Figure 10.

Working model describing the LACS-mediated transfer of fatty acids from the ER to the chloroplast envelope. By the Lands cycle at the ER, mainly de novo synthesized oleic acid is incorporated into the sn-2 position of PC. The oleic acid is converted by the desaturase FAD2 to linoleic acid (18:2), and PC molecules with this signature are the preferred substrate for lipid transfer toward the plastid. We propose that 18:2 of remodeled PC is released through deacylation by phospholipase A₂ to establish a pool of lyso-PC and a pool of free linoleic acid, both destined for lipid transfer to the plastid. As shown earlier (Bessoule et al., 1995), the lyso-PC is well suited to migrate between membranes and is able to move from the ER to the plastid. In parallel, LACS4 and LACS9 are both involved in converting the free linoleic acid into a tightly coupled pool of linoleoyl-CoA, which is used by plastidial acyl-CoA-lyso-PC acyltransferase to resynthesize PC in the plastidial envelope. Parts of the resynthesized PC stay in the envelope, whereas another portion is converted via PA into DAG, which is the immediate substrate for glycolipid biosynthesis. X, C₁₆, or C₁₈ fatty acid.