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Efficacy of antifreeze protein types in protecting liposome membrane integrity depends on phospholipid class
1A/F Protein Canada, Inc., St. John's, Nfld., Canada.
Abstract:
Antifreeze proteins have been reported to be capable of maintaining the membrane integrity of cold sensitive mammalian cells when exposed to hypothermic 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 (glyco)protein types (AFP I, II, III and AFGP) and albumin, with lipid membranes. Fluorescein isothiocyanate labelling indicated that all of the proteins bound to the three liposome types (dielaidoylphosphatidylcholine (DEPC), dielaidoylphosphatidylethanolamine (DEPE) and dielaidoylphosphatidylglycerol (DEPG)). AFGP was found to be highly effective at preventing leakage from all three liposome compositions as they were cooled through their phase transition temperatures. This was not the case for the other proteins. All four antifreeze types prevented zwitterionic DEPC liposomes from leaking as they were cooled through their phase transition temperature. However, albumin was equally as effective, indicating that this capacity was not unique to antifreeze proteins. All of the proteins, except AFGP, induced the negatively charged DEPG liposomes to leak prior to cooling, and were less effective 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 thereby maintaining the ordered structure of the membrane during phase transition. However, when the membrane contains a negatively charged polar group, such as with DEPE and DEPG, proteins, although bound to them, may not be able to maintain sufficient membrane organization to prevent leakage during phase transition or, they may gain entry into the lipid bilayer, disrupt the structure and induce leakage. These results imply that the efficacy of antifreeze proteins in the cold protection of mammalian cells will not only depend on protein structure, but also on the lipid composition of the cell membrane.
Insights
Antifreeze proteins and albumin protect lipid membranes from cold damage by binding to them. However, their effectiveness varies with membrane lipid composition, especially for negatively charged membranes.
Area of Science:
- Biochemistry
- Cell Biology
- Membrane Biophysics
Background:
- Antifreeze proteins (AFPs) protect cold-sensitive cells from hypothermic damage.
- The precise mechanisms by which AFPs maintain membrane integrity are not fully understood.
Purpose of the Study:
- To investigate the interactions between different antifreeze proteins (AFPs) and albumin with lipid membranes.
- To elucidate the role of lipid composition in AFP-mediated membrane protection.
Main Methods:
- Liposomes composed of DEPC, DEPE, and DEPG were used as model membrane systems.
- Fluorescein isothiocyanate labeling tracked protein binding to liposomes.
- Liposome leakage assays assessed membrane integrity during cooling through phase transition temperatures.
Main Results:
- All tested proteins (AFGP, AFP I, II, III, and albumin) bound to all liposome types.
- AFGP effectively prevented leakage from all liposome compositions during cooling.
- Albumin and AFPs protected zwitterionic DEPC liposomes but induced leakage in negatively charged DEPG liposomes.
Conclusions:
- Protein interaction with lipid membranes is crucial for maintaining integrity during cold exposure.
- Membrane lipid composition significantly influences the protective efficacy of AFPs and other proteins.
- AFP effectiveness in cold protection of mammalian cells depends on both protein structure and cell membrane lipid composition.