THE SPECIFICITY OF GLYCOLIPID-PREFERREDOXIN INTERACTION - REQUIREMENTS FOR MEMBRANE-BINDING

Preferredoxin (prefd) is a precursor protein that is imported into chl oroplasts. Monolayer experiments have shown that prefd has a high affi nity for monogalactosyldiglyceride (MGalDG) isolated from chloroplasts , which contains polyunsaturated fatty acid constituents and is theref ore in a liquid-expanded state, but has been found to interact also wi th MGalDG with long-chain saturated fatty acids, which exist in a gel state. For an optimal interaction, the fatty acid chain length and the extent of unsaturation are also important parameters, whereas the con formation of the sugar moiety, the sugar-glycerol or glycerol-hydrocar bon chain linkages are of little influence on the pressure changes mea sured in monomolecular layers. Conversely, steric hindrance of a methy l group at position 3 of the sugar largely inhibits the interaction. Q uantification of the interaction with radiolabelled prefd shows that o nly a smalt part of the molecule is able to penetrate MGalDG in the ge l state, whereas a nearly four-times larger part is able to penetrate MGalDG isolated from chloroplasts. It is likely that interactions of t he transit sequence of prefd with the glycolytic head group MGalDG are involved in targeting and binding to the chloroplast membrane.