Bioinformatics Reveal Five Lineages of Oleosins and the Mechanism of Lineage Evolution Related to Structure/Function from Green Algae to Seed Plants

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.

Subcellular Lipid Droplets in Vanilla Leaf Epidermis and Avocado Mesocarp Are Coated with Oleosins of Distinct Phylogenic Lineages

“…Vanilla transcripts of 2 U (U1 and U2), 2 SL (SL1 and SL2), and 1 SH oleosins are present in seed, leaves, shoots, and/or roots. The SL1-2 and SH1 oleosin transcripts are abundant in read per kilobase per million mapped reads (RPKMs) and largely restricted to seed, as expected for SL and SH oleosins in various plant species (Huang and Huang, 2015). U2 oleosin transcript at a low level is also restricted to seed.…”

“…In addition, we used RNA-seq to construct transcriptomes of leaves of Scadoxus (yielding one transcript encoding a full-length U oleosin [Supplemental Table S1] and another encoding an incomplete-length oleosin of unknown oleosin lineage) and Ornithogalum caudatum (yielding a transcript encoding an incomplete-length oleosin). Oleosins obtained from these sequence databases are categorized into the U, SL, and SH oleosin phylogenic lineages (Supplemental Table S1) in accordance with the established sequence characteristics of each lineage (Huang and Huang 2015) and their placement in a phylogenic tree of oleosins (Fig. 2).…”

“…These findings inspired us to examine the LD clusters in Asparagales and the LDs in avocado fruit mesocarp for their possible association with oleosins and the characteristics of these putative oleosins. Avocado mesocarp was not known to possess oleosins, but a recent report found oleosin transcripts in the tissue (Huang and Huang, 2015); whether the LDs in mesocarp are coated with oleosin has not been explored.…”

“…T oleosins are restricted to Brassicaceae and are absent in the closest family of Cleomaceae (Huang et al, 2013;Huang and Huang, 2015). They are associated with LDs in tapetosomes for the above-mentioned specific metabolic and physiological function.…”

“…A comprehensive bioinformatics analysis of oleosins based on available information in Huang and Huang (2015) revealed five oleosin phylogenic lineages. Primitive (P) oleosins are present on LDs in primitive organisms including green algae, mosses, and ferns.…”