10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Laboratory Studies on Mechanisms Behind Bacterial Ice-Nucleation Activity
Meilee Ling, Heike Wex, Sarah Grawe, Jonas Jakobsson, Susan Hartmann, Jakob Löndahl, Kai Finster, Thomas Boesen, TINA SANTL-TEMKIV, Aarhus University, Denmark
Abstract Number: 827 Working Group: Unraveling the Many Facets of Ice Nucleating Particles and Their Interactions with Clouds
Abstract Ice nucleation active (INA) bacteria are one type of atmospheric ice nuclei that have attracted particular attention due to their unique ability to produce specific INA proteins (INprot). Several studies have shown that these proteins are the most efficient ice nuclei known as they induce nucleation at temperatures close to 0°C. We have just started understanding the impacts of aerosolized INA bacteria on atmospheric processes. We present the outcome of a study that focused on elucidating the largely unknown molecular mechanisms behind ice-nucleation induced by INA bacteria.
It is known, that ice nucleation in bacteria is induced by certain proteins. These proteins consist of three parts: N-terminal, C-terminal and the central repeat domain. The central repeat domain is involved in ice-formation and is essential for the protein to function as ice nucleating particle. To understand the role of the central repeat domain of the INprot as well as the role of intermolecular interactions between INprot molecules for their ice-nucleation behaviour, we produced purified recombinant proteins with reducted theoretical ice-binding surface. The native protein that contains 67 amino acid tandem repeats was truncated to only contain 9, 16, and 28 amino acid repeats. The recombinant proteins interacted with bacterial lipids to form particles that are between 30 nm and 100 nm in size. The particle size corresponded roughly with the size of the central repeat domain of INprot.
For one of the INprot constructs that contained 16 repeats (INprot16R), ice nucleation assays were performed using both the Leipzig Aerosol Cloud Interaction Simulator (LACIS) and droplet freezing assays. We found that the truncated bacterial ice nucleation protein was ice active although its ice-binding site was more than 4 times smaller than the sizes of native protein. The onset ice nucleation temperature was between −24°C and −26°C, which strongly indicated that the number of amino-acid tandem repeats determined the ice-nucleation temperature. In addition, we demonstrated that ice nucleation between -9°C to -10°C, which was comparable to the ice-nucleation activity of full-length INprot, was caused by large INprot16R oligomers. These results support previous observations that oligomerization of INprot increases their ice-binding surface, thus changing their freezing behaviour. In conclusion, both the repeat number and the degree of oligomerization contribute in an independent manner to the nucleation mechanism of INA proteins.