Membrane Lipids Constituents in Chlamydomonas Chloroplast and Potential Vital Function of Betaine Lipid DGTS

INTRODUCTION: Elucidation of membrane lipids compositions in chloroplasts and their dynamic distributions is pivotal to understand the acyl-lipid metabolism in microalgae. More importantly, the efficient isolation of massive intact chloroplasts is an essential prerequisite for quantitative analysis of membrane lipids constituents at subcellular levels RATIONALE: A rapid cell disruption method driven by high pressure nitrogen was adopted to isolate highly purified intact chloroplasts from autotrophically grown Chlamydomonas reinhardtii. The transmission electron microscope and immunoblotting analysis were used to assess the structural integrity and purity of isolated chloroplasts. The components of membrane lipids and their fatty acyl profiles in cells and chloroplasts were analyzed and compared dependent on the thin-layer chromatography coupled with gas chromatography. RESULTS: Approximately 10% of intact chloroplasts based on chlorophyll content were isolated from cell-wall-deficient C. reinhardtii in this study. The fatty acyl amounts in the chloroplasts occupied nearly 72% of that in the cells. The membrane lipids in the chloroplasts predominantly included galactolipids (i.e., MGDG and DGDG) and sulfolipid (i.e., SQDG) (74%). The isolated chloroplasts also contained 9% of betaine lipid DGTS. In addition, the fatty acyl profiles in the chloroplast fractions were also prominently distinct from that in the extra-chloroplastidic fractions. Specifically, approximately equivalent proportions of C16 and C18 acyls were distributed in the chloroplasts, while the extra-chloroplastic fractions consisted of significantly more C18 acyls. These results revealed the subcellular compartment differentiation between the procaryotic (chloroplast) and eukaryotic (endoplasmic reticulum) biosynthetic pathways. The notable distribution of betaine lipid DGTS within the chloroplast (40%), and the distribution of its characteristic polyunsaturated fatty acyls (C18:3n6 and C18:4n3) in DGDG indicated the crucial function of DGTS as a potential transfer carrier to shift these fatty acyls to DGDG. CONCLUSION: The findings provide valuable insights into the underlying function of chloroplastidic DGTS in transfer and turnover of glycerolipids across subcellular compartments, further beneficial for understanding the dynamic glycerolipid remodeling and storage triacylglycerol biosynthesis.