Oleosin of Subcellular Lipid Droplets Evolved in Green Algae    

Abstract

In primitive and higher plants, intracellular storage lipid droplets (LDs) of triacylglycerols are stabilized with a surface layer of phospholipids and oleosin. In chlorophytes (green algae), a protein termed major lipid-droplet protein (MLDP) rather than oleosin on LDs was recently reported. We explored whether MLDP was present directly on algal LDs and whether algae had oleosin genes and oleosins. Immunofluorescence microscopy revealed that MLDP in the chlorophyte Chlamydomonas reinhardtii was associated with endoplasmic reticulum subdomains adjacent to but not directly on LDs. In C. reinhardtii, low levels of a transcript encoding an oleosin-like protein (oleolike) in zygotes-tetrads and a transcript encoding oleosin in vegetative cells transferred to an acetate-enriched medium were found in transcriptomes and by reverse transcription-polymerase chain reaction. The C. reinhardtii LD fraction contained minimal proteins with no detectable oleolike or oleosin. Several charophytes (advanced green algae) possessed low levels of transcripts encoding oleosin but not oleolike. In the charophyte Spirogyra grevilleana, levels of oleosin transcripts increased greatly in cells undergoing conjugation for zygote formation, and the LD fraction from these cells contained minimal proteins, two of which were oleosins identified via proteomics. Because the minimal oleolike and oleosins in algae were difficult to detect, we tested their subcellular locations in Physcomitrella patens transformed with the respective algal genes tagged with a Green Fluorescent Protein gene and localized the algal proteins on P. patens LDs. Overall, oleosin genes having weak and cell/development-specific expression were present in green algae. We present a hypothesis for the evolution of oleosins from algae to plants.Eukaryotes and prokaryotes contain neutral lipids in subcellular lipid droplets (LDs) in diverse cell types for food reserves and other purposes (Hsieh and Huang, 2004; Bickel et al., 2009;Murphy, 2012). These LDs are present in seeds, flowers, pollen, and fruits of higher plants; the vegetative and reproductive organs of primitive plants, algae, fungi, and nematodes; mammalian organs/tissues such as mammalian glands and adipose tissues; and bacteria. Of all the LDs, those in seeds, usually called oil bodies, are the most prominent and were studied extensively early on (Huang, 1992;Frandsen et al., 2001).Seeds of most plant species store oils (triacylglycerols [TAGs]) as a food reserve for germination and postgerminative growth. The TAGs are present in subcellular spherical LDs of approximately 0.5 to 2 mm in diameter (Frandsen et al

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

“…After omitting these likely redundant sequences, we retained 800 to 850 DNA-deduced oleosin sequences (Supplemental Table S1) for all subsequent analyses. Our extensive searches confirmed what our laboratory had reported earlier (Huang et al, 2013b): that organisms other than plants and their ancestor green algae do not possess oleosin genes.…”

Proteome Analysis of Cytoplasmatic and Plastidic β-Carotene Lipid Droplets in Dunaliella bardawil

“…In earlier work, we found that proteolysis of CGP destabilizes bC-plastoglobuli, suggesting that CGP may have a similar role to fibrillins in stabilizing the plastoglobules (Katz et al, 1995;Youssef et al, 2010;Singh and McNellis 2011). We did not identify in our proteomes homologs of green algal oleosins (Huang et al, 2013), Chlorella spp. caleosin (Lin et al, 2012), Nannochloropsis spp.…”

Major Lipid Body Protein: A Conserved Structural Component of Lipid Body Accumulated during Abiotic Stress in S. quadricauda CASA-CC202

“…Here, we report that C. reinhardtii MLDP polyclonal antibody (Huang et al, 2013) recognizes and cross-react with the S. quadricauda MLDP, indicating the presence and conservation of MLDP-like protein in green microalga linage (Figure 1). Additionally, we also tested the relative abundance of MLDP in two important stress conditions like nitrogen starvation and salt stress condition; confocal imaging reveals the accumulation of MLDP in both the stress condition (Figure 1).…”