Oleosins as Lipid Biosynthetic Enzymes and its Regulation by Phosphorylation

Oils are generally stored in spherical intracellular organelles referred to as oil bodies that are covered by proteins like oleosin.  Seeds with high oil content have more oleosins than those with low content.  However, the exact role of oleosins in oil biosynthesis and accumulation is unclear.  A multiprotein complex containing monoacylglycerol acyltransferase (MGAT) from microsomal membranes of immature Arachis hypogaea seeds was isolated.  MGAT and phosphatidylcholine hydrolase activities are associated with oleosin-3 (OLE3). Overexpression of OLE3 gene in Saccharomyces cerevisiae showed an increased accumulation of triacylglycerols. A serine/threonine/tyrosine protein kinase phosphorylates OLE3 and the phospho-OLE3 exhibited a low acyltransferase and a high phospholipase A2 activities. A homologue of oleosin 3 in Arabidopsis thaliana is At4g25140 (oleosin 1, OLE1) is unique in having phosphatase (DXDXT) motif at the C-terminal end. OLE1 is capable of dephosphorylating lysophosphatidic acid tomonoacylglycerol. Overexpression of OLE1 in S. cerevisiae caused an accumulation of TAG. The phosphatase activity of oleosin 1 is tightly phosphoregulated by serine/threonine/tyrosine protein kinase. These findings provide a direct role for a storage protein in the biosynthesis and mobilization of fatty oils.