The conformational basis of energy transduction in membrane systems. V. Measurement of configurational changes by light scattering

Determinants, and implications, of the shape and size of thylakoids and cristae

Thylakoids are flattened sacs isolated from other membranes; cristae are attached to the rest of the inner mitochondrial membrane by the crista junction, but the crista lumen is separated from the intermembrane space. The shape of thylakoids and cristae involves membranes with small (5-30 nm) radii of curvature. While the mechanism of curvature is not entirely clear, it seems to be largely a function of Curt proteins in thylakoids and Mitochondrial Organising Site and Crista Organising Centre proteins and oligomeric F_OF₁ ATP synthase in cristae. A subordinate, or minimal, role is attributable to lipids with areas of their head group area greater (convex leaflet) or smaller (concave leaflet) than the area of the lipid tail; examples of the latter group are monogalactosyldiglyceride in thylakoids and cardiolipin in cristae. The volume per unit area on the lumen side of the membrane is less than that of the chloroplast stroma or cyanobacterial cytosol for thylakoids, and mitochondrial matrix for cristae. A low volume per unit area of thylakoids and cristae means a small lumen width that is the average of wider spaces between lipid parts of the membranes and the narrower gaps dominated by extra-membrane components of transmembrane proteins. These structural constraints have important implications for the movement of the electron carriers plastocyanin and cytochrome c₆ (thylakoids) and cytochrome c (cristae) and hence the separation of the membrane-associated electron donors to, and electron acceptors from, these water-soluble electron carriers. The donor/acceptor pairs, are the cytochrome fb₆Fe_nh complex and P₇₀₀⁺ in thylakoids, and Complex III and Complex IV of cristae. The other energy flux parallel to the membranes is that of the proton motive force generated by redox-powered H⁺ pumps into the lumen to the proton motive force use in ATP synthesis by H⁺ flux from the lumen through the ATP synthase. For both the electron transport and proton motive force movement, concentration differences of reduced and oxidised electron carriers and protonated and deprotonated pH buffers are involved. The need for diffusion along a congested route of these energy transfer agents may limit the separation of sources and sinks parallel to the membranes of thylakoids and cristae.

The C-terminal transmembrane domain of Bcl-xL mediates changes in mitochondrial morphology

We investigate the effect of mitochondrial localization and the Bcl-x(L) C-terminal transmembrane (TM) domain on mitochondrial morphology and subcellular light scattering. CSM 14.1 cell lines stably expressed yellow fluorescent protein (YFP), YFP-Bcl-x(L,) YFP-Bcl-x(L)-DeltaTM, containing the remainder of Bcl-x(L) after deletion of the last 21 amino acids corresponding to the TM domain, or YFP-TM, consisting of YFP fused at its C-terminal to the last 21 amino acids of Bcl-x(L). YFP-Bcl-x(L) and YFP-TM localized to the mitochondria. Their expression decreased the intensity ratio of wide-to-narrow angle forward scatter by subcellular organelles, and correlated with an increase in the proportion of mitochondria with an expanded matrix having greatly reduced intracristal spaces as observed by electron microscopy. Cells expressing YFP-TM also exhibited significant autophagy. In contrast, YFP-Bcl-x(L)-DeltaTM was diffusely distributed in the cells, and its expression did not alter light scattering or mitochondrial morphology compared with parental cells. Expression of YFP-Bcl-x(L) or YFP-Bcl-x(L)-DeltaTM provided significant resistance to staurosporine-induced apoptosis. Surprisingly however, YFP-TM expression also conferred a moderate level of cell death resistance in response to staurosporine. Taken together, our results suggest the existence of a secondary Bcl-x(L) function that is mediated by the transmembrane domain, alters mitochondrial morphology, and is distinct from BH3 domain sequestration.

Calcium-induced alterations in mitochondrial morphology quantified in situ with optical scatter imaging

Optical scatter imaging (OSI), a technique we developed recently, was used to measure the ratio of wide-to-narrow angle scatter (OSIR) within endothelial cells subjected to calcium overload (1.6 mM) after permeabilization by ionomycin. Within a few minutes of calcium overload, the mitochondria, which started as elongated organelles, rounded up into spherically shaped particles. This change in morphology was accompanied by a statistically significant 14% increase in OSIR in the cells' cytoplasm. Mitochondrial rounding and OSIR increase were suppressed by cyclosporin A (25 microM), implying that the observed geometrical and scattering changes were directly attributable to the mitochondrial permeability transition. The angular scattering properties of a long mitochondrion rounding up were approximated by numerical simulations of light scatter from an ellipsoid rounding up into a sphere. The simulations predicted a relative increase in OSIR comparable to that measured experimentally for the case where the shape transition takes place with little or no volume increase. The simulations also suggested that mitochondrial refractive index changes could not account for the OSIR changes observed. Our data show that changes in OSIR correlate with mitochondrial morphology change in situ. OSI provides a new tool for subcellular imaging and complements other microscopy methods, such as fluorescence.

Role of calcium ions in the control of embryogenesis of Xenopus. Changes in the subcellular distribution of calcium in early cleavage embryos after treatment with the ionophore A23187

Treatment of stage 5 Xenopus embryos with the ionophore A23187 for only 10 min, in the absence of extracellular Mg2+ and Ca2+, causes cortical contractions and a high incidence of abnormal embryos during subsequent development. Cation analysis shows that divalent ions are not lost from the embryos, but that Ca2+ is redistributed within the subcellular fractions. Ca2+ is probably released from yolk platelets and/or pigment granules by the action of A23187, [Ca2+] rises in the cytosol, and the mitochondria attempt to take up this free Ca2+. The mitochondria concomitantly undergo characteristic ultrastructural transformations, changing towards energized-twisted and energized-zigzag conformations. A23187 allows these changes to be demonstrated in situ. Extracellular divalent cations (10(-4) M) interfere with this intracellular action of A23187. Intracellular accumulation of Na+ (by treatment with ouabain) or Li+ also causes abnormal development, probably by promoting a release of Ca2+ from the mitochondria. It is suggested (a) that all these treatments cause a rise in [Ca2+]i which interferes with normal, integrated cell division, so causing, in turn, abnormal embryogenesis, (b) that levels of [Ca2+]i are of importance in regulating cleavage, (c) that the mitochondria could well have a function in regulating [Ca2+]i during embryogenesis in Xenopus, and (d) that vegetalizing agents may well act by promoting a rise in [Ca2+]i in specific cells in the amphibian embryo.

Dynamics of structural changes in biological particles from rapid light scattering measurements

A unique light scattering photometer has been developed for continuous measurement of the complete angular spectrum of light scattered by dynamically changing systems. Although primarily designed for work with biological particles such as large viruses, subcellular organelles, and bacteria, it can be used to study polymerization and depolymerization of macromolecules, synthetic polymers, etc. The instrument has been applied to the direct measurement of the diameter of phospholipid vesicles and micelles in chromatographic column effluents and to the dynamics of polymerization of microtubular protein and osmotic lysis of chromaffin of granules.

Ion movements during energy-linked mitochondrial structural changes

The structure of isolated rat liver mitochondria has been observed in the electron microscope following incubation of the mitochondria in vitro under a variety of conditions. The results show that ultrastructural changes are only associated with the energization and deenergization of isolated mitochondria if the composition of the incubation medium permits ion movements in or out of the matrix. The mechanism of energy coupling does not appear to depend on these major mitochondrial structural changes. The addition of low levels of valinomycin greatly increases the rate at which the matrix compartment swells and shrinks on energization and deenergization even at low K+ concentrations.

Ultrastructural studies of beef heart mitochondria. 3. The inequality of gross morphological change and oxidative phosphorylation

The relationships between membranes and intramembrane compartments of isolated heart mitochondria are inadequately defined to express the induced morphological changes associated with the structural organization. The inner membrane and matrix are the major structural entities which undergo transformation upon alteration of metabolism or incubation conditions. To better express these morphological changes within a mitochondrion, two inner membranes plus enclosed matrix are defined as an inmerix (plural inmerices). Three general morphological forms of mitochondria can be distinguished by the size and shape of inmerices. These are distended, condensed, and coalesced inmerixal configurations. Hypotonic conditions and P(i) in isotonic sucrose generate distended configurations. This P(i) distention is apparently dependent on utilization of energy. It does not occur under anaerobic conditions. Oxidizable substrates generate condensed configurations. ADP and dADP generate coalesced configurations and stop formation of condensed and distended inmerixal configurations in the absence of inhibitors. ADP coalescence is apparently not dependent on an energy input. It occurs under aerobic and anaerobic conditions, and in isotonic and hypotonic media. Atractyloside completely inhibits the effects of ADP on inmerixal membranes whereas oligomycin does not. Distention by P(i) is unaffected by the two inhibitors. Distended inmerices, without added P(i) (12 mM and 62 mM sucrose), are coalesced by ADP. These studies indicate that coalescence of inmerixal membranes probably reflects the consequences of specific stoichiometric binding or translocation of adenine nucleotides.

Ion transport underlying metabolically controlled volume changes of isolated mitochondria

An earlier study has indicated that the swelling of isolated mitochondria induced by inorganic phosphate (P(i)) can be accounted for largely or completely by the accumulation of ions. In the present study, a similar uptake was shown to be induced by a wider range of P(i) concentrations. Addition of ADP, KCN, or 2,4-dinitrophenol initiates a shrinkage which can be accounted for by the efflux of ions. The results are consistent with the explanations that (a) P(i) induces a transport of ions and a concomitant osmotic swelling, and (b) the addition of substances which compete or interfere with the energy available for transport results in ion efflux and a corresponding mitochondrial shrinkage. The results are not consistent with proposals that the changes in the light scattered by mitochondrial suspensions with alterations in metabolic states reflect a mechanochemical coupling phenomenon.

Topological and functional organization of the mitochondrion

The morphological and biochemical properties of isolated mitochondrial inner and outer membranes are summarized and discussed in relation to the functional organization of the intact mitochondrion. The enzymatic composition of the mitochondrial inner compartment is compared to over-all mitochondrial function. The role of the mitochondrial outer compartment is discussed with reference to both the inner membrane-matrix fraction and the intact mitochondrion.

Energized configurations of heart mitochondria in situ

Changes in the configurational state of the cristal membranes of rat heart mitochondria within the living cell can be induced by imposing energizing conditions. An exact correlation has been established between the configurational states of the cristal membrane and the energy states of the mitochondrion. The configurational changes observed in mitochondria in situ are comparable to those established for beef heart mitochondria in vitro and are consistent with the postulate of the conformational basis of energy transductions in membrane systems. The formation of paracrystalline arrays is one of the noteworthy features of configurational changes of mitochondria in situ.