A theoretical model of a plant antifreeze protein from Lolium perenne

Antifreeze proteins (AFPs), found in certain organisms enduring freezing en vironments, have the ability to inhibit damaging ice crystal growth. Recent ly, the repetitive primary sequence of the AFP of perennial ryegrass, Loliu m, perenne, was reported. This macromolecular antifreeze has high ice recry stallization inhibition activity but relatively low thermal hysteresis acti vity. We present here a theoretical three-dimensional model of this 118-res idue plant protein based on a beta -roll domain with eight loops of 14-15 a mino acids. The fold is supported by a conserved valine hydrophobic core an d internal asparagine ladders at either end of the roll. Our model, which i s the first proposed for a plant AFP, displays two putative, opposite-facin g, ice-binding sites with surface complementarity to the prism face of ice. The juxtaposition of the two imperfect ice-binding surfaces suggests an ex planation for the protein's inferior thermal hysteresis but superior ice re crystallization inhibition activity and activity when compared with fish an d insect AFPs.