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In plants, de novo synthesis of fatty acids (FAs) occurs in plastids, whereas assembly and modification of acyl lipids is accomplished in the endoplasmic reticulum (ER) and plastids as well as in mitochondria. Subsequently, lipophilic compounds are distributed within the cell and delivered to their destination site. Thus, constant acyl-exchanges between subcellular compartments exist. These can occur via several modes of transport and plant membrane-intrinsic proteins for FA/lipid transfer have been shown to play an essential role in delivery and distribution. Lately, substantial progress has been made in identification and characterization of transport proteins for lipid compounds in plant organelle membranes. In this review, we focus on our current understanding of protein mediated lipid traffic between organelles of land plants.

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Dehydroepiandrosterone sulfate (DHEAS) is the most abundant circulating steroid in human, with the highest concentrations between age 20 and 30, but displaying a significant decrease with age. Many beneficial functions are ascribed to DHEAS. Nevertheless, long-term studies are very scarce concerning the intake of DHEAS over several years, and molecular investigations on DHEAS action are missing so far. In this study, the role of DHEAS on the first and rate-limiting step of steroid hormone biosynthesis was analyzed in a reconstituted in vitro system, consisting of purified CYP11A1, adrenodoxin and adrenodoxin reductase. DHEAS enhances the conversion of cholesterol by 26%. Detailed analyses of the mechanism of DHEAS action revealed increased binding affinity of cholesterol to CYP11A1 and enforced interaction with the electron transfer partner, adrenodoxin. Difference spectroscopy showed K d-values of 40±2.7 µM and 24.8±0.5 µM for CYP11A1 and cholesterol without and with addition of DHEAS, respectively. To determine the K d-value for CYP11A1 and adrenodoxin, surface plasmon resonance measurements were performed, demonstrating a K d-value of 3.0±0.35 nM (with cholesterol) and of 2.4±0.05 nM when cholesterol and DHEAS were added. Kinetic experiments showed a lower Km and a higher kcat value for CYP11A1 in the presence of DHEAS leading to an increase of the catalytic efficiency by 75%. These findings indicate that DHEAS affects steroid hormone biosynthesis on a molecular level resulting in an increased formation of pregnenolone.

In algae and land plants, transport of fatty acids (FAs) from their site of synthesis in the plastid stroma to the endoplasmic reticulum (ER) for assembly into acyl lipids is crucial for cellular lipid homeostasis, including the biosynthesis of triacylglycerol (TAG) for energy storage. In the unicellular green alga Chlamydomonas reinhardtii, understanding and engineering of these processes is of particular interest for microalga-based biofuel and biomaterial production. Whereas in the model plant Arabidopsis thaliana, FAX (fatty acid export) proteins have been associated with a function in plastid FA-export and hence TAG synthesis in the ER, the knowledge on the function and subcellular localization of this protein family in Chlamydomonas is still scarce. Among the four FAX proteins encoded in the Chlamydomonas genome, we found Cr-FAX1 and Cr-FAX5 to be involved in TAG production by functioning in chloroplast and ER membranes, respectively. By in situ immunolocalization, we show that Cr-FAX1 inserts into the chloroplast envelope, while Cr-FAX5 is located in ER membranes. Severe reduction of Cr-FAX1 or Cr-FAX5 proteins by an artificial microRNA approach results in a strong decrease of the TAG content in the mutant strains. Further, overexpression of chloroplast Cr-FAX1, but not of ER-intrinsic Cr-FAX5, doubled the content of TAG in Chlamydomonas cells. We therefore propose that Cr-FAX1 in chloroplast envelopes and Cr-FAX5 in ER membranes represent a basic set of FAX proteins to ensure shuttling of FAs from chloroplasts to the ER and are crucial for oil production in Chlamydomonas.

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The human mitochondrial cytochrome P450 enzymes CYP11A1, CYP11B1, and CYP11B2 are involved in the biosynthesis of steroid hormones. CYP11A1 catalyzes the side-chain cleavage of cholesterol, and CYP11B1 and CYP11B2 catalyze the final steps in the biosynthesis of gluco-and mineralocorticoids, respectively. This study reveals their additional capability to metabolize the xenobiotic steroid oral turinabol (OT; 4-chlor-17b-hydroxy-17a-methylandrosta-1,4-dien-3-on), which is a common doping agent. By contrast, microsomal steroid hydroxylases did not convert OT. Spectroscopic binding assays revealed dissociation constants of 17.7 mM and 5.4 mM for CYP11B1 and CYP11B2, respectively, whereas no observable binding spectra emerged for CYP11A1. Catalytic efficiencies of OT conversion were determined to be 46 min 21 mM 21 for CYP11A1, 741 min 21 mM 21 for CYP11B1, and 3338 min 21 mM 21 for CYP11B2, which is in the same order of magnitude as for the natural substrates but shows a preference of CYP11B2 for OT conversion. Products of OT metabolism by the CYP11B subfamily members were produced at a milligram scale with a recombinant Escherichia coli-based whole-cell system. They were identified by nuclear magnetic resonance spectroscopy to be 11b-OH-OT for both CYP11B isoforms, whereby CYP11B2 additionally formed 11b,18-diOH-OT and 11b-OH-OT-18-al, which rearranges to its tautomeric form 11b,18-expoxy-18-OH-OT. CYP11A1 produces six metabolites, which are proposed to include 2-OH-OT, 16-OH-OT, and 2,16-diOH-OT based on liquid chromatography-tandem mass spectrometry analyses. All three enzymes are shown to be inhibited by OT in their natural function. The extent of inhibition thereby depends on the affinity of the enzyme for OT and the strongest effect was demonstrated for CYP11B2. These findings suggest that steroidogenic cytochrome P450 enzymes can contribute to drug metabolism and should be considered in drug design and toxicity studies.