Cryoprotective activity of Arabidopsis KS-type dehydrin depends on the hydrophobic amino acids of two active segments

Dehydrins are intrinsically disordered proteins which are related to cold tolerance in plants. Dehydrins show potent cryoprotective activities for freeze-sensitive enzymes such as lactate dehydrogenase (LDH). Previous studies demonstrated that K-segments conserved in dehydrins had cryoprotective activities and that K-segment activities depended on the hydrophobic amino acids in the segment. However, the cryoprotective roles of hy-drophobic amino acids in dehydrin itself have not been reported. Here, we demonstrated that hydrophobic amino acids were required for the cryoprotective activity of Arabidopsis dehydrin AtHIRD11. Cryoprotective activities were compared between AtHIRD11 and the corresponding mutant in which all hydrophobic residues were changed to T (AtHIRD1 Phi/T) by using LDH. The change strikingly reduced AtHIRD11 activity. A seg-mentation analysis indicated that the conserved K-segment (Kseg) and a previously unidentified segment (non-K-segment 1, NK1) showed cryoprotective activities. Circular dichroism indicated that the secondary structures of all peptides showed disorder, but only cryoprotective peptides changed to the ordered forms by sodium dodecyl sulfate. Ultracentrifuge analysis indicated that AtHIRD11 and AtHIRD1 Phi/T had similar molecular sizes in solution. These results suggest that not only structural disorder but also hydrophobic amino acids contributed to the cryoprotective activity of AtHIRD11. A possible mechanism based on an extended molecular shield model is proposed.