Type I 'antifreeze' proteins - Structure-activity studies and mechanisms of ice growth inhibition

The type I 'antifreeze' proteins, found in the body fluids of fish inhabiti ng polar oceans, are alanine-rich or-helical proteins that are able to inhi bit the growth of ice. Within this class there are two distinct subclasses of proteins: those related to the winter flounder sequence HPLC6 and which contain Ii-residue repeat units commencing with threonine; and those from t he sculpins that are unique in the N-terminal region that contains establis hed helix breakers and lacks the 11-residue repeat structure present in the rest of the protein. Although 14 type I proteins have been isolated, almos t all research has focused on HPLC6, the 37-residue protein from the winter flounder Pseudopleuronectes americanus. This protein modifies both the rat e and shape (or 'habit') of ice crystal growth, displays hysteresis and acc umulates specifically at the (2 0 (2) over bar 1) ice plane. Until very rec ently, all models to explain the mechanism for this specific interaction ha ve relied on the interaction of the four threonine hydroxyls, which are spa ced equally apart on one face of the helix, with the ice lattice. In contra st, proteins belonging to the sculpin family accumulate specifically at the (2 (1) over bar (1) over bar 0) plane. The molecular origin of this differ ence in specificity between the flounder and sculpin proteins is not unders tood. This review will summarize the structure-activity and molecular model ling and dynamics studies on HPLC6, with an emphasis on recent studies in w hich the threonine residues have been mutated. These studies have identifie d important hydrophobic contributions to the ice growth inhibition mechanis m. Some 50 mutants of HPLC6 have been reported and the data is consistent w ith the following requirements for ice growth inhibition: (a) a minimum len gth of approx. 25 residues; (b) an alanine-rich sequence in order to induce a highly helical conformation; (c) a hydrophobic face; (d) a number of cha rged/polar residues which are involved in solubility and/or interaction wit h the ice surface. The emerging picture, that requires further dynamics stu dies including accurate modelling of the ice/water interface, suggests that a hydrophobic interaction between the surface of the protein and ice is th e key to explaining accumulation at specific ice planes, and thus the molec ular level mechanism for ice growth inhibition.