Importance of phosphatidylethanolamine for the interaction of apocytochrome c with model membranes containing phosphatidylserine

The effect of phosphatidylethanolamine (PE) on the binding of apocytochrome c to model membranes was examined. When 1-palmitoyl-2-oleoyl-sn-glycero-3- phosphocholine (POPC) of the standard vesicles composed of 80% of this lipi d and 20% of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) was gra dually replaced with upward of 50% of 1-palmitoyl-2-oleoyl-sn-glycero-3-pho sphoethanolamine (POPE), the binding increased appreciably. Ca2+, causing t he phase separation of PS, also brought about increased binding of apocytoc hrome c in the PC/PS system, underlining the importance of PS properties in membranes for the protein binding. The resonance energy transfer between T rp-59 in apocytochrome c and pyrene-PS incorporated into bilayers showed th at the replacement of PC with PE increased the extent of apocytochrome c pe netration into membranes by a PE concentration-dependent manner. However, i n the absence of PS, PE had no apparent effect on these functions of apocyt ochrome c, suggesting that PE-induced change(s) of acidic membrane properti es is important to the association of apocytochrome c with vesicles. From t he observations that the excimer to monomer fluorescence ratio of pyrene-PS increased and the fluorescence of NBD-PS was quenched with increasing conc entration of PE, it was deduced that PE caused PS-enriched domains in PC/PE /PS membranes. The colocalization of pyrene-PS with BODIPY-PS by PE further supported the possibility. We suggest that PE-induced formation of PS-enri ched domains acts as binding sites for apocytochrome c in membranes.