“…We have recently demonstrated that fluorescence measurements can be used to monitor reliably and specifically the lateral distribution of lipids in vesicles containing phosphatidic acid (PA)' and phosphatidylcholine (PC) in the presence and absence of divalent cations (Graham et al, 1985). In this study, we have correlated fluorometric observations of divalent cation induced mixing of lipids and aqueous contents (which we define operationally as "fusion") between PA-PC vesicles with direct electron microscopic observations of vesicle-vesicle ' Abbreviations: ANTS, 8-aminonaphthalene-1,3,6-trisulfonic acid disodium salt; C,,-NBD-PC, 1-palmitoyl-2-[ 12-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]dodecanoyl]-sn-glycero-3-phosphocholine; DO, dioleoyl; DPX, p-xylene bis(pyridinium bromide); EDTA, ethylenediaminetetraacetic acid disodium salt; egg PA, phosphatidic acid prepared from egg yolk phosphatidylcholine; egg PC, phosphatidylcholine from egg yolk; LUV, large unilamellar vesicle(s); NBD-PE, N-(7-nitro-2,1,3-benzoxadiazol-4-y1)phosphatidylethanolamine: PA, 1,2-diacyl-snglycero-3-phosphatidic acid: PC, 1,2-diacyI-sn-glycero-3-phosphocholine; PE, 1,2-diacyl-sn-glycero-3-phosphoethanolamine; PS, 1,2-diacyl-snglycero-3-phosphoserine; Rho-PE, N-(lissamine rhodamine B sulfonyl)-phosphatidylethanolamine; Tes, N-[tris(hydroxymethyl)methyl]-2-aminoethanesulfonic acid; EGTA, [ethylenebis(oxyethylenenitrilo)]-tetraacetic acid; DE, dielaidoyl. interactions and with calorimetric and fluorometric measurements of PA-PC miscibility, under conditions that promote interactions between vesicles.…”