Electron microscopic investigation of the morphology and calcium-induced fusion of lipid vesicles with an oligomerised inner leaflet.

The lipid head groups in the inner leaflet of unilamellar bilayer vesicles of the synthetic lipids DHPBNS and DDPBNS can be selectively oligomerised. Earlier studies have established that these vesicles fuse much slower and less extensively upon oligomerisation of the lipid head groups. The morphology and calcium-induced fusion of vesicles of DHPBNS and DDPBNS were investigated using cryo-electron microscopy. DHPBNS vesicles are not spherical but flattened, ellipsoidal structures. Upon addition of CaCl(2), DHPBNS vesicles with an oligomerised inner leaflet were occasionally observed in an arrested hemifused state. However, the evidence for hemifusion is not equivocal due to potential artefacts of sample preparation. DDPBNS vesicles show the expected spherical morphology. Upon addition of excess CaCl(2), DDPBNS vesicles fuse into dense aggregates that show a regular spacing corresponding to the bilayer width. Upon addition of EDTA, the aggregates readily disperse into large unilamellar vesicles. At low concentration of calcium ion, DDPBNS vesicles with an oligomerised inner leaflet form small multilamellar aggregates, in which a spacing corresponding to the bilayer width appears. Addition of excess EDTA results in slow dispersal of the Ca2+-lipid aggregates into a heterogeneous mixture of bilamellar, spherical vesicles and networks of thread-like vesicles. These lipid bilayer rearrangements are discussed within the context of shape transformations and fusion of lipid membranes.

Interaction of a spin-labeled long chain acylcholine with the cholinergic receptor protein in its membrane environment.

The choline ester of a spin-labeled fatty acid, 8-doxylpalmitocylcholine, CH3--(CH2)7--CR-(CH2)6-- + COO--(CH2)2--N(CH3)3, where R is the paramagnetic 4',4'-dimethyloxazolidine-N-oxyl (doxyl) ring has been synthesized. 8-Doxylpalmitoylcholine blocks reversibly the depolarization of Electrophorus electroplaque elicited by the bath application of carbamylcholine. It slows down the initial rate of binding of the alpha-[3-H]toxin from Naja nigricollis to receptor-rich membranes fragments from Torpedo, and it displaces [3-H]acetylcholine bound to the cholinergic receptor site present in these fragments. Electron spin resonance spectra of 8-doxylpalmitoylcholine in the presence of the receptor-rich membrane fragments show complete immobilization of the spin label. Various cholinergic agents tested, including N. nigricollis alpha-toxin, reverse this immobilization, probably by displacing the 8-doxylpalmitoylcholine from its complex with the cholinergic receptor protein to the lipid phase of the membrane.

Interaction between spin-labeled acyl-coenzyme A and the mitochondrial adenosine diphosphate carrier.

Spin-labeled long-chain (m,n)acyl-CoA's (general formula: CH3(CH2)mCR(CH2)nCOSCoA, where R is an oxazolidine ring containing a nitroxide) inhibit anion transports through the inner mitochondrial membrane at low concentrations as ordinary long-chain acyl-CoA's do. The inhibition constant relative to the inhibition of the ADP transport in heart mitochondria by spin-labeled palmityl-CoA and stearyl-CoA is of the order of 10-7 M, a value which is similar to that found for natural long-chain acyl-CoA's. A short-chain spin-labeled acyl-CoA (C5) showed no inhibitory effect in the range of concentrations tested (up to 30 muM). (2) (10,3)Acyl-CoA added to heart mitochondria at low concentrations exhibits spectra corresponding to an immobilized probe. The corresponding free fatty acid shows a higher freedom of motion between 0 and 30 degrees. The same differences in spectra of spin-labeled acyl-CoA and spin-labeled free fatty acid were found in inner membrane vesicles from rat liver mitochondria, but not in outer membrane preparations. (3) The selective interaction of spin-labeled acyl-CoA with the ADP carrier is indicated by the release of this interaction by specific ligands of the ADP carrier, such as ADP or ATP, carboxyatractyloside, adn bongkrekic acid. ADP (or ATP) and carboxyatractyloside rendered the spin-labeled (10,3)acyl-CoA nearly as mobile as the (10,3) free fatty acid. No effect was obtained with AMP, GDP, or UDP which are not transported by the ADP carrier. Bongkrekic acid, another specific inhibitor of the ADP carrier, was inactive when added alone; however, it was effective when added together with amounts of ADP which are ineffective per se. (4) The electron spin resonance (esr) spectrum observed at low concentrations of (10,3)acyl-CoA arises from (10,3)acyl-CoA bound to the ADP carrier. At higher concentrations the (10,3)-acy-CoA is more suggesting that the bulk of the label is also present in the lipid phase of the membrane. Spin-labeled acylCoA's incorporated into a sonicated dispersion of lipids extracted from heart mitochondria exhibited similar mobile spectra. (5) When the oxazolidine ring is moved down the hydrocarbon chain of the acyl-CoA, the binding features tended to disappear. Whereas nitroxide-protein interactions could be easily measured with the (10,3)acyl-CoA and the (7,6)acyl-CoA, much less or even no significant interactions could be detected with the (5,10)acyl-CoA or the (1,14)acyl-CoA. (6) The above results suggest that spin-labeled long-chain acylCoA added to mitochondria binds by its polar moiety to the ADP carrier. The acyl chain interacts with the ADP carrier protein over a length of 10-15 A. The remaining portion of the acyl chain experiences a fluid lipid environment.