The transfer of fatty acids across biological membranes is a largely uncharacterized process, although it is essential at membranes of several higher plant organelles like chloroplasts, peroxisomes, or the endoplasmic reticulum. Here, we analyzed loss-of-function mutants of the unicellular cyanobacterium Synechocystis sp. PCC 6803 as a model system to circumvent redundancy problems encountered in eukaryotic organisms. Cells deficient in the only cytoplasmic Synechocystis acyl-acyl carrier protein synthetase (SynAas) were highly resistant to externally provided α-linolenic acid, whereas wild-type cells bleached upon this treatment. Bleaching of wild-type cells was accompanied by a continuous increase of α-linolenic acid in total lipids, whereas no such accumulation could be observed in SynAas-deficient cells (Ɗsynaas). When SynAas was disrupted in the tocopherol-deficient, α-linolenic acid-hypersensitive Synechocystis mutant Ɗslr1736, double mutant cells displayed the same resistance phenotype as Ɗsynaas. Moreover, heterologous expression of SynAas in yeast (Saccharomyces cerevisiae) mutants lacking the major yeast fatty acid import protein Fat1p (Ɗfat1) led to the restoration of wild-type sensitivity against exogenous α-linolenic acid of the otherwise resistant Ɗfat1 mutant, indicating that SynAas is functionally equivalent to Fat1p. In addition, liposome assays provided direct evidence for the ability of purified SynAas protein to mediate α-[14C]linolenic acid retrieval from preloaded liposome membranes via the synthesis of [14C]linolenoyl-acyl carrier protein. Taken together, our data show that an acyl-activating enzyme like SynAas is necessary and sufficient to mediate the transfer of fatty acids across a biological membrane.

Addendum to: Kunz HH, Scharnewski M, Feussner K, Feussner I, Flügge UI, Fulda M, Gierth M. The ABC transporter PXA1 and peroxisomal β-oxidation are vital for metabolism in mature leaves of Arabidopsis during extended darkness.