The ice-binding proteins of a snow alga, Chloromonas brevispina: probable acquisition by horizontal gene transfer.

All ice-and snow-related unicellular algae examined so far secrete ice-binding proteins (IBPs) to mitigate freezing damage. Two types of IBP have been identified in chlorophytes. Type 1 IBPs are members of a large family of proteins that share a large domain of unknown function (DUF3494). Previous studies have suggested that the type 1 algal IBP genes were acquired by horizontal gene transfer. To test this hypothesis I sequenced the IBP genes of a snow alga, Chloromonas brevispina. The IBPs were identified by ice affinity purification, de novo sequencing of a tryptic peptide and large-scale sequencing of the transcriptome and genome. C. brevispina has genes for over 20 IBP isoforms, which strongly indicates their importance. The IBPs are all of type 1 and match fungal and bacterial proteins more closely than they match known algal IBPs, providing further evidence that the genes were acquired by horizontal transfer. Modeling of the 3D structures of the IBPs based on the known structure of a homologous protein suggests that the ice-binding site has characteristics that are shared by all DUF3494 proteins.