Previously, we showed that the proton permeability of small
unilamellar vesicles (SUVs) composed of polar lipid fraction E (PLFE)
from the thermoacidophilic archaeon Sulfolobus
acidocaldarius was remarkably low and insensitive to
temperature (Komatsu and Chong
1998). In this study, we used photon correlation spectroscopy
to investigate the time dependence of PLFE SUV size as a function of
Ca2+ concentration. In the absence of Ca2+,
vesicle diameter changed little over 6 months. Addition of
Ca2+, however, immediately induced formation of vesicle
aggregates with an irregular shape, as revealed by confocal
fluorescence microscopy. Aggregation was reversible upon addition of
EDTA; however, the reversibility varied with temperature as well as
incubation time with Ca2+. Freeze-fracture electron
microscopy showed that, after a long period of incubation (2 weeks)
with Ca2+, the PLFE vesicles had not just aggregated, but
had fused or coalesced. The initial rate of vesicle aggregation varied
sigmoidally with Ca2+ concentration. At pH 6.6, the
threshold calcium concentration (Cr) for
vesicle aggregation at 25 and 40 °C was 11 and 17 mM,
respectively. At pH 3.0, the Cr at 25
°C increased to 25 mM. The temperature dependence of
Cr may be attributable to changes in
membrane surface potential, which was –22.0 and –13.2 mV
at 25 and 40 °C, respectively, at pH 6.6, as determined by
2-(p-toluidinyl)naphthalene-6-sulfonic acid
fluorescence. The variation in surface potential with temperature is
discussed in terms of changes in lipid conformation and membrane
organization.
