Abstract. An N-terminal truncated form of the ice nucleation protein (INP) of Pseudomonas syringae lacking the transport sequence for the localization of InaZ in the outer membrane was fused to N- and C- terminal inner membrane (IM) anchors and expressed in Escherichia coli C41. The ice nucleation (IN) activity of the corresponding living recombinant E. coli catalyzing heterogeneous ice formation of super-cooled water at high subzero temperatures was tested by droplet freezing assay. Median freezing temperature (T50) of the parental living E. coli C41 cells without INP was detected at −20.1 °C and with inner membrane anchored INPs at T50 value between −7 °C and −9 °C demonstrating that IM anchored INPs facing the luminal IM site are able to induce IN from the inside of the bacterium almost similar to bacterial INPs located at the outer membrane. Bacterial Ghosts (BGs) derived from the different constructs showed first droplet freezing values between −6 °C and −8 °C whereas C41 BGs alone without carrying IM anchored INPs exhibit a T50 of −18.9 °C. The more efficient IN of INP-BGs compared to their living parental strains can be explained by the free access of IM anchored INP constructs to ultrapure water filling the inner space of the BGs. The cell killing rate of -NINP carrying E. coli at subzero temperatures is higher when compared to survival rates of the parental C41 strain.
Molecular Characterization of an Ice Nucleation Protein Variant (InaQ) from Pseudomonas syringae and the Analysis of Its Transmembrane Transport Activity in Escherichia coli