# A previously unrecognized class of fungal ice-nucleating proteins with bacterial ancestry

## TL;DR

Scientists discovered a new type of ice-nucleating proteins in fungi that function similarly to bacterial proteins, likely due to a shared evolutionary origin.

## Contribution

Identification of a novel class of fungal ice-nucleating proteins with bacterial ancestry through structural and phylogenetic analysis.

## Key findings

- Fungal proteins from Mortierellaceae efficiently promote ice formation without membranes.
- Structural modeling suggests β-solenoid folds and multimerization in fungal INpros.
- Fungal INpros are orthologs of bacterial InaZ, implying horizontal gene transfer.

## Abstract

Ice-nucleating proteins (INpros) catalyze ice formation at high subzero temperatures, with major biological and environmental implications. While bacterial INpros have been structurally characterized, their counterparts in other organisms have remained largely unknown. Here, we identify membrane-independent proteins in fungi of the Mortierellaceae family that promote ice formation with high efficiency. These proteins are predicted to adopt β-solenoid folds and multimerize to form extended ice-binding surfaces, exhibiting mechanistic parallels with bacterial INpros. Structural modeling, phylogenetic analysis, and heterologous gene expression leading to ice nucleation in Escherichia coli and Saccharomyces cerevisiae show that the fungal INpros are encoded by orthologs of the bacterial InaZ gene, which was likely acquired by a fungal ancestor through horizontal gene transfer. The discovery of cell-free fungal INpros provides tools for innovative freezing applications and reveals biophysical constraints on ice nucleation across life.

Discovery of potent soluble fungal ice nucleators uncovers a conserved molecular approach to promoting ice formation.