Genome-Wide Analysis of Oleosin Gene Family in Tree Species

Triacylglycerols (TAGs) are the major molecules of energy storage in eukaryotes. Trees contribute to part of enormous plant oil reserves because fruits and kernels of many trees contain up to 50-80% of oil. TAGs accumulate in oil bodies in plants, similar to oil droplets in animals. Oleosins (OLEs) are a group of mostly low-molecular-mass hydrophobic proteins localized on the surfaces of plant oil bodies.  OLEs/oils ratio determines the size and shape of intracellular oil bodies. However, there is a lack of comprehensive sequence analysis of OLEs among diverse trees. The objectives of this study were to identify OLEs from tree species, to perform genome-wide analysis of tree OLEs, to classify tree OLEs, to identify conserved sequence motifs and amino acid residues in tree OLEs and OLE subfamilies and compared them with OLEs of Arabidopsis thaliana. Data mining identified 65 OLEs with perfect conservation of the “proline knot” motif (PX5SPX3P) from 19 trees including 5 sequenced tree genomes of Prunus persica (peach), Populus trichocarpa (poplar), Ricinus communis (castor bean), Theobroma cacao (cacao) and Vitis vinifera (grapevine). These OLEs were hydrophobic proteins with > 40% hydrophobic amino acid residues. They had similar properties and amino acid composition. Genome-wide phylogenetic analysis and multiple sequence alignment demonstrated that these proteins were separated into 5 OLE subfamilies. There were distinct patterns of sequence conservation among the OLE subfamilies and within individual species. These analyses should facilitate studying the structure-function relationship and identify critical amino acid residues in OLE for metabolic engineering of tree TAGs.