Efficacy of antifreeze protein types in protecting liposome membrane integrity depends on phospholipid class

Antifreeze proteins have been reported to be capable of maintaining the mem brane integrity of cold sensitive mammalian cells when exposed to hypotherm ic temperatures. However the mechanism(s) whereby these proteins exert this protective effect is unknown. The present study used liposomes as a model system to examine the nature of the interactions between four antifreeze (g lyco)protein types (AFP I, II, III and AFGP) and albumin, with lipid membra nes. Fluorescein isothiocyanate labelling indicated that all of the protein s bound to the three liposome types (dielaidoylphosphatidylcholine (DEPC), dielaidoylphosphatidylethanolamine (DEPE) and dielaidoylphosphatidylglycero l (DEPG)). AFGP was found to be highly effective at preventing leakage from all three liposome compositions as they were cooled through their phase tr ansition temperatures. This was not the case for the other proteins. All fo ur antifreeze types prevented zwitterionic DEPC liposomes from leaking as t hey were cooled through their phase transition temperature. However, albumi n was equally as effective, indicating that this capacity was not unique to antifreeze proteins. All of the proteins, except AFGP, induced the negativ ely charged DEPG liposomes to leak prior to cooling, and were less effectiv e than AFGP in preventing phase transition leakage from DEPE liposomes. It is proposed that many proteins, including antifreeze proteins, can protect zwitterionic liposomes, such as DEPC, by binding to the lipid bilayer there by maintaining the ordered structure of the membrane during phase transitio n. However, when the membrane contains a negatively charged polar group, su ch as with DEPE and DEPG, proteins, although bound to them, may not be able to maintain sufficient membrane organization to prevent leakage during pha se transition or, they may gain entry into the lipid bilayer, disrupt the s tructure and induce leakage. These results imply that the efficacy of antif reeze proteins in the cold protection of mammalian cells will not only depe nd on protein structure, but also on the lipid composition of the cell memb rane. (C) 2000 Elsevier Science B.V. All rights reserved.