Involvement of surface potential in regulation of polar membrane lipids in Acholeplasma laidlawii.

The process of secretory granule-plasma membrane fusion can be studied in sea urchin eggs. Micromolar calcium concentrations are all that is required to bring about exocytosis in vitro. I discuss recent experiments with sea urchin eggs that concentrate on the biophysical aspects of granule-membrane fusion. The backbone of biological membranes is the lipid bilayer. Sea urchin egg membrane lipids have negatively charged head groups that give rise to an electrical potential at the bilayer-water interface. We have found that this surface potential can affect the calcium required for exocytosis. Effects on the surface potential may also explain why drugs like trifluoperazine and tetracaine inhibit exocytosis: they absorb to the bilayer and reduce the surface potential. The membrane lipids may also be crucial to the formation of the exocytotic pore through which the secretory granule contents are released. We have measured calcium-induced production of the lipid, diacylglycerol. This lipid can induce a phase transition that will promote fusion of apposed lipid bilayers. The process of exocytosis involves the secretory granule core as well as the lipids of the membrane. The osmotic properties of the granule contents lead to swelling of the granule during exocytosis. Swelling promotes the dispersal of the contents as they are extruded through the exocytotic pore. The movements of water and ions during exocytosis may also stabilize the transient fusion intermediate and consolidate the exocytotic pore as fusion occurs.

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Modifications of membrane lipid structure and their influence on cell growth, passive permeability, and enzymatic and transport activities in Acholeplasma laidlawii B

Biochemistry and Cell Biology ◽

10.1139/o86-009 ◽

1986 ◽

Vol 64 (1) ◽

pp. 58-65 ◽

Cited By ~ 6

Author(s):

Ronald N. McElhaney

Keyword(s):

Cell Growth ◽

Atpase Activity ◽

Phase State ◽

Membrane Lipids ◽

Cholesterol Content ◽

Membrane Lipid ◽

Passive Permeability ◽

Acholeplasma Laidlawii ◽

Lipid Fatty Acid ◽

Chain Lengths

Acholeplasma laidlawii B is a simple procaryotic microorganism, without a cell wall, whose membrane lipid fatty acid composition and cholesterol content can be dramatically modified. It is thus possible to markedly vary the temperature and cooperativity of the gel to lipid-crystalline phase transition, thereby altering the phase state and "fluidity" of the A. laidlawii membrane lipids. By varying the chain length of the exogenous fatty acids supplied, it is also possible to modify the thickness of the membrane lipid bilayer. Acholeplasma laidlawii B cell growth and most membrane functions are strongly dependent on the phase state of the membrane lipids. When gel-state lipid predominates at physiological temperatures, cell growth declines sharply, nonelectrolyte permeability exhibits a pronounced local maximum, net glucose transport is markedly reduced, and ATPase activity declines moderately. Upon complete conversion of the lipid to the gel state, cell growth ceases, passive permeability is markedly reduced, and net glucose transport is abolished, but appreciable ATPase activity remains. Provided that the membrane lipid is predominantly or exclusively in the liquid-crystalline state, cell growth and ATPase activity are almost independent of membrane lipid fatty acid composition and cholesterol content, except that cell growth is inhibited by the presence of "hyperfluid" lipid. In contrast, passive permeability and active glucose uptake are influenced by alterations in membrane lipid composition, with increases in lipid fluidity resulting in higher nonelectrolyte translocation rates. Alterations in the length of the membrane lipid hydrocarbon chains per se also appear to affect A. laidlawii growth and membrane function. Thus lipids containing fatty acids with effective chain lengths of 13 carbons or less do not support cell growth or normal enzymatic and transport activities, and lipids with effective hydrocarbon chain lengths of 19 carbons or more do not support normal cell growth.

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