The insertion of soluble proteins into membranes has been a topic of considerable interest. We have studied the insertion of bovine alpha-lactalbumin into single-bilayer vesicles prepared from egg phosphatidylcholine (PC). Fluorescence studies indicated rapid and tight binding of apo-alpha-lactalbumin (apo-alpha-LA) to PC vesicles as a function of pH. The binding was maximal at pH values which favor the formation of the molten globule state. As an increase of hydrophobic surface is observed in the molten globule state, this conformational state can provide a molecular basis for insertion of soluble proteins into membranes. The membrane-bound complex formed at low pH (3.0) could be isolated and was found to be stable at neutral pH. The structural characterization of the apo-alpha-LA-PC complex was studied by fluorescence quenching using iodide, acrylamide, and 9,10-dibromostearic acid. The results obtained indicated that some of the tryptophans of apo-alpha-LA were buried in the membrane interior and some were exposed on the outer side. Fluorescence quenching and CD studies indicated the membrane-bound conformation of apo-alpha-LA was some conformational state that is between the soluble, fully folded conformation and the molten globule state.

-Thirteen dairy propionibacteria were screened for catalase-insensitive, protease-sensitive inhibition of various Gram-positive and Gram-negative bacteria. Production of bacteriocin-like agents was identified in 8 ofthese cultures. The bacteriocin produced by Propionibacteriumjensenii B 1264 inhibited closely related propionibacteria and lactic acid bacteria and was bactericidal to Lactobacillus delbrueckii subsp. lactis ATCC 4797. It was produced in sodium lactate broth cultures during stationary phase; maximum activity was detected after 10 d of growth but only after IO-fold concentration of culture supematants. The bacteriocin was precipated by ammonium sulphate at 70 % saturation. The ammonium sulphate-concentrated, dialyzed bacteriocin was stable to 0.1-1.0 rnol-Lr' NaCI, 0.1-2.0 % SOS, 4 mol-L:' urea, 100 "C (60 min), pH 2-10 and organic sol vents (methanol, ethanol and isopropanol at 0-50 %). Batch anion exchange chromatography (pH 6.5) of the bacteriocin yielded 101 % activity and 15-fold increased purity. Flat bed analytical isoelectric focussing showed that the isoelectric point was between pH 3 and 3.5. SOS-PAGE yielded an estimated molecular mass between 6 and 9 kOa. The evidence suggests that this anionic bacteriocin is unique. © Inra/Eisevier, Paris.