The function, characterization of expression, localization and activity of a divergent ice nucleating protein from Pseudomonas borealis
Vanderveer, Tara Lynn
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An ice nucleating protein (INP) with 66% amino acid sequence identity to the better-known INP of Pseudomonas syringae has been described in an environmental isolate of P. borealis and designated InaPb. Despite the fact that INPs are classified as ice-binding proteins, InaPb showed little affinity for pre-formed ice and showed incorporation rates similar to Ina- strains. Additionally, it appeared to lack in the ability to shape ice or limit its growth. However, it was an effective ice nucleator. Using the coding sequence for InaPb and a green fluorescent protein tag (GFP), an InaPb-GFP fusion protein construct was inserted into a broad host expression vector in order to visualize the expression and localization of the protein in E. coli and an Ina- strain of P.syringae. The InaPb-GFP protein appears to localize at the poles of E. coli, but the nucleation temperature for these cells was only marginally above -9°C, which indicated poor nucleation activity. When expressed in Ina- P. syringae, the proteins showed clustering throughout the cell and an increased ability to nucleate ice following cold conditioning. The ability to nucleate ice was further increased by the removal of the GFP tag resulting in an average nucleation temperature more consistent with that seen in the native host P. borealis. Since inaPb transcript levels did not appear to change after cold conditioning, the clustering seen using fluorescence microscopy was likely the result of increased aggregation of protein in the membrane. Most INP- producing bacteria are associated with plant disease, but experiments with P. borealis suggested that the Ina+ phenotype was not indicative of pathogenicity in this strain. It is hoped that my contribution to the functional characterization of this INP will lead to a better understanding of these special proteins and their importance to the handful of bacteria that exhibit this activity.