Fungi proteins help water freeze more easily, study finds

An international research team led by Konrad Meister from Boise State University's Department of Chemistry and Biochemistry and the Max Planck Institute for Polymer Research has identified a new class of ice crystal-forming proteins in lower fungi.

The study shows that fungi of the family Mortierellaceae use a genetic blueprint that originates from bacteria. Unlike bacteria, however, the fungi use the gene to form water-soluble proteins. This structural adaptation explains the high stability and efficiency of ice formation by the fungi. According to the researchers, the fungal proteins are promising for applications in the field of freezing technology.

• Genetic origin: The study shows that the fungi produce proteins that originally come from bacteria to freeze ice.
• Ice nucleation: The ability to nucleate ice could be evolutionarily important for microorganisms, offering them survival advantages in the atmosphere.
• Practical applications: The newly identified proteins could be important in the cryopreservation of cells and organs, in food processing, and in snow production.

Water freezes at 0 °C - at least according to school textbooks. But under ideal conditions, pure water remains liquid down to a temperature of 40 °C. A small shock or a dust particle is then enough to cause the liquid to abruptly and suddenly turn into ice. Some types of bacteria are also good ice formers because they produce special proteins that promote freezing at temperatures around 0 °C. For example, proteins from the bacterium Pseudomonas syringae cause water to freeze better than any other known material. Such ice nucleation proteins are found not only in bacteria, but also in some fungi. While the structure of bacterial proteins has been well studied, that of fungi has remained unclear until now.

The international team led by Meister describes for the first time a new class of ice nucleation proteins from the Mortierellaceae family of fungi. This family belongs to the lower fungi, which also include yeasts. Researchers from the Max Planck Institute for Chemistry and researchers at Boise State, including Biomolecular Sciences Ph.D. students Rosemary Eufemio and Kaden Shaw, collaborated to carry out the work.

To uncover the structure of the fungal proteins, the researchers sequenced the genomes of ice-active fungi isolated from water samples and lichens collected during previous polar expeditions. In doing so, they discovered genes closely related to a gene already known from ice-active bacteria such as Pseudomonas syringae: The gene InaZ is the template for makin ice nucleation proteins.

Gene transfer across species

However, during structural analysis, the researchers discovered significant differences. Unlike the bacterial proteins, which must be embedded in a membrane to function, fungal proteins are water-soluble and unusually stable.

Based on phylogenetic analyses - i.e., analyses of the origin of a gene - the team concluded that the InaZ gene was most likely transferred from bacteria to a fungal ancestor across species in the distant past. Instead of developing ice nucleation independently, the fungi adopted a highly effective trait of the bacteria and adapted it to their own physiological requirements.

"It's a bit the same and yet different," Eufemio said. "Fungi use the same repetitive sequence architecture as bacteria for their ice-forming sites, but have made them more soluble and stable, which probably benefits their ecological function."

To prove that the identified fungal genes are indeed the template for ice-nucleating proteins, the research team transferred two of the identified genes into non-ice-active yeast and bacteria. The modified microorganisms then became ice-active, which confirmed the functional connection.

Applications in cryopreservation conceivable

In addition to the biological significance of the discovery, Meister also sees concrete practical applications in technologies based on controlled freezing. "Soluble ice-nucleating proteins are easier to isolate, handle and integrate into formulations and technological processes than membrane-bound ones. This opens up new possibilities for controlled freezing in the cryopreservation of cells and organs, food processing and snow production."

Ice nucleation: Properties and significance

The ability to nucleate ice - i.e., to form ice nuclei in a targeted manner - is of great evolutionary importance for certain microorganisms. It gives them survival advantages, especially in the atmosphere. When ice forms in clouds, the frozen droplets fall to earth as precipitation. "This allows bacteria and fungal spores to be transported over long distances and reach new habitats such as plant surfaces, soils or other geographical regions," explained biologist and Earth system researcher Janine Fröhlich from the Max Planck Institute for Chemistry.

A well-known example is Pseudomonas syringae, which is commonly found on plant leaves. By triggering ice formation on the leaf cells, it causes frost damage. This causes plant sap to leak out, which serves as a nutrient source for the bacteria - in other words, they deliberately damage the plant in order to feed.

In addition, ice-nucleating bacteria have a climatic significance: they are among the most effective natural triggers of ice formation in clouds and can thus influence precipitation, weather events and the global water cycle.

The researchers report their findings in the study "A Previously Unrecognized Class of Fungal Ice-Nucleating Proteins with Bacterial Ancestry" in Science Advances.

A version of this press release was originally produced by the Max Planck Institute.

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