Abstract

Though anhydrobiotic tardigrades (micro‐animals also known as water bears) possess many genes of secretory abundant heat soluble (SAHS) proteins unique to Tardigrada, their functions are unknown. A previous crystallographic study revealed that a SAHS protein (RvSAHS1) from one of the toughest tardigrades, Ramazzottius varieornatus, has a β‐barrel architecture similar to fatty acid binding proteins (FABPs) and two putative ligand binding sites (LBS1 and LBS2) where fatty acids can bind. However, some SAHS proteins such as RvSAHS4 have different sets of amino acid residues at LBS1 and LBS2, implying that they prefer other ligands and have different functions. Here RvSAHS4 was crystallized and analyzed under a condition similar to that for RvSAHS1. There was no electron density corresponding to a fatty acid at LBS1 of RvSAHS4, where a putative fatty acid was observed in RvSAHS1. Instead, LBS2 of RvSAHS4, which was composed of uncharged residues, captured a putative polyethylene glycol molecule. These results suggest that RvSAHS4 mainly uses LBS2 for the binding of uncharged molecules.

Highlights

  • One of Secretory abundant heat soluble (SAHS) proteins from R. varieornatus (RvSAHS1) is secreted into the culture medium when it is expressed in human cells; SAHS proteins are thought to protect extracellular components and/or secretory organelles on anhydrobiosis.[23]

  • Residues found in the b-barrel of RvSAHS1 were bulky and hydrophilic, while smaller and/or hydrophobic residues are assembled in fatty acid binding proteins (FABPs)

  • The overall structure of RvSAHS4 shared a typical FABP fold having an antiparallel b-barrel composed of 10 b-strands, and a helix-turn-helix lid between bA and bB

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Summary

Introduction

Water is indispensable for all living things. severe loss of water results in death for almost all organisms. Results and Discussion The unit cell of the RvSAHS4 crystal contained two RvSAHS4 molecules (MolA and MolB) [Fig. 1(B)]. Because main chain atoms could not form hydrogen bonds between bD and bE, there was a gap as is found in RvSAHS1 and FABPs.[28] The N-terminal region of the adjacent molecule was inserted into this gap in the crystal structure [Fig. 1(B)].

Results
Conclusion

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