Transit sequence-dependent binding of the chloroplast precursor protein ferredoxin to lipid vesicles and its implications for membrane stability

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Abstract
The binding of the transit peptide (trfd) and precursor of the chloroplast protein ferredoxin (prefd) to large unilamellar lipid vesicles was investigated in relation to the lipid composition of the bilayer. Prefd binds with a dissociation constant of 0.27 μM to vesicles with a composition corresponding to the chloroplast envelope outer membrane. Binding is mediated by the transit sequence. From an analysis of binding to vesicles containing the individual lipid components it could be concluded that anionic lipids are mainly responsible for binding, emphasizing the importance of electrostatics for the transit sequence-lipid interaction. Binding is also mediated by the specific chloroplast glycolipid monogalactosyldiacylglycerol. Monolayer experiments revealed that in this case a more extended domain of the transit sequence inserts into the lipid layer. Precursor binding does not result in a loss of vesicle barrier function. However, high concentrations of trfd do cause release of vesicle-enclosed carboxyfluorescein. The results are discussed in the light of the chloroplast protein import process, with special emphasis on the role of monogalactosyldiacylglycerol

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