The seed oleosins: Structure, properties and biological role

The oleosins are a group of proteins that are present on the surface of oil bodies present in seeds and other oil-storing tissues. They appear to form a monolayer with phospholipids that acts to maintain the size and stability of the oil body. Although most studies indicate that oleosin synthesis is closely integrated with triacylglycerol synthesis, oleosins occur in isoforms that may differ in theirpattern of synthesis during development or in their tissue specificity. Comparisons of oleosin sequences show that all comprise three structural domains, a highly conserved central hydrophobic domain of about 70 amino acid residues flanked by more variable N- and C-terminal domains. There is general agreement that the C-terminal domain forms an amphipathic α-helix located on the oil body surface while the N-terminal domain may contain a mixture of structures and be located in the cytoplasm. In contrast, the hydrophobic domain is thought to be buried within the triacylglycerol matrix and both α-helical and β-sheet structures have been proposed. We present a new model for this domain based on an α-helical hairpin structure. A combination of in vivo and in vitro studies has demonstrated that oleosins are initially targeted to the endoplasmic reticulum (ER) by an SRP (signal recognition particle) -mediated process, and have identified parts of the oleosin molecule that are required for this targeting, for insertion into the ER membrane and for the correct orientation and topology once inserted. However, the precise structure of the oleosin hydrophobic domain in the ER membrane and whether this structure changes during oil body ontology are not known.