© 2003 Heron Publishing—Victoria, Canada
Calcium-induced aggregation of archaeal bipolar tetraether liposomes derived from the thermoacidophilic archaeon Sulfolobus acidocaldarius
Roby Kanichay (1), Lawrence T. Boni (2), Peter H. Cooke (3), Tapan K. Khan (1) and Parkson Lee-Gau Chong (1, 4)
1. Department of Biochemistry, Temple University School of Medicine, Philadelphia, PA 19140, USA / 2. Transave Inc., 11 Deer Park Drive, Suite 117, Monmouth, NJ 08852, USA / 3. USDA, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, PA 19038, USA / 4. Author to whom correspondence should be addressed ([email protected]) / Received November 18, 2002; accepted June 22, 2003; published online July 18, 2003
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.
Keywords: fluorescence, light scattering, membranes, microscopy, pH, surface potential, temperature, vesicle size.
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