Analysis of a Super-Complex at Contact Sites Between Mitochondria and Plastids.

Plastids are organelles playing fundamental roles in different cellular processes such as energy metabolism or lipid biosynthesis. To fulfill their biogenesis and their function in the cell, plastids have to communicate with other cellular compartments. This communication can be mediated by the establishment of direct contact sites between plastids envelop and other organelles. These contacts are dynamic structures regulated in response to stress. For example, during phosphate (Pi) starvation, the number of contact sites between plastids and mitochondria significantly increases. In this situation, these contacts play an important role in the transfer of galactoglycerolipids from plastids to mitochondria. Recently, Pi starvation stress was used to identify key proteins involved in the traffic of galactoglycerolipids from plastids to mitochondria in Arabidopsis thaliana. A mitochondrial lipoprotein complex called MTL (Mitochondrial Transmembrane Lipoprotein) was identified. This complex contains mitochondrial proteins but also proteins located in the plastid envelope, suggesting its presence at the plastid-mitochondria junction. This chapter describes the protocol to isolate the MTL complex by clear-native polyacrylamide gel electrophoresis (CN-PAGE) from the mitochondrial fraction of Arabidopsis cell cultures and the methods to study different features of this complex.

Substitution of Deoxycholate with the Amphiphilic Polymer Amphipol A8-35 Improves the Stability of Large Protein Complexes during Native Electrophoresis.

Native polyacrylamide gel electrophoresis (PAGE) is a powerful technique for protein complex separation that retains both their activity and structure. In photosynthetic research, native-PAGE is particularly useful given that photosynthetic complexes are generally large in size, ranging from 200 kD to 1 MD or more. Recently, it has been reported that the addition of amphipol A8-35 to solubilized protein samples improved protein complex stability. In a previous study, we found that amphipol A8-35 could substitute sodium deoxycholate (DOC), a conventional electrophoretic carrier, in clear-native (CN)-PAGE. In this study, we present the optimization of amphipol-based CN-PAGE. We found that the ratio of amphipol A8-35 to α-dodecyl maltoside, a detergent commonly used to solubilize photosynthetic complexes, was critical for resolving photosynthetic machinery in CN-PAGE. In addition, LHCII dissociation from PSII-LHCII was effectively prevented by amphipol-based CN-PAGE compared with that of DOC-based CN-PAGE. Our data strongly suggest that majority of the PSII-LHCII in vivo forms C2S2M2 at least in Arabidopsis and Physcomitrella. The other forms might appear owing to the dissociation of LHCII from PSII during sample preparation and electrophoresis, which could be prevented by the addition of amphipol A8-35 after solubilization from thylakoid membranes. These results suggest that amphipol-based CN-PAGE may be a better alternative to DOC-based CN-PAGE for the study of labile protein complexes.

Formation of a PSI-PSII megacomplex containing LHCSR and PsbS in the moss Physcomitrella patens.

Mosses are one of the earliest land plants that diverged from fresh-water green algae. They are considered to have acquired a higher capacity for thermal energy dissipation to cope with dynamically changing solar irradiance by utilizing both the "algal-type" light-harvesting complex stress-related (LHCSR)-dependent and the "plant-type" PsbS-dependent mechanisms. It is hypothesized that the formation of photosystem (PS) I and II megacomplex is another mechanism to protect photosynthetic machinery from strong irradiance. Herein, we describe the analysis of the PSI-PSII megacomplex from the model moss, Physcomitrella patens, which was resolved using large-pore clear-native polyacrylamide gel electrophoresis (lpCN-PAGE). The similarity in the migration distance of the Physcomitrella PSI-PSII megacomplex to the Arabidopsis megacomplex shown during lpCN-PAGE suggested that the Physcomitrella PSI-PSII and Arabidopsis megacomplexes have similar structures. Time-resolved chlorophyll fluorescence measurements show that excitation energy was rapidly and efficiently transferred from PSII to PSI, providing evidence of an ordered association of the two photosystems. We also found that LHCSR and PsbS co-migrated with the Physcomitrella PSI-PSII megacomplex. The megacomplex showed pH-dependent chlorophyll fluorescence quenching, which may have been induced by LHCSR and/or PsbS proteins with the collaboration of zeaxanthin. We discuss the mechanism that regulates the energy distribution balance between two photosystems in Physcomitrella.

Measurement of Lipid Transport in Mitochondria by the MTL Complex.

Membrane biogenesis requires an extensive traffic of lipids between different cell compartments. Two main pathways, the vesicular and non-vesicular pathways, are involved in such a process. Whereas the mechanisms involved in vesicular trafficking are well understood, fewer is known about non-vesicular lipid trafficking, particularly in plants. This pathway involves the direct exchange of lipids at membrane contact sites (MCSs) between organelles. In plants, an extensive traffic of the chloroplast-synthesized digalactosyldiacylglycerol (DGDG) to mitochondria occurs during phosphate starvation. This lipid exchange occurs by non-vesicular trafficking pathways at MCSs between mitochondria and plastids. By a biochemical approach, a mitochondrial lipoprotein super-complex called MTL (Mitochondrial Transmembrane Lipoprotein complex) involved in mitochondria lipid trafficking has been identified in Arabidopsis thaliana. This protocol describes the method to isolate the MTL complex and to study the implication of a component of this complex (AtMic60) in mitochondria lipid trafficking.

Analysis of the MTL Supercomplex at Contact Sites Between Mitochondria and Plastids.

Plastids are organelles playing fundamental roles in different cellular processes such as energy metabolism or lipid biosynthesis. To fulfill their biogenesis and their function in the cell, plastids have to communicate with other cellular compartments. This communication can be mediated by the establishment of direct contact sites between plastids envelop and other organelles. These contacts are dynamic structures that are modified in response to stress. As example, during phosphate (Pi) starvation, the number of contact sites between plastids and mitochondria significantly increases. In this situation, these contacts play an important role in the transfer of galactoglycerolipids from plastids to mitochondria. Recently, Pi starvation stress was used to identify key proteins involved in the traffic of galactoglycerolipids from plastids to mitochondria in Arabidopsis thaliana. A mitochondrial lipoprotein complex called MTL (mitochondrial transmembrane lipoprotein complex) was identified. This complex contains mitochondrial proteins but also proteins located in the plastid envelope, suggesting its presence at the plastid-mitochondria junction. This chapter describes the protocol to isolate the MTL complex by clear-native polyacrylamide gel electrophoresis (CN-PAGE) from the mitochondrial fraction of Arabidopsis cell cultures and the methods to study different features of this complex.

Evidence of the supercomplex organization of photosystem II and light-harvesting complexes in Nannochloropsis granulata.

Diverse light-harvesting complexes (LHCs) have been found in photosynthetic microalgae that originated from secondary endosymbiosis involving primary red algae. However, the associations between LHCs and photosystem I (PSI) and photosystem II (PSII) in these microalgae are not fully understood. Eustigmatophyta is a red algal lineage that appears to have a unique organization in its photosynthetic machinery, consisting of only chlorophyll a and carotenoids that are atypical compared with other closely related groups. In this study, the supramolecular organization of pigment-protein complexes in the eustigmatophyte alga, Nannochloropsis granulata was investigated using Clear Native (CN) PAGE coupled with two-dimensional (2D) SDS-PAGE. Our results showed two slowly migrating green bands that corresponded to PSII supercomplexes, which consisted of reaction centers and LHCs. These green bands were also characterized as PSII complexes by their low temperature fluorescence emission spectra. The protein subunits of the PSII-LHC resolved by 2D CN/SDS-PAGE were analyzed by mass spectrometry, and four different LHC proteins were identified. Phylogenetic analysis of the identified LHC protein sequences revealed that they belonged to four different Lhc groups; (1) stress-related Lhcx proteins, (2) fucoxanthin chlorophyll a/c-binding Lhcf proteins, (3) red-shifted Chromera light-harvesting proteins (Red-CLH), and (4) Lhcr proteins, which are commonly found in organisms possessing red algal plastids. This is the first report showing evidence of a pigment-protein supercomplex consisting of PSII and LHCs, and to identify PSII-associated LHC proteins in Nannochloropsis.