for some other proteins. Late Embryogenesis protein was found soluble after it was heat shocked, its heat solubility was seen as some connections with its desiccation protection ability.

Scientists focused on the heat soluble characteristic, tried to identify any other protein which is relevant to tardigrade’s anhydrobiosis.

Finally, a family of heat soluble protein was found in tardigrades, they were no sequence similarity to the Late Embryogenesis Abundant proteins, and variously found in tardigrades. It was named by where they were found, Cytosolic Abundant Heat Soluble (CAHS) protein, Secretory Abundant Heat Soluble (SAHS) protein and Mitochondrial Abundant Heat Soluble (MAHS) protein. These proteins were then characterized as Intrinsically Disordered Protein (IDP), and then, they called them Tardigrade Specific Intrinsically Disordered Protein (TDP).

The IDPs are disordered, which they do not have a fixed structure, unless they are attached to a macromolecule, which is critical to its function.

The entire mechanism of this protein family remains unknown, but still, scientists discovered some characteristic of this protein family.

Most significant one is the conformational change of the protein structure under water deficient condition, some motif will change from a β-sheet to an α-helix. On CAHS, more specifically, a conserved 19-mer motif will change to an amphophilic α-helix, compare to the Late Embryogenesis Abundant protein’s LEA motif, it has an extra negative charged region, which makes the α-helix more narrow, excluding its interaction with static electricity interaction. On SAHS, there is a signal peptide on its N-terminus, which is used as a tag for exocytotic secretion. From research, the affinity between the SAHS protein and the cell membrane is low, so it is not only for membrane protection, but for the entire tissue. On MAHS, there is a mitochondrial localizing peptide, which is for localizing the protein, to specifically protect mitochondria. In the middle of the protein, there is a conserved region, forming an α-helix, which might be similar to the CAHS and SAHS.

There are to hypothesis of the mechanism of TDPs, the vitrification hypothesis and the water replacement hypothesis.

The vitrification hypothesis claimed that, the proteins will form a glass like matrix in the cell, under water deficient conditions. The matrix can embed cell organelles, which can prevent them from aggregation together, or denature of proteins, or the fusion of cell membrane, under water deficient conditions, This biological glass has its maximum tolerance temperature, the glass transition temperature of this protein is 65°C, when temperature goes beyond 65°C, it will enters a rubbery state, which the biological glass matrix could not recover, unlike they can dissolve in water, under room temperature. It had been detected that there was glass formed when Tardigrades enters anhydrobiosis state.

The water replacement hypothesis is based on its similarity to trehalose. Trehalose was proved that there were hydrogen bonds formed, when water is lost, between trehalose and the macromolecules. The hydrogen bonds basically sustain the dimensional position of each macromolecules, so the newly formed bonds can prevent protein denaturation, aggregation and cell membrane fusion as well. There are hydrogen bonds formed between trehalose and cell membrane, under water deficient conditions, and the cell membrane is under and liquid crystalline state, but there is no report, yet, about the hydrogen bonds formed between TDPs and the macromolecules.

Although the two hypothesizes are not fully proved, but scientist thought that the vitrification and the water replacement plays a part of each other.