Lipid mixing during membrane aggregation and fusion: why fusion assays disagree

The kinetics of lipid mixing during membrane aggregation and fusion was monitored by two assays employing resonance energy transfer between N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine (NBD-PE) and N-(lissamine Rhodamine B sulfonyl)phosphatidylethanolamine (Rh-PE). For the "probe mixing" assay, NBD-PE and Rh-PE were incorporated into separate populations of phospholipid vesicles. For the "probe dilution" assay, both probes were incorporated into one population of vesicles, and the assay monitored the dilution of the molecules into the membrane of unlabeled vesicles. The former assay was found to be very sensitive to aggregation, even when the internal aqueous contents of the vesicles did not intermix. Examples of this case were large unilamellar vesicles (LUV) composed of phosphatidylserine (PS) in the presence of Mg2+ and small unilamellar vesicles (SUV) composed of phosphatidylserine in the presence of high concentrations of Na+. No lipid mixing was detected in these cases by the probe dilution assay. Under conditions where membrane fusion (defined as the intermixing of aqueous contents with concomitant membrane mixing) was observed, such as LUV (PS) in the presence of Ca2+, the rate of probe mixing was faster than that of probe dilution, which in turn was faster than the rate of contents mixing. Two assays monitoring the intermixing of aqueous contents were also compared. The Tb/dipicolinic acid assay reported slower fusion rates than the 1-aminonaphthalene-3,6,8-trisulfonic acid/N,N'-p-xylylene-bis(pyridinium bromide) assay for PS LUV undergoing fusion in the presence of Ca2+. These observations point to the importance of utilizing contents mixing assays in conjunction with lipid mixing assays to obtain the rates of membrane destabilization and fusion.(ABSTRACT TRUNCATED AT 250 WORDS)