Proteomic profiling of oil bodies isolated from the unicellular green microalga Chlamydomonas reinhardtii: With focus on proteins involved in lipid metabolism

Abstract

Oil bodies are sites of energy and carbon storage in many organisms including microalgae. As a step toward deciphering oil accumulation mechanisms in algae, we used proteomics to analyze purified oil bodies from the model microalga Chlamydomonas reinhardtii grown under nitrogen deprivation. Among the 248 proteins (≥ 2 peptides) identified by LC-MS/MS, 33 were putatively involved in the metabolism of lipids (mostly acyl-lipids and sterols). Compared with a recently reported Chlamydomonas oil body proteome, 19 new proteins of lipid metabolism were identified, spanning the key steps of the triacylglycerol synthesis pathway and including a glycerol-3-phosphate acyltransferase (GPAT), a lysophosphatidic acid acyltransferase (LPAT) and a putative phospholipid:diacylglycerol acyltransferase (PDAT). In addition, proteins putatively involved in deacylation/reacylation, sterol synthesis, lipid signaling and lipid trafficking were found to be associated with the oil body fraction. This data set thus provides evidence that Chlamydomonas oil bodies are not only storage compartments but also are dynamic structures likely to be involved in processes such as oil synthesis, degradation and lipid homeostasis. The proteins identified here should provide useful targets for genetic studies aiming at increasing our understanding of triacyglycerol synthesis and the role of oil bodies in microalgal cell functions.

Three Acyltransferases and Nitrogen-responsive Regulator Are Implicated in Nitrogen Starvation-induced Triacylglycerol Accumulation in Chlamydomonas

“…Previous studies have provided evidence for fatty acid recycling, and this is consistent with increased expression of lipases in nitrogen-starved cells (18,42,73). Some of these are annotated as TAG lipases, which would be counterintuitive, but they may well be membrane lipid lipases that release fatty acids that are subsequently incorporated into TAGs.…”

“…Surprisingly, genes LIPG2, Cre05.g234800, and Cre02.g127300, encoding candidate TAG lipases, are up-regulated, which has also been noted in previous transcriptome and proteome studies (18,42). One possibility is that these enzymes are not TAG lipases but rather play a role in releasing fatty acids from membrane lipids for de novo TAG synthesis.…”

“…DGTT1 is up-regulated in response to nitrogen starvation, but its function could not be validated (18,41). Nevertheless, proteomic studies of membrane fractions from Chlamydomonas enriched for lipid droplets identified DGTT1, PDAT1, and a structural protein (named MLDP for major lipid droplet protein) (19,42). To assess the role of these proteins in TAG accumulation in Chlamydomonas, we used RNA-Seq to describe the transcriptome during a time course of nitrogen starvation.…”

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

“…MLDP stabilizes mature LDs and acts as a recruiting platform for other proteins and enzymes required for the maturation of LDs. In C. reinhardtii, MLDP is identified as an abundant protein on the outer surface of the LDs proteome (Moellering and Benning, 2010;Nguyen et al, 2011). MLDP abundance positively correlates with the accumulation of TAGs (Tsai et al, 2014).…”

The Evolutionary Conserved Oil Body Associated Protein OBAP1 Participates in the Regulation of Oil Body Size

“…with structural similarities to other OB-associated proteins (Vieler et al, 2012), and a major lipid droplet protein was also identified in Chlamydomonas reinhardtii and other green algae species (Moellering and Benning, 2010;Peled et al, 2011). Proteomic analysis in different plants and algae identified a wide variety of proteins associated with OBs (Katavic et al, 2006;Jolivet et al, 2009;Nguyen et al, 2011;Tnani et al, 2011Tnani et al, , 2012. Different roles have been proposed to the OB-associated proteins, including OB biogenesis, stability, trafficking, and mobilization.…”