Antifreeze proteins differentially affect model membranes during freezing

Over the past decade antifreeze proteins from polar fish have been shown ei ther to stabilize or disrupt membrane structure during low temperature and freezing stress. However, there has been no systematic study on how membran e composition affects the interaction of antifreeze proteins with membranes under stress conditions. Therefore, it is not possible at present to predi ct which antifreeze proteins will protect, and which will damage a particul ar membrane during chilling or freezing. Here, we analyze the effects of Fr eezing on spinach thylakoid membranes and on model membranes of varying lip id composition in the presence of antifreeze protein type I (AFP I) and spe cific fractions of antifreeze glycoproteins (AFGP). We find that the additi on of galactolipids to phospholipid model membranes changes the effect each protein has on the membrane during freezing. However, the greatest differe nces observed in this study an between the different types of antifreeze pr oteins. We find that AFP type I and the largest molecular weight fractions of AFGP induce concentration dependent leakage from, and are fusogenic to t he liposomes. This is the first report that an antifreeze protein induces m embrane fusion. In contrast, the smallest fraction of AFGP offers a limited degree of protection during freezing and does not induce membrane fusion a t concentrations up to 10 mg/ml. (C) 2001 Elsevier Science B.V. All rights reserved.