An ice-binding protein from the glacier ice alga Ancylonema nordenskioeldii

Abstract

Response to Bowles et al. (2024) ‘Metagenome-assembled genome of the glacier alga Ancylonema yields insights into the evolution of streptophyte life on ice and land’

As plants began to colonize the land c. 470–450 million years ago, they had to overcome many abiotic stresses not experienced by their marine ancestors (Rensing, 2018). One such stress was freezing and thawing, which can damage plant cell walls. Bacteria, which had established their presence on land well before the arrival of plants, greatly aided the transition of plants to land by the horizontal transfer of key genes (Yue et al., 2012; Ma et al., 2020). Bacteria are likely donors of genes that can mitigate freeze–thaw injury, as proteins with ice-binding activity have been found in several bacteria (Raymond et al., 2007, 2008; Vance et al., 2018). Each of the proteins in those studies contains a c. 200-aa domain called DUF3494 that has a beta solenoid structure with one side that binds to ice crystals (Vance et al., 2019). At very low concentrations, these proteins can drastically prevent the recrystallization of ice that occurs during thawing, which is thought to damage cell walls. Similar proteins have been identified in hundreds of species of bacteria, although not all of them have been examined for ice-binding activity. Horizontally acquired genes of this type appear to be the source of freeze–thaw tolerance in a number of algae and fungi that live in icy habitats (Raymond & Kim, 2012).

Zygnematophyceae, being the closest known relatives of all land plants (Cheng et al., 2019), are of interest because of their remarkable ability to find solutions for the stresses encountered by early land plants (Kunz et al., 2024). In this vein, Bowles et al. (2024) recently obtained the metagenome of algae inhabiting the Morteratsch Glacier in Switzerland to investigate the adaptation of the early streptophyte alga Ancylonema nordenskioeldii to life in ice. Among the survival mechanisms investigated, the authors looked for genes encoding ice-binding proteins (IBPs). They found several candidates in the protein kinase superfamily, ATP-binding cassette protein family and heat shock protein family, although none were confirmed to have ice-binding activity. Apparently, they did not see our earlier paper in which we identified an IBP in A. nordenskioeldii (AnIBP) from the Morteratsch Glacier (Procházková et al., 2024). Here, we summarize the main findings of this paper, in which we showed that AnIBP has ice-binding activity and that this activity could be attributed to a protein of the DUF3494 family.