BBA - Bioenergetics (v.1837, #4)

Functional role of cardiolipin in mitochondrial bioenergetics by Giuseppe Paradies; Valeria Paradies; Valentina De Benedictis; Francesca M. Ruggiero; Giuseppe Petrosillo (408-417).
Cardiolipin is a unique phospholipid which is almost exclusively located in the inner mitochondrial membrane where it is biosynthesized. Considerable progress has recently been made in understanding the role of cardiolipin in mitochondrial function and bioenergetics. This phospholipid is associated with membranes designed to generate an electrochemical gradient that is used to produce ATP, such as bacterial plasma membranes and inner mitochondrial membrane. This ubiquitous and intimate association between cardiolipin and energy transducing membranes indicates an important role for cardiolipin in mitochondrial bioenergetic processes. Cardiolipin has been shown to interact with a number of proteins, including the respiratory chain complexes and substrate carrier proteins. Over the past decade, the significance of cardiolipin in the organization of components of the electron transport chain into higher order assemblies, termed respiratory supercomplexes, has been established. Moreover, cardiolipin is involved in different stages of the mitochondrial apoptotic process, as well as in mitochondrial membrane stability and dynamics. This review discusses the current understanding of the functional role that cardiolipin plays in several reactions and processes involved in mitochondrial bioenergetics. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.
Keywords: Cardiolipin; Mitochondrial bioenergetics;

Oxidative phosphorylation (OXPHOS) is the main source of energy in eukaryotic cells. This process is performed by means of electron flow between four enzymes, of which three are proton pumps, in the inner mitochondrial membrane. The energy accumulated in the proton gradient over the inner membrane is utilized for ATP synthesis by a fifth OXPHOS complex, ATP synthase. Four of the OXPHOS protein complexes associate into stable entities called respiratory supercomplexes. This review summarises the current view on the arrangement of the electron transport chain in mitochondrial cristae. The functional role of the supramolecular organisation of the OXPHOS system and the factors that stabilise such organisation are highlighted. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.
Keywords: Oxidative phosphorylation; Mitochondria; Supercomplex; ATP synthase; Electron microscopy;

Functional role of mitochondrial respiratory supercomplexes by Maria Luisa Genova; Giorgio Lenaz (427-443).
Recent experimental evidence has replaced the random diffusion model of electron transfer with a model of supramolecular organisation based upon specific interactions between individual respiratory complexes. These supercomplexes were found to be functionally relevant by flux control analysis and to confer a kinetic advantage to NAD-linked respiration (channelling). However, the Coenzyme Q pool is still required for FAD-linked oxidations and for the proper equilibrium with Coenzyme Q bound in the supercomplex. Channelling in the cytochrome c region probably also occurs but does not seem to confer a particular kinetic advantage. The supramolecular association of individual complexes strongly depends on membrane lipid amount and composition and is affected by lipid peroxidation; it also seems to be modulated by membrane potential and protein phosphorylation. Additional properties of supercomplexes are stabilisation of Complex I, as evidenced by the destabilising effect on Complex I of mutations in either Complex III or IV, and prevention of excessive generation of reactive oxygen species. The dynamic character of the supercomplexes allows their involvement in metabolic adaptations and in control of cellular signalling pathways. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.
Keywords: Respiratory chain; Supercomplexes; Reactive oxygen species; Mitochondria; Cell signaling; Electron transfer;

The function of the respiratory supercomplexes: The plasticity model by Rebeca Acin-Perez; Jose A. Enriquez (444-450).
Mitochondria are important organelles not only as efficient ATP generators but also in controlling and regulating many cellular processes. Mitochondria are dynamic compartments that rearrange under stress response and changes in food availability or oxygen concentrations. The mitochondrial electron transport chain parallels these rearrangements to achieve an optimum performance and therefore requires a plastic organization within the inner mitochondrial membrane. This consists in a balanced distribution between free respiratory complexes and supercomplexes. The mechanisms by which the distribution and organization of supercomplexes can be adjusted to the needs of the cells are still poorly understood. The aim of this review is to focus on the functional role of the respiratory supercomplexes and its relevance in physiology. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.
Keywords: Mitochondrial supercomplex; Respirasome; OXPHOS; Metabolism;

Mitochondrial dismissal in mammals, from protein degradation to mitophagy by Silvia Campello; Flavie Strappazzon; Francesco Cecconi (451-460).
Mitochondria are double-membraned highly dynamic organelles; the shape, location and function of which are determined by a constant balance between opposing fusion and fission events. A fine modulation of mitochondrial structure is crucial for their correct functionality and for many physiological cell processes, the status of these organelles, being thus a key aspect in a cell's fate. Indeed, the homeostasis of mitochondria needs to be highly regulated for the above mentioned reasons, and since a) they are the major source of energy; b) they participate in various signaling pathways; albeit at the same time c) they are also the major source of reactive oxygen species (ROS, the main damaging detrimental players for all cell components). Elaborate mechanisms of mitochondrial quality control have evolved for maintaining a functional mitochondrial network and avoiding cell damage. The first mechanism is the removal of damaged mitochondrial proteins within the organelle via chaperones and protease; the second is the cytosolic ubiquitin–proteasome system (UPS), able to eliminate proteins embedded in the outer mitochondrial membrane; the third is the removal of the entire mitochondria through mitophagy, in the case of extensive organelle damage and dysfunction. In this review, we provide an overview of these mitochondria stability and quality control mechanisms, highlighting mitophagy, and emphasizing the central role of mitochondrial dynamics in this context. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.
Keywords: Mitochondrion; Mitochondria fusion/fission; Autophagy; Ubiquitylation; Neurodegeneration; Mitochondria Quality Control;

The endoplasmic reticulum–mitochondria connection: One touch, multiple functions by Saverio Marchi; Simone Patergnani; Paolo Pinton (461-469).
The endoplasmic reticulum (ER) and mitochondria are tubular organelles with a characteristic “network structure” that facilitates the formation of interorganellar connections. The ER and mitochondria join together at multiple contact sites to form specific domains, termed mitochondria-ER associated membranes (MAMs), with distinct biochemical properties and a characteristic set of proteins. The functions of these two organelles are coordinated and executed at the ER–mitochondria interface, which provides a platform for the regulation of different processes. The roles played by the ER–mitochondria interface range from the coordination of calcium transfer to the regulation of mitochondrial fission and inflammasome formation as well as the provision of membranes for autophagy. The novel and unconventional processes that occur at the ER–mitochondria interface demonstrate its multifunctional and intrinsically dynamic nature. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.
Keywords: Endoplasmic reticulum; mitochondria; MAM; Inflammation; Calcium; Autophagy;

Glycerolipids in photosynthesis: Composition, synthesis and trafficking by Laurence Boudière; Morgane Michaud; Dimitris Petroutsos; Fabrice Rébeillé; Denis Falconet; Olivier Bastien; Sylvaine Roy; Giovanni Finazzi; Norbert Rolland; Juliette Jouhet; Maryse A. Block; Eric Maréchal (470-480).
Glycerolipids constituting the matrix of photosynthetic membranes, from cyanobacteria to chloroplasts of eukaryotic cells, comprise monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol and phosphatidylglycerol. This review covers our current knowledge on the structural and functional features of these lipids in various cellular models, from prokaryotes to eukaryotes. Their relative proportions in thylakoid membranes result from highly regulated and compartmentalized metabolic pathways, with a cooperation, in the case of eukaryotes, of non-plastidic compartments. This review also focuses on the role of each of these thylakoid glycerolipids in stabilizing protein complexes of the photosynthetic machinery, which might be one of the reasons for their fascinating conservation in the course of evolution. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.
Keywords: Galactolipid; Monogalactosyldiacylglycerol; Digalactosyldiacylglycerol; Phosphatidylglycerol; Sulfolipid; Sulfoquinovosyldiacylglycerol;

Chloroplast thylakoid membranes accommodate densely packed protein complexes in ordered, often semi-crystalline arrays and are assembled into highly organized multilamellar systems, an organization warranting a substantial degree of stability. At the same time, they exhibit remarkable structural flexibility, which appears to play important – yet not fully understood – roles in different short-term adaptation mechanisms in response to rapidly changing environmental conditions. In this review I will focus on dynamic features of the hierarchically organized photosynthetic machineries at different levels of structural complexity: (i) isolated light harvesting complexes, (ii) molecular macroassemblies and supercomplexes, (iii) thylakoid membranes and (iv) their multilamellar membrane systems. Special attention will be paid to the most abundant systems, the major light harvesting antenna complex, LHCII, and to grana. Two physical mechanisms, which are less frequently treated in the literature, will receive special attention: (i) thermo-optic mechanism —elementary structural changes elicited by ultrafast local heat transients due to the dissipation of photon energy, which operates both in isolated antenna assemblies and the native thylakoid membranes, regulates important enzymatic functions and appears to play role in light adaptation and photoprotection mechanisms; and (ii) the mechanism by which non-bilayer lipids and lipid phases play key role in the functioning of xanthophyll cycle de-epoxidases and are proposed to regulate the protein-to-lipid ratio in thylakoid membranes and contribute to membrane dynamics. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.
Keywords: Circular dichroism; LHCII; Non-bilayer lipid; Small angle neutron scattering; Thermo-optic mechanism; Thylakoid membrane;

The survival and fitness of photosynthetic organisms is critically dependent on the flexible response of the photosynthetic machinery, harbored in thylakoid membranes, to environmental changes. A central element of this flexibility is the lateral diffusion of membrane components along the membrane plane. As demonstrated, almost all functions of photosynthetic energy conversion are dependent on lateral diffusion. The mobility of both small molecules (plastoquinone, xanthophylls) as well as large protein supercomplexes is very sensitive to changes in structural boundary conditions. Knowledge about the design principles that govern the mobility of photosynthetic membrane components is essential to understand the dynamic response of the photosynthetic machinery. This review summarizes our knowledge about the factors that control diffusion in thylakoid membranes and bridges structural membrane alterations to changes in mobility and function. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.
Keywords: Photosynthesis; Thylakoid membrane; Diffusion coefficient; Percolation theory; Macromolecular crowding;

The thylakoid membranes of cyanobacteria are the major sites of respiratory electron transport as well as photosynthetic light reactions. The photosynthetic and respiratory electron transport chains share some components, and their presence in the same membrane opens up the possibility for a variety of “unorthodox” electron transport routes. Many of the theoretically possible electron transport pathways have indeed been detected in particular species and circumstances. Electron transport has a crucial impact on the redox balance of the cell and therefore the pathways of electron flow in the cyanobacterial thylakoid membrane must be tightly regulated. This review summarises what is known of cyanobacterial electron transport components, their interactions and their sub-cellular location. The role of thylakoid membrane organisation in controlling electron transport pathways is discussed with respect to recent evidence that the larger-scale distribution of complexes in the membrane is important for controlling electron exchange between the photosynthetic and respiratory complexes. The distribution of complexes on scales of 100 nm or more is under physiological control, showing that larger-scale thylakoid membrane re-arrangement is a key factor in controlling the crosstalk between photosynthetic and respiratory electron transport. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.
Keywords: Cyanobacteria; Electron transport; Photosynthesis; Respiration; Thylakoid membrane;

Roles of autophagy in chloroplast recycling by Hiroyuki Ishida; Masanori Izumi; Shinya Wada; Amane Makino (512-521).
Chloroplasts are the primary energy suppliers for plants, and much of the total leaf nitrogen is distributed to these organelles. During growth and reproduction, chloroplasts in turn represent a major source of nitrogen to be recovered from senescing leaves and used in newly-forming and storage organs. Chloroplast proteins also can be an alternative substrate for respiration under suboptimal conditions. Autophagy is a process of bulk degradation and nutrient sequestration that is conserved in all eukaryotes. Autophagy can selectively target chloroplasts as whole organelles and or as Rubisco-containing bodies that are enclosed by the envelope and specifically contain the stromal portion of the chloroplast. Although information is still limited, recent work indicates that chloroplast recycling via autophagy plays important roles not only in developmental processes but also in organelle quality control and adaptation to changing environments. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.
Keywords: Autophagy; Chloroplast; Leaf senescence; Nutrient recycling; Rubisco;

Plants are photosynthetic organisms that have evolved unique systems to adapt fluctuating environmental light conditions. In addition to well-known movement responses such as phototropism, stomatal opening, and nastic leaf movements, chloroplast photorelocation movement is one of the essential cellular responses to optimize photosynthetic ability and avoid photodamage. For these adaptations, chloroplasts accumulate at the areas of cells illuminated with low light (called accumulation response), while they scatter from the area illuminated with strong light (called avoidance response). Plant-specific photoreceptors (phototropin, phytochrome, and/or neochrome) mediate these dynamic directional movements in response to incident light position and intensity. Several factors involved in the mechanisms underlying the processes from light perception to actin-based movements have also been identified through molecular genetic approach. This review aims to discuss recent findings in the field relating to how chloroplasts move at molecular levels. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.
Keywords: Actin filament; Blue light; Chloroplast movement; Cytoskeleton dynamics; Photoreceptor; Phototropin;