Pressure as a Probe of Vesicle Fusion

Abstract

Fusion between two lipid membranes is superficially easy to understand: Two lipid bilayers come into contact and meld into one seamless membrane. But even at this basic level, one must distinguish whether fusion simply means the merging of the two membranes or whether the internal contents (of the liposomes or vesicles) must also mix. Certainly, for the case of vesicles, the requirement that “true” fusion involves the mixing of the internal volumes as well as lipid mixing is a reasonable and practical way of defining fusion. There are other situations, however, where the above definition is inadequate. For instance, in modeling exocytosis, where the internal contents of a liposome is excreted outward, there is no mixing per se of two internal volumes. In this case, the essence of membrane fusion has to be defined slightly differently. Leaving aside such interesting considerations concerning the definition of fusion, there is also much unknown about the process of fusion. Certainly there are many pathways for initiating vesicle membrane fusion; a partial listing would include induction by cations (Papahadjopoulos et al., 1977), by pH (Ellens et al., 1984), by proteins (Morero et al., 1985), by temperature (Verkleij, 1980), by electric field (Zimmermann, 1982; Sowers, 1984), or even spontaneous fusion between vesicles (Suurkuusk et al., 1976; Schullery et al., 1980; Chang et al., 1982).