Abstract
The halotolerant microalgae Dunaliella bardawil accumulates under nitrogen deprivation two types of lipid droplets: plastoglobuli rich in β-carotene (βC-plastoglobuli) and cytoplasmatic lipid droplets (CLDs). We describe the isolation, composition, and origin of these lipid droplets. Plastoglobuli contain β-carotene, phytoene, and galactolipids missing in CLDs. The two preparations contain different lipid-associated proteins: major lipid droplet protein in CLD and the Prorich carotene globule protein in βC-plastoglobuli. The compositions of triglyceride (TAG) molecular species, total fatty acids, and sn-1+3 and sn-2 positions in the two lipid pools are similar, except for a small increase in palmitic acid in plastoglobuli, suggesting a common origin. The formation of CLD TAG precedes that of βC-plastoglobuli, reaching a maximum after 48 h of nitrogen deprivation and then decreasing. Palmitic acid incorporation kinetics indicated that, at early stages of nitrogen deprivation, CLD TAG is synthesized mostly from newly formed fatty acids, whereas in βC-plastoglobuli, a large part of TAG is produced from fatty acids of preformed membrane lipids. Electron microscopic analyses revealed that CLDs adhere to chloroplast envelope membranes concomitant with appearance of small βC-plastoglobuli within the chloroplast. Based on these results, we propose that CLDs in D. bardawil are produced in the endoplasmatic reticulum, whereas βC-plastoglobuli are made, in part, from hydrolysis of chloroplast membrane lipids and in part, by a continual transfer of TAG or fatty acids derived from CLD.
Highlights
Eukaryotic cells accumulate neutral lipids in different tissues mainly in the form of lipid droplets (Murphy 2012)
Pulse-labeling experiments with 14C-palmitic acid (14C-PA) were designed to answer 3 questions: Are TAG in the two lipid pools synthesized de novo, from newly-incorporated fatty acids? Are TAG produced from degradation of membrane polar lipids in the chloroplast or in microsomal cytoplasmic membranes? Are TAG in βC-plastoglobuli made from pre-formed TAG in cytoplasmatic lipid droplets (CLD)? 14C-PA was chosen because we found that this fatty acid, which is a major component in all polar and neutral glycerolipid fractions in D. bardawil, is rapidly incorporaed into both cytoplasmic and chloroplastic membane lipids, as shown below
In this work we tried to clarify the interrelations between two neutral lipid droplets in Dunaliella bardawil, CLD and βC-plastoglobuli, the latter being unique to this species
Summary
Eukaryotic cells accumulate neutral lipids in different tissues mainly in the form of lipid droplets (Murphy 2012). Most lipid droplets consist of a core of triglycerides (TAG) and/or sterol esters coated by a phospholipids monolayer and embedded with proteins (Zweytick et al 2000). Plants accumulate triglycerides (TAG) in different tissues, primarily in seeds, and in fruit, such as in palm oil, in flowers and in leaves. Plants contain plastoglobuli, small chloroplastic lipid droplets consisting primarily of storage lipids and pigments. Proteome analyses of plastoglobuli suggest that they are involved in synthesis and degradation of lipids, pigments and coenzymes (Ytterberg et al 2006, Lundquist et al 2012). It has been shown that plant plastoglobuli are associated with thylakoid membranes (Austin et al 2006, Ytterberg et al 2006)
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