FORMATION OF 2-DIMENSIONAL ARRAYS OF ANNEXIN-V ON PHOSPHATIDYLSERINE-CONTAINING LIPOSOMES

Annexins are intracellular proteins which bind to membranes in a Ca2+-dependent manner and which have been proposed to play regulatory roles in different membrane processes. In the present study, the stoichiometry of the Ca2+-dependent binding of annexin V to phosphatidylserine molecules incorporated into liposomes was studied by fluorescence spectroscopy. The Ca2+-dependence of the binding was determined using liposomes made of dioleoylphosphatidylserine (PS) and dioleoylphosphatidylcholine (PC), with a PC/PS molar ratio ranging from 1 to 800. These liposomes were shown to be mostly unilamellar by cryoelectron microscopy. [Ca2+](1/2) concentrations required for half-maximal binding of annexin V range from 57 μM at PC/PS = 1 up to 96 mM at PC/PS = 800. Titration of accessible PS molecules showed that annexin V molecules bind equally well to liposomes of PC/PS ratio ranging from 1 to 400. The stoichiometry of the binding between annexin V and PS, determined at low PS content, is eight annexin V molecules per one PS molecule. We propose a novel model of the Ca2+-dependent interaction between annexin V and lipid membranes, based on the formation of two-dimensional arrays of annexin V molecules, stabilized by both protein-lipid and protein-protein interactions. © 1994 Academic Press Limited.