9.B.133 The Ice Nucleation Protein Secretion System (INP-SS) Family

The ice nucleation protein (INP) of Pseudomonas syringae is involved in pathogenicity of foliar necroses. It may be useful in snow making, frozen food preparation, and surface-display system development. The gene, inaQ, from a P. syringae strain MB03 encodes InaQ which shares similar protein domains with three P. syringae INPs, namely, InaK, InaZ, and InaV, which have an N-terminal hydrophobic domain, a central repeating domain, and a short hydrophilic C-terminal domain. Expression of the full-length InaQ and of various truncated variants has been induced in E. coli to analyze their transmembrane transport and surface-binding activities, while using the green fluorescence protein (GFP) as the fusion partner (Li et al., 2012). With two transmembrane segments and a weak secretion signal, the N-terminal domain (InaQ-N) alone was responsible for the transport process, whereas the C-terminal region was nonfunctional in protein transport. The first 18aas were essential for secretion. The mechanism of secretion was defined, but possibly, the N-terminal domain self-catalyzes secretion, or the Sec translocon could do it in spite of the lack of a typical signal peptide sequence. INA proteins show sequence similarity to members of the autotransporter-1 (1.B.12), autotransporter-2 (1.B.40), and PE-PGRS (9.B.96) families in their central repeat domains. 

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