Incorporation of spin-labeled fatty acids into bovine brain clathrin coated vesicles

Stearic acids with a nitroxide radical at selected positions have been incorporated in the phospholipid bilayers of clathrin coated vesicles, uncoated vesicles and sonicated liposomes made from the lipids extracted from the uncoated vesicles. The extent of incorporation was found minimum for stearic acids labeled on C-12 and for bilayers of uncoated vesicles. The ESR spectra of the spin-labeled fatty acids incorporated in the bilayers showed a pronounced temperature dependence (without discontinuity) and a decrease in the hyperfine splitting as the nitroxide group was inserted deeper in the hydrophobic core of the membranes. An abrupt phospholipid phase transition or a phase separation could be excluded. The presence of the external proteins (the clathrin coat) on the membranes was not found to noticeably influence the gradient of flexibility of the fatty acid chains of the phospholipids. The influence of the internal proteins embedded in the bilayers was evidenced by a detailed analysis of the ESR spectra of (7,8)SA in terms of two components: one component arising from the labels surrounded exclusively by phospholipids, the other component arising from labels of reduced mobility perturbed by the vicinity of the proteins. These results support the persistence of lipidic domains in the endocytic vesicles despite the accumulation of receptors which follows their formation.

Triggering of mannosyltransferase activity in inner mitochondrial membranes by dolichyl-monophosphate incorporation mediated through phospholipids or fatty acids

The activity of GDPmannose:dolichyl monophosphate mannosyltransferase in inner mitochondrial membranes can be triggered by dolichyl-monophosphate incorporation mediated through phospholipids or fatty acids. The efficiency of this incorporation and the efficiency of the enzyme activity are not equivalent. Among a variety of amphiphiles which were tested, the highest mannosyltransferase activity was obtained with the mixture of lipids extracted from the outer mitochondrial membranes. The results presented here appear consistent only with a mechanism involving collisional contacts of the phospholipid vesicles and fusion with the membranes. ESR spectroscopy confirms that (a) the incorporation process is followed by solubilization of dolichyl monophosphate molecules in the lipid phase and (b) the general organization of the inner mitochondrial membranes is not perturbed by the addition of dolichyl monophosphate.

Structural studies on transmembrane proteins. 2. Spin labeling of bacteriorhodopsin mutants at unique cysteines

Site-directed mutagenesis was used to produce mutants of bacteriorhodopsin where either glycine-72, threonine-90, leucine-92, or serine-169 was replaced by a cysteine. Two different spin labels were then covalently attached to these sites. The selection of attachment sites covered two postulated loops (72,169) and a membrane-spanning segment (90,92). It was not possible to properly refold the protein labeled at position 90, presumably due to steric problems, but the EPR spectra of the other mutants that were successfully reconstituted in phospholipid vesicles provided information on the dynamics of protein side chains in the vicinity of the label site. A power saturation approach was used to investigate the spin relaxation times, which in turn can be influenced by collisions with paramagnetic species. The differential effect of oxygen and a water-soluble chromium complex on the power-saturation behavior of the spin-labeled mutants was used to obtain topographical information on the sites in the membrane-bound protein. The results are consistent with residues 72 and 169 being located in structured loops exposed to the aqueous phase and residue 92 being localized in the membrane interior, possibly near a helix-helix contact region.

Oxygen diffusion-concentration in erythrocyte plasma membranes studied by the fluorescence quenching of anionic and cationic pyrene derivatives

Fluorescence quenching by oxygen of cationic [pyrene-(CH2)nN(CH3)3+; n = 1, 4, and 11] and anionic [pyrene-(CH2)nCO2-, n = 3, 8, 11, and 15] probes was investigated in erythrocyte plasma membranes (leaky) in order to assess the ability of oxygen molecules to interact with solutes located at different positions in the membrane. The pseudounimolecular quenching rate constants measured increase, both for cationic and anionic probes, when n increases. These results are interpreted in terms of an increased oxygen solubility toward the center of the membrane interior, and imply that lateral diffusion contributes more than transverse diffusion to total oxygen mobility. For all of the probes considered, quenching rates increase when n-alkanols are added. The effect observed increases when n decreases and when the size of the n-alkanol alkyl chain increases. Arrhenius-type plots for the quenching rate constants show noticeable downward curvatures. Average (0-40 degrees C) activation energies are approximately 6 kcal/mol.

A radioprotector: cysteamine, inhibits oxygen transport in lipidic membranes

Spin labeling techniques make possible the observation of oxygen diffusion or concentrations in phospholipid membranes. In such a system, cysteamine, depending upon the molecular cysteamine/DPPC ratio and the pH conditions, inhibits oxygen transport, and this result provides an original explanation for cellular hypoxia after cysteamine administration.