Ca²⁺-induced fusion of phospholipid vesicles monitored by mixing of aqueous contents

Abstract

Ca²⁺ has a central role in various cellular phenomena involving membrane fusion^1–3. However, little is known about the mechanisms involved. Model membrane systems such as phospholipid vesicles have been used extensively to study the mechanism of membrane fusion at the molecular level⁴. For example, phosphatidylserine (PS) vesicles have been shown to undergo massive aggregation and structural rearrangements on addition of Ca²⁺, with eventual formation of large cochleate structures^5–8. Although these structures do not retain appreciable internal volume, their formation has been proposed to result from fusion of the initial vesicles^5–8. The significance of the PS–Ca²⁺ system as a model for biological membrane fusion has been questioned recently by Ginsberg⁹. Based on the observation that divalent cations induce the release of contents from PS vesicles but fail to bring about the uptake of a marker from the medium, he proposes that the vesicles are ruptured completely during interaction with divalent cations and reassemble subsequently to form large non-vesicular structures. The present study demonstrates that the question raised by Ginsberg⁹ is not particularly relevant to the phenomenon concerned, and that his experimental observations do not allow the exclusive conclusion that Ca²⁺ induces lysis of PS vesicles rather than fusion.