Abstract
Plant cells contain subcellular lipid droplets with a triacylglycerol matrix enclosed by a layer of phospholipids and the small structural protein oleosin. Oleosins possess a conserved central hydrophobic hairpin of approximately 72 residues penetrating into the lipid droplet matrix and amphipathic amino- and carboxyl (C)-terminal peptides lying on the phospholipid surface. Bioinformatics of 1,000 oleosins of green algae and all plants emphasizing biological implications reveal five oleosin lineages: primitive (in green algae, mosses, and ferns), universal (U; all land plants), and three in specific organs or phylogenetic groups, termed seed low-molecular-weight (SL; seed plants), seed high-molecular-weight (SH; angiosperms), and tapetum (T; Brassicaceae) oleosins. Transition from one lineage to the next is depicted from lineage intermediates at junctions of phylogeny and organ distributions. Within a species, each lineage, except the T oleosin lineage, has one to four genes per haploid genome, only approximately two of which are active. Primitive oleosins already possess all the general characteristics of oleosins. U oleosins have C-terminal sequences as highly conserved as the hairpin sequences; thus, U oleosins including their C-terminal peptide exert indispensable, unknown functions. SL and SH oleosin transcripts in seeds are in an approximately 1:1 ratio, which suggests the occurrence of SL-SH oleosin dimers/multimers. T oleosins in Brassicaceae are encoded by rapidly evolved multitandem genes for alkane storage and transfer. Overall, oleosins have evolved to retain conserved hairpin structures but diversified for unique structures and functions in specific cells and plant families. Also, our studies reveal oleosin in avocado (Persea americana) mesocarp and no acyltransferase/lipase motifs in most oleosins.