Modeling Pseudomonas syringae ice-nucleation protein as a beta-helical protein

Antifreeze proteins (AFPs) inhibit the growth of ice, whereas ice-nucleatio n proteins (INPs) promote its formation. Although the structures of several AFPs are known, the structure of INP has been modeled thus far because of the difficulty in determining membrane protein structures. Here, we present a novel model of an INP structure from Pseudomonas syringae based on compa rison with two newly determined insect AFP structures. The results suggest that both this class of AFPs and INPs may have a similar P-helical fold and that they could interact with water through the repetitive TXT motif. By t heoretical arguments, we show that the distinguishing feature between an ic e inhibitor and an ice nucleator lies in the size of the ice-interacting su rface. For INPs, the larger surface area acts as a template that is larger than the critical ice embryo surface area required for growth. In contrast, AFPs are small enough so that they bind to ice and inhibit further growth without acting as a nucleator.