Helicity of a tardigrade disordered protein promotes desiccation tolerance

Abstract

In order to survive extreme drying (anhydrobiosis), many organisms, spanning every kingdom of life, accumulate intrinsically disordered proteins (IDPs). For decades, the ability of anhydrobiosis-related IDPs to form transient amphipathic helices has been suggested to be important for promoting desiccation tolerance. However, evidence empirically supporting the necessity and/or sufficiency of helicity in mediating anhydrobiosis is lacking. Here we demonstrate that the linker region of CAHS D, a desiccation-related IDP from tardigrades that contains significant helical structure, is the protective portion of this protein. Perturbing the sequence composition and grammar of the linker region of CAHS D, through the insertion of helix-breaking prolines, modulating the identity of charged residues, sequence scrambling, or replacement of hydrophobic amino acids with serine or glycine residues results in variants with different degrees of helical structure. Importantly, the resulting helicity of these variants generated through similar helix breaking modalities correlates strongly with their ability to promote desiccation tolerance, providing direct evidence that helical structure is necessary for robust protection conferred by this desiccation-related IDP. However, correlation of protective capacity and helical content in variants generated through different helix perturbing modalities do not show as strong a trend, suggesting that while helicity is important it is not the only property that makes a protein protective during desiccation. These results provide direct evidence for the decades old theory that helicity of desiccation-related IDPs is linked to their anhydrobiotic capacity.

Structural adaptability and surface activity of tardigrade-inspired peptides

“…Interestingly, CAHS1M1 adopts mainly a helical configuration already at 0 % TFE. Upon mimicking desiccation (either by adding TFE or by exposing the peptides to a water/air interface), we observed that all model peptides adopt helical structures, similar to what is observed for the entire protein (5,15). The control peptide, however, does not show any tendency to form a helical structure, only a slightly more propensity to bend.…”

“…We note that the control peptide, expected to have different physicochemical properties, has a more ordered structure, consisting of a bend/turn that is stabilized under drying conditions. Furthermore, sequence-structure analysis suggests that specific patterns in the sequence, such as alternating of positive and negative charges, might be crucial in stabilizing alpha-helical conformations, as suggested in literature (15,16). Finally, our observations indicate that the four model peptides not only exhibit surface activity but also, consistent with bulk findings, adopt a helical structure under partially dehydrated conditions, suggesting that this conformation is the most preferred in these circumstances.…”