Cold survival in freeze-intolerant insects

The structure and function of β-helical antifreeze proteins

Antifreeze proteins (AFPs) designate a class of proteins that are able to bind to and inhibit the growth of macromolecular ice. These proteins have been characterized from a variety of organisms. Recently, the structures of AFPs from the spruce budworm (Choristoneura fumiferana) and the yellow mealworm (Tenebrio molitor) have been determined by NMR and X-ray crystallography. Despite nonhomologous sequences, both proteins were shown to consist of β-helices. We review the structures and dynamics data of these two insect AFPs to bring insight into the structure–function relationship and explore their β-helical architecture. For the spruce budworm protein, the fold is a left-handed β-helix with 15 residues per coil. The Tenebrio molitor protein consists of a right-handed β-helix with 12 residues per coil. Mutagenesis and structural studies show that the insect AFPs present a highly rigid array of threonine residues and bound water molecules that can effectively mimic the ice lattice. Comparisons of the newly determined ryegrass and carrot AFP sequences have led to models suggesting that they might also consist of β-helices, and indicate that the β-helix might be used as an AFP structural motif in nonfish organisms.