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Method Article
A fast and efficient protocol is presented for the isolation of plastoglobule lipid droplets associated with various photosynthetic organisms. The successful preparation of isolated plastoglobules is a crucial first step that precedes detailed molecular investigations such as proteomic and lipidomic analyses.
Plastoglobule lipid droplets are a dynamic sub-compartment of plant chloroplasts and cyanobacteria. Found ubiquitously among photosynthetic species, they are believed to serve a central role in the adaptation and remodeling of the thylakoid membrane under rapidly changing environmental conditions. The capacity to isolate plastoglobules of high purity has greatly facilitated their study through proteomic, lipidomic, and other methodologies. With plastoglobules of high purity and yield, it is possible to investigate their lipid and protein composition, enzymatic activity, and protein topology, among other possible molecular characteristics. This article presents a rapid and effective protocol for the isolation of plastoglobules from chloroplasts of plant leaf tissue and presents methodological variations for the isolation of plastoglobules and related lipid droplet structures from maize leaves, the desiccated leaf tissue of the resurrection plant, Eragrostis nindensis, and the cyanobacterium, Synechocystis sp. PCC 6803. Isolation relies on the low density of these lipid-rich particles, which facilitates their purification by sucrose density flotation. This methodology will prove valuable in the study of plastoglobules from diverse species.
The current understanding of plastoglobule composition and function has emerged through detailed proteomic and lipidomic studies1,2,3,4,5. Such studies have been greatly aided by a rapid and effective method of isolation that relies on their very low density for efficient separation using sucrose gradients. Initial methods of plastoglobule isolation were achieved from species such as the beech tree (Fagus sylvatica), scotch broom (Sarothamnus scoparius), onion (Allium cepa), spinach (Spinacia oleracea), pansy (Viola tricolor), pepper (Capsicum annuum), and pea (Pisum sativum)6,7,8,9,10,11,12,13. An updated method to isolate chloroplast plastoglobules in a more efficient and better yielding manner was later presented by Ytterberg et al.3,14. While initially employed for the study of the plastoglobules of Arabidopsis thaliana leaf chloroplasts, we have successfully employed this updated method for the healthy leaf tissue of other plant species, both monocot and dicot, including maize (Zea mays), tomato (Solanum lycopersicum), lovegrass (Eragrostis nindensis), purple false brome (Brachypodium distachyon), and wild tobacco (Nicotiana benthamiana; unpublished results). Furthermore, the isolation method has been successfully adapted to the plastoglobules of cyanobacteria, including Synechocystis sp. PCC 6803 and Anabaena sp. PCC 712015, and the desiccated leaf tissue of the resurrection plant, E. nindensis.
Chloroplast plastoglobules of healthy leaf tissue are physically connected to the thylakoid membranes16. Despite this physical continuity, the two chloroplast sub-compartments maintain distinct lipid and protein compositions, although the regulated exchange of lipid and protein between the two compartments has been proposed2,4,17,18,19. In fact, an interesting hemifusion model has recently been proposed for the trafficking of neutral lipids between chloroplasts and cytosol19. Due to the physical continuity of plastoglobules and thylakoids, the isolation method with healthy leaf tissue begins with the collection of a pelleted crude thylakoid preparation, which is subsequently sonicated to separate the plastoglobules from the thylakoids, which is in contrast to methods used for isolating cytosolic lipid droplets20. Ultracentrifugation on a sucrose cushion then floats the low-density plastoglobules up through the sucrose, effectively separating them from the thylakoids, nuclei, and other high-density material. In contrast, plastoglobules in cyanobacteria, as well as those of desiccated leaf tissue, evidently exist in vivo in a free-floating form. Hence, their isolation involves directly floating on a sucrose gradient. This article demonstrates the isolation method from healthy leaf tissue and further demonstrates two variations that can be used to isolate plastoglobules from desiccated leaf tissue or cyanobacterial cultures, greatly expanding the physiological breadth and evolutionary context in which plastoglobules can be studied.
Isolated plastoglobules can subsequently be used for any number of downstream analyses to investigate molecular characteristics. We have used the isolated plastoglobules from A. thaliana leaf tissue for extensive proteomic and lipidomic analysis under differing environmental conditions or genotypes, demonstrating the selective modification of protein and lipid composition in adaptation to stress2,4,21,22. In addition, in vitro kinase assays that demonstrate trans-phosphorylation activity associated with isolated plastoglobules have been performed22, the oligomeric states of protein components has been investigated using native gel electrophoresis 21, and protease-shaving assays have been performed23.
The primary benefit of this method is the relative speed of the procedure. In our experience, the protocols outlined below can be fully completed within approximately 4 h. An alternate method to isolate plastoglobules from leaf tissue has been described, which allows the simultaneous isolation of other chloroplast sub-compartments24. This alternative method offers some clear advantages when quantitative comparison to the other chloroplast sub-compartments is necessary or desired. However, this alternative method is also more tedious and will provide a significantly lower yield of isolated plastoglobules from comparable quantities of leaf tissue. When a focused study of plastoglobules is the aim, the methodology outlined here is the optimal choice. Nonetheless, total leaf and crude thylakoid aliquots can be collected during the sample preparation, and it is highly recommended to do so, to have reference samples for subsequent comparison.
1. Crude plastoglobule isolation
2. Harvesting pure plastoglobules
Upon completion of step 1 of the protocol, one should be able to readily see a considerable amount of plastoglobule/lipid droplet material floating on (or near) the top layer of the sucrose cushion (Figure 1B-C). Other fractions could also be collected at this stage. For example, the thylakoids will be pelleted and can be re-suspended with medium R 0.2 for subsequent analyses. After subsequent centrifugation, purified plastoglobules will be obtained at or ne...
To minimize physiological/biochemical changes to the material and protect certain photo- and thermo-labile prenyl-lipid pigments that are a rich component of plastoglobules, it is critical to perform the isolation at 4 °C and protected from light. As indicated above, the initial steps are performed in the cold room under a safety lamp using a green-emitting light bulb. The subsequent steps performed in the laboratory are under dimmed lights and use ice or refrigerated centrifugation. For similar reasons, the inclusi...
No conflicts of interest to declare.
Research in the Lundquist lab group is supported by grants from the NSF (MCB-2034631) and USDA (MICL08607) to P.K.L. The authors thank Dr. Carrie Hiser (MSU) for support in the development of the cyanobacterial plastoglobule isolation method.
Name | Company | Catalog Number | Comments |
AEBSF | Milipore Sigma | P7626 | |
Antipain.2HCl | Bachem | H-1765.0050BA | |
Aprotinin | Milipore Sigma | A6106 | |
Ascorbate | BDH | BDH9242 | |
Bestatin | Sigma Aldrich | B8385 | |
Beta-Glycerophosphate. 2Na5H2O | EMD Millipore | 35675 | |
Bovine Serum Albumin | Proliant Biological | 68700 | |
Chymostatin | Sigma Aldrich | C7268 | |
Eragrostis nindensis | N/A | N/A | |
E-64 | Milipore Sigma | E3132 | |
French Pressure cell (model FA-079) | SLM/Aminco | N/A | |
HEPES | Sigma Aldrich | H3375 | |
Leupeptin | Sigma Aldrich | L2884 | |
Magnesium Chloride | Sigma Aldrich | M8266 | |
Multitron shaking incubator | Infors HT | N/A | |
Phospho-ramidon.2 Na | Sigma Aldrich | R7385 | |
Potassium Hydroxide | Fisher Chemicals | M16050 | |
Reduced Cysteine | MP Biochemicals | 101444 | |
Sodium Fluoride | Sigma Aldrich | S7920 | |
Sodium Ortho-vanadate | Sigma Aldrich | 450243 | |
Sodium Pyrophosphate · 10H2O | Sigma Aldrich | 3850 | |
Sorbitol | Sigma Aldrich | S3889 | |
Sucrose | Sigma Aldrich | S9378 | |
Sylvania 15 W fluorescent Gro-Lux tube light bulb, 18" | Walmart | N/A | |
Synechocystis sp. PCC 6803 | N/A | N/A | |
Optima MAX-TL Ultracentrifuge | Beckman Coulter | A95761 | |
Waring Blender (1.2 L) | VWR | 58977-227 | Commercial blender |
Zea mays | N/A | N/A |
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