Abstract
Ice-nucleating proteins (INPs) found in bacteria are the most effective ice nucleators known, enabling the crystallization of water at temperatures close to 0 °C. Although their function has been known for decades, the underlying mechanism is still under debate. Here, we show that INPs from Pseudomonas syringae in aqueous solution exhibit a defined solution structure and show no significant conformational changes upon cooling. In contrast, irreversible structural changes are observed upon heating to temperatures exceeding ∼55 °C, leading to a loss of the ice-nucleation activity. Sum-frequency generation (SFG) spectroscopy reveals that active and heat-inactivated INPs impose similar structural ordering of interfacial water molecules upon cooling. Our results demonstrate that increased water ordering is not sufficient to explain INPs' high ice-nucleation activity and confirm that intact three-dimensional protein structures are critical for bacterial ice nucleation, supporting a mechanism that depends on the INPs' supramolecular interactions.
Highlights
Ice-nucleating proteins (INPs) found in bacteria are the most effective ice nucleators known, enabling the crystallization of water at temperatures close to 0 °C
Ice crystals can be formed either by homogeneous nucleation at lower temperatures or by heterogeneous nucleation catalyzed by compounds that serve as ice nucleators (IN)
They are typically present as monomers but have repeatedly been shown to aggregate in the bacterial outer membranes.[5−8] Large INP aggregates are thought to be responsible for freezing at temperatures between −2 and −4 °C and smaller INP aggregates at temperatures between −7 and −12 °C.9
Summary
Ralph Schwidetzky − Max Planck Institute for Polymer Research, 55128 Mainz, Germany. Ellen H.G. Backus − Max Planck Institute for Polymer Research, 55128 Mainz, Germany; Department of Physical Chemistry, University of Vienna, 1090 Vienna, Austria; orcid.org/0000-0002-6202-0280. Ulrich Pöschl − Max Planck Institute for Chemistry, 55128 Mainz, Germany; orcid.org/0000-0003-1412-3557. Mischa Bonn − Max Planck Institute for Polymer Research, 55128 Mainz, Germany; orcid.org/0000-0001-68518453. Notes The authors declare no competing financial interest
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