Calcium-induced aggregation of archaeal bipolar tetraether liposomes derived from the thermoacidophilic archaeon Sulfolobus acidocaldarius

• Published in: Archaea Vol. 2003; no. 3; pp. 175 - 183
• Main Authors: Kanichay, Roby, Boni, Lawrence T, Cooke, Peter H, Khan, Tapan K, Chong, Parkson Lee-Gau
• Format: Journal Article
• Language: English
• Published: United States Hindawi Limiteds 2003; Heron Publishing; Hindawi Limited
• Subjects: Calcium - pharmacology; Freeze Fracturing; Liposomes - chemistry; Membrane Lipids - chemistry; Microscopy, Confocal; Microscopy, Electron; Molecular Structure; Sulfolobus acidocaldarius - chemistry; membranes; fluorescence; pH; temperature; vesicle size; light scattering; microscopy; surface potential
• ISSN: 1472-3646; 1472-3654
• DOI: 10.1155/2003/603528

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 Ca^(2+) concentration. In the absence of Ca^(2+), vesicle diameter changed little over 6 months. Addition of Ca^(2+), 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 Ca^(2+). Freeze-fracture electron microscopy showed that, after a long period of incubation (2 weeks) with Ca^(2+), the PLFE vesicles had not just aggregated, but had fused or coalesced. The initial rate of vesicle aggregation varied sigmoidally with Ca^(2+) concentration. At pH 6.6, the threshold calcium concentration (C_r) for vesicle aggregation at 25 and 40 °C was 11 and 17 mM, respectively. At pH 3.0, the C_r at 25 °C increased to 25 mM. The temperature dependence of C_r 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.