Giant vesicles as models to study the interactions between membranes and proteins

The interaction between polypeptides and membranes is a fundamental aspect of cell biochemistry. Liposomes have been used in this context as in vitro systems to study such interactions. We present here the case of giant vesic les (GVs), which, due to their size (radius larger than 10 microns), mimic more closely the situation observed in cell membranes and furthermore permi t to study protein-membrane interactions by direct optical monitoring. It i s shown that GVs formed from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholi ne by electroformation are permeable to certain low molecular weight molecu les such as the nucleic acid dye YO-PRO-1 and fluorescein diphosphate where as conventional liposomes (large or small unilamellar liposomes) are not. I n addition, it is shown that non-membrane proteins, such as DNases or RNase s, added to the selected GVs from the outside, are able to convert their su bstrate, which is strictly localized on the internal side of the membrane. This effect is only seen in GVs (also when they are removed from the origin al electroformation environment) and is absent in conventional liposomes. T he fact that these effects are only present in GVs obtained by electroforma tion and not in conventional small liposomes is taken as an indication that certain physico-chemical properties of the bilayer are affected by the mem brane curvature, although the mechanism underlying such differences could n ot be established as yet. (C) 2000 Elsevier Science B.V. All rights reserve d.