Effect of cytochrome c on the phase behavior of charged multicomponent lipid membranes

We studied the effect of submicromolar concentrations of cytochrome c (cyt c) on the phase behavior of ternary lipid membranes composed of charged dioleoylphosphatidylglycerol, egg sphingomyelin and cholesterol. The protein was found to induce micron-sized domains in membranes belonging to the single-fluid-phase region of the protein-free ternary mixture and, as a result, to expand the region of coexistence of liquid ordered (L) and liquid disordered (L-d) phases. Direct observations on individual vesicles revealed that protein adsorption increases the area of L-d domains. Measurements using a fluorescent analog of cyt c showed that the protein preferentially adsorbs onto domains belonging to the L-d phase. The adsorption was quantitatively characterized in terms of partitioning ratios between the L-d and the L-o phases. The protein was also found to induce vesicle leakage even at relatively low concentrations. In eukaryotic cells under normal physiological conditions, cyt c is localized within the intermembrane space of mitochondria. During cell apoptotis, cyt c is released into the cytosol and its adsorption to intracellular membranes may strongly perturb the lipid distribution within these membranes as suggested by our results. (C) 2014 Elsevier B.V. All rights reserved.