New Cysteine-Rich Ice-Binding Protein Secreted from Antarctic Microalga, <i>Chloromonas</i> sp.

<div><p>Many microorganisms in Antarctica survive in the cold environment there by producing ice-binding proteins (IBPs) to control the growth of ice around them. An IBP from the Antarctic freshwater microalga, <i>Chloromonas</i> sp., was identified and characterized. The length of the <i>Chloromonas</i> sp. IBP (<i>ChloroIBP</i>) gene was 3.2 kb with 12 exons, and the molecular weight of the protein deduced from the <i>ChloroIBP</i> cDNA was 34.0 kDa. Expression of the <i>ChloroIBP</i> gene was up- and down-regulated by freezing and warming conditions, respectively. Western blot analysis revealed that native ChloroIBP was secreted into the culture medium. This protein has fifteen cysteines and is extensively disulfide bonded as shown by in-gel mobility shifts between oxidizing and reducing conditions. The open-reading frame of <i>ChloroIBP</i> was cloned and over-expressed in <i>Escherichia coli</i> to investigate the IBP’s biochemical characteristics. Recombinant ChloroIBP produced as a fusion protein with thioredoxin was purified by affinity chromatography and formed single ice crystals of a dendritic shape with a thermal hysteresis activity of 0.4±0.02°C at a concentration of 5 mg/ml. <i>In silico</i> structural modeling indicated that the three-dimensional structure of ChloroIBP was that of a right-handed β-helix. Site-directed mutagenesis of <i>ChloroIBP</i> showed that a conserved region of six parallel T-X-T motifs on the β-2 face was the ice-binding region, as predicted from the model. In addition to disulfide bonding, hydrophobic interactions between inward-pointing residues on the β-1 and β-2 faces, in the region of ice-binding motifs, were crucial to maintaining the structural conformation of ice-binding site and the ice-binding activity of ChloroIBP.</p>