Ice-binding proteins isolated from Arctic microalgae play a role in freezing tolerance and its regulatory pathway could be associated with AP2 domain containing transcription factor

Abstract

Polar microalgae have built their own strategies to protect the cells under an environment with repeated freeze-thaw cycles of ice crystals. Microalgal ice-binding proteins (IBPs) are poorly understood at the genome-wide level, although they are essential for cold adaptation. We examined the biological functions of IBPs in the Arctic green alga Chloromonas sp. KNF0032. We identified six IBP genes (called CmIBPs) in a transcriptome study and characterized their structural features. Progressively CmIBP genes were more expressed at low temperature. The CmIBPs were predicted to have β-solenoid forms with parallel β-sheets with repeated TXT motifs. CmIBP1 showed effective ice recrystallization inhibition (IRI) activities both in vitro and in vivo. CmIBP2 and CmIBP3 had moderate or low IRI effects as recombinant proteins, and transgenic plants containing them showed a freezing tolerance phenotype after cold acclimation with increased protein stability. The repeated TXT structure of CmIBPs appears similar to the AidA domain-containing adhesin protein from methanogens. Additionally, it is highlighted that they were organized with multi-exon/intron structures and plant type low-temperature-responsive cis-elements in their promoters. These results suggest that low temperature response pathway in microalgae could be resembled with plants. The regulatory pathway associated with AP2 domain containing transcription factors will be discussed.