beta-helix structure and ice-binding properties of a hyperactive antifreeze protein from an insect

Insect antifreeze proteins (AFP) are considerably more active at inhibiting ice crystal growth than AFP from fish or plants. Several insect AFPs, also known as thermal hysteresis proteins, have been cloned(1-3) and expressed( 1,2). Their maximum activity is 3-4 times that of fish AFPs(1) and they are 10-100 times more effective at micromolar concentrations. Here we report t he solution structure of spruce budworm (Choristoneura fumiferana) AFP and characterize its ice-binding properties. The 9-kDa AFP is a beta-helix with a triangular cross-section and rectangular sides that form stacked paralle l beta-sheets; a fold which is distinct from the three known fish AFP struc tures. The ice-binding side contains 9 of the 14 surface-accessible threoni nes organized in a regular array of TXT motifs that match the ice lattice o n both prism and basal planes. In support of this model, ice crystal morpho logy and ice-etching experiments are consistent with AFP binding to both of these planes and thus may explain the greater activity of the spruce budwo rm antifreeze.