Cytochrome C – cardiolipin interactions: extended lipid anchorage revisited

Resonance energy transfer (RET) from antrylvinyl-labeled (AV) lipids to the heme moiety of cytochrome c (cyt c) has been employed to assess the molecular level details of cyt c interactions with negatively charged lipid membranes composed of phosphatidylcholine (PC) and its mixture with 10 or 20 mol % of cardiolipin (CL). At the lowest ionic strength used here (20 mM) RET profiles from neutral (AV-PC) and anionic (AV-CL) donor were virtually indistinguishable, suggesting that the peculiarities of cyt c association with lipid membranes containing different donors are identical. In contrast, elevating ionic strength up to 40 and 60 mM resulted in expected decrease of energy transfer efficiency in the case of AV-PC containing liposomes, but not for those with AV-CL where RET exhibited an unexpected enhancement with increasing ionic strength. Monte Carlo analysis of the results obtained allowed us to attribute this untypical behavior to the transition of CL into extended lipid conformation. The revealed peculiarities of cyt c – CL interactions are of great interest not only from the viewpoint of regulating cyt c electron transfer and apoptotic propensities but also for elucidating the general mechanisms by which membrane functional activities can be modulated by protein-lipid interactions