Contact sites between the outer and inner membrane of mitochondria-role in protein transport

Many essential functions of mitochondrial metabolism have been studied in the past three decades in considerable depth: oxidative phosphorylation, catabolism of fatty acids, role in nitrogen metabolism, and amino acid metabolism. More recently, other aspects attracted much attention like protein translocation into mitochondria, inheritance of mitochondrial DNA, movement of mitochondria, their fusion and fission, and their involvement in apoptosis, ageing, cancer and other cellular processes. Together with these new views on the function of mitochondria, new ideas on the structure of mitochondria emerged. Here we will discuss the current knowledge about how the membranes of mitochondria are organized and how they interact. Interactions between components of the inner and the outer membrane are necessary for a number of central mitochondrial functions such as the channeling of metabolites, coordinated fusion and fission of mitochondria, and protein transport. Some of these interactions appear stable such as the so-called morphological contact sites; others are quite dynamic. Direct evidence that a certain protein is part of morphologically defined contact sites is lacking. Nevertheless, protein translocase complexes of the outer and the inner membrane exhibit stable interactions between the two membranes when precursor proteins are arrested during import into mitochondria. Finally, we discuss possible roles of cristae junctions, another morphologically defined membrane structure in mitochondria.     

The role of contact sites between inner and outer mitochondrial membrane in energy transfer

Three functions have been suggested to be localized in contact sites between the inner and the outer membrane of mitochondria from mammalian cells: (i) transfer of energy from matrix to cytosol through the action of peripheral kinases; (ii) import of mitochondrial precursor proteins; and (iii) transfer of lipids between outer and inner membrane. In the contact site-related energy transfer a number of kinases localized in the periphery of the mitochondrion play a crucial role. Two examples of such kinases are relevant here: (i) hexokinase isoenzyme I which is capable of binding to the outer aspect of the outer membrane; and (ii) the mitochondrial isoenzyme of creatine kinase which is localized in the intermembrane space. Recently, evidence was presented that both hexokinase and creatine kinase are preferentially localized in contact sites (Adams, V. et al. (1989) Biochim. Biophys. Acta 981, 213-225). The aim of the present experiments was two-fold. First, to establish methods which enable the bioenergetic aspects of energy transfer mediated by kinases in contact sites to be measured. In these experiments emphasis was on hexokinase, while 31P-NMR was the major experimental technique. Second, we wanted to develop methods which can give insight into factors playing a role in the formation of contact sites involved in energy transfer. In the latter approach, mitochondrial creatine kinase was studied using monolayer techniques.     

Identification of a GTP-binding protein in the contact sites between inner and outer mitochondrial membranes

Whilst investigating whether GTP hydrolysis may be required for the import of preproteins into mitochondria we have found that a GTP-binding protein is located at the contact sites between mitochondrial inner and outer membranes. When mitochondrial outer membranes purified from rat liver were UV-irradiated in the presence of [alpha-32P]GTP, a 52 kDa protein was radiolabelled, whereas [alpha-32P]ATP did not label this protein. GTP-binding proteins were also labelled in the cytosolic and microsomal fractions, but the 52 kDa protein was concentrated in mitochondrial membranes and was the only protein specifically labelled by GTP in these membranes. Fractionation of mitochondrial membrane vesicles into outer membranes, inner membranes and contact sites between outer and inner membranes showed that the GTP-binding activity was highly enriched in contact sites, the location at which preprotein import is believed to occur. A protein of almost identical size was also found to be labelled in mitochondria from yeast.     

Transport of proteins into mitochondria: translocational intermediates spanning contact sites between outer and inner membranes

Translocational intermediates of precursor proteins of ATPase F1 beta subunit and cytochrome c1 across mitochondrial membranes were analyzed using two different approaches, transport at low temperature and transport after binding of precursor proteins to antibodies. Under both conditions precursors were partially transported into mitochondria in an energy-dependent manner. They were processed by the metalloprotease in the matrix but a major proportion of the polypeptide chains was still present at the outer face of the outer mitochondrial membrane. We conclude that transfer of precursors into the inner membrane or matrix space occurs through "translocation contact sites"; precursor polypeptides to F1 beta and cytochrome c1 enter the matrix space with the amino terminus first; and a membrane potential is required for the transmembrane movement on an amino-terminal "domain-like" structure but not for completing translocation of the major part of the polypeptides.     

Seasonal and interannual variation in high-latitude estuarine fish community structure along a glacial to non-glacial watershed gradient in Southeast Alaska

Environmental Biology of Fishes - Along the Gulf ofAlaska, rapid glacier retreat has driven changes in transport of freshwater, sediments, and nutrients to estuary habitats. Over the coming...     

Electron transfer between outer and inner membranes in plant mitochondria

Experiments on dissociation and reassociation of outer and inner membranes of cauliflower (Brassica oleracea L.) mitochondria have been carried out in order to investigate the possible occurrence of an intermembrane electron transfer. With NADH as electron donor, it has been shown that electron transfer can take place between an antimycin-insensitive NADH-cytochrome c reductase, on the outer membrane, and the cyanide-sensitive cytochrome oxidase, on the inner membrane, provided a mobile carrier such as cytochrome c is present in the intermembrane space. In intact plant mitochondria, part of the oxidation of exogenous NADH could be carried out by this pathway.     

莱州湾东岸潮下带大型底栖动物群落beta多样性格局及其驱动因素

群落β多样性格局的形成与维持机制一直是群落生态学研究的热点与核心。然而, 与陆生生态系统相比, 海洋生态系统尤其是海洋底栖生态系统中生物群落β多样性的研究明显滞后。本研究从分类(即物种组成)和功能(即性状组成)两个方面出发, 应用Mantel分析和基于矩阵的多元回归(multiple regression on distance matrices, MRM)分析, 探究了莱州湾东岸潮下带大型底栖动物群落β多样性及其周转(turnover)和嵌套(nestedness)组分与环境因子和空间距离的关系, 揭示了环境过滤和扩散限制两种生态学过程对其群落构建机制的影响。结果显示: (1)莱州湾东岸潮下带大型底栖动物群落的分类与功能性状β多样性均维持在较高水平且均以周转组分占主导, 表明研究区域大型底栖动物群落在物种和功能性状组成上差异较大, 而这种差异大部分来自物种或功能性状在空间或群落间的更替; (2)空间地理距离对大型底栖动物群落分类与功能性状β多样性及其组分无显著影响(Mantel检验, P > 0.05), 表明扩散限制对莱州湾东岸潮下带大型底栖动物群落的影响有限; (3) MRM分析表明, 沉积物总有机质(total organic matter, TOM)和粉砂含量显著影响大型底栖动物群落分类β多样性, 而TOM则显著影响功能性状β多样性。上述结果表明, 环境过滤是驱动莱州湾东岸潮下带大型底栖动物群落构建机制的首要因素。本研究阐明了莱州湾东岸潮下带大型底栖动物群落构建机制, 同时也为理解我国其他海域大型底栖动物群落的形成与维持机制提供了参考。     

天津沿海水鸟群落格局分析

2007年1～12月,利用样点法对天津沿海水鸟进行调查.共观察到水鸟6目11科71种,其中古北界40种,东洋界1种,广布种30种;鸟类居留型以旅鸟为主,有部分夏候鸟、冬候鸟和留鸟.水鸟群落格局时空差异明显,时间格局表现为季节变化较大,种数、数量及多样性均以春秋季高于冬夏季;空间格局表现为种数和Shannon-Weaner指数(H')均以青坨子最高,数量以北大港水库最多,均匀度指数(E)以唐家河口最高,4个群落参数的最小值均出现在滨海大道和独流减河.     

Ectomycorrhizal fungal community structure across a bog-forest ecotone in southeastern Alaska

We examined the ectomycorrhizal (ECM) fungal community across a bog-forest ecotone in southeastern Alaska. The bog and edge were both characterized by poorly drained Histosols and a continuous layer of Sphagnum species, ericaceous shrubs, Carex species, and shore pine [Pinus contorta Dougl. ex Loud. var. contorta]. The forest had better-drained Inceptisols and Spodosols, a tree community comprised of western hemlock [Tsuga heterophylla (Raf.) Sarg.], yellow cedar (Thuja plicata Donn ex D. Don.), Sitka spruce [Picea sitchensis (Bong.) Carr.] and shore pine, and an understorey of ericaceous shrubs and herbs. ECM root tip density (tips cm^–3 soil) was significantly greater in the forest than the edge or bog and ECM colonization was significantly different in all three plant communities. The below ground ECM fungal taxa were analyzed using molecular techniques (PCR-RFLP and DNA sequencing). Three ECM fungal taxa, Suillus tomentosus (Kauffman) Singer, Cenococcum geophilum Fr.:Fr, and a Russula species, differed in relative frequency, yet were among the four most frequent in all three plant communities. Although differences in ECM fungal richness were observed across plant communities, unequal sampling of ECM roots due to root density and colonization differences confounded richness comparisons. Using resampling procedures for creating taxon-accumulation curves as a function of sampled ECM roots revealed similarities in cumulative ECM fungal taxa richness across the ecotone.     

Detecting genetic structure in migrating bowhead whales off the coast of Barrow, Alaska

We develop a general framework for analysing and testing genetic structure within a migratory assemblage that is based on measures of genetic differences between individuals. We demonstrate this method using microsatellite DNA data from the Bering-Chukchi-Beaufort stock of bowhead whales (Balaena mysticetus), sampled via Inuit hunting during the spring and autumn migration off Barrow, Alaska. This study includes a number of covariates such as whale ages and the time separation between captures. Applying the method to a sample of 117 bowhead whales, we use permutation methods to test for temporal trends in genetic differences that can be ascribed to age-related effects or to timing of catches during the seasons. The results reveal a pattern with elevated genetic differences among whales caught about a week apart, and are statistically significant for the autumn migration. In contrast, we find no effects of time of birth or age-difference on genetic differences. We discuss possible explanations for the results, including population substructuring, demographic consequences of historical overexploitation, and social structuring during migration.     

Nearshore benthic community structure at the Bounty and Antipodes Islands, Subantarctic New Zealand

Management decisions aimed at protecting biodiversity ideally should be based on biological information, but for remote and logistically difficult sites, such as are found at high latitudes, these data may be lacking. During March 2009, surveys were completed of the nearshore rocky reef communities around the Bounty and Antipodes Islands, in New Zealand??s subantarctic region. Previously considered to support the same habitat types (which used physical variables as surrogates for biological communities), analysis of photoquadrats taken at both island groups showed that the rocky reef communities were significantly different, both in terms of their species composition and in terms of their potential ecological function. While Antipodes Island supported fairly typical subantarctic shallow subtidal marine communities dominated by nongeniculate coralline algae, the rocky reefs at the Bounty Islands were dominated by filter- and suspension-feeding invertebrates, in particular encrusting sponges, barnacles and mussels. The mobile invertebrate fauna associated with these communities were also significantly different between the two island groups. Contrasting geology, oceanographic conditions and nutrient input from seabird and pinniped colonies may all contribute to the observed nearshore community structures at the Bounty and Antipodes Islands. Our research provides a baseline for assessing change in the subantarctic region and highlights the importance of using biological community data where available, to inform conservation management decisions.     

枸杞岛潮下带沙地生境鱼类群落结构和季节变化

为了解岛礁水域沙地生境的鱼类群落结构特征,评估该生境对鱼类资源养护的潜在作用,于2009年采用多网目组合刺网对枸杞岛潮下带沙质区域进行了逐月采样,同步设置岩礁为对照生境.应用α和β多样性指数结合相对重要性指数、相对渔获率、ABC曲线和聚类排序方法对两种生境中的鱼类组成、多样性变化和群落格局与变化进行了全面探讨.全年在沙地和岩礁生境共采集鱼类63种,隶属11目38科56属,2种生境各自出现的鱼类皆为46种.受暖水种频繁出现在沙地生境的影响,潮下带沙质区域鱼类区系比岩礁生境略显丰富,且春夏季的渔获量普遍高于岩礁生境;由于种类组成均匀度较低,沙地生境各季节的α多样性普遍较低,夏季显著低于岩礁生境.日本须鳎是沙地生境的指示种,为早春、夏末和秋冬季沙地底层优势鱼种.5-7月鳀、多数月份鲻和10月份鳗鲇等种类对沙地生境的阶段性利用,使其形成了区别于以趋礁性鱼类为优势类群的岩礁生境的群落格局和季节动态.沙地是多种鱼类幼鱼阶段的庇护所和饵料场,是鲆鲽类的良好栖息地.沙地生境在维持鱼类区系和养护鱼类资源方面具有重要作用.     

Transfer of phosphatidic acid between microsomal and mitochondrial outer and inner membranes

A protein fraction from rat liver cytoplasm, precipitable at 50-95% saturation of ammonium sulphate, binds phosphatidic acid from mitochondrial and microsomal membranes. Protein-bound phosphatidic acid was eluted from Sephadex G-75 in fractions corresponding to a molecular weight of about 10 000. No such binding was observed with mitochondrial soluble proteins, either total or precipitated with ammonium sulphate between 50 and 95% saturation. The transfer of phosphatidic acid from microsomes to mitochondria was increased by liver cytoplasmic proteins precipitable at 50-95% saturation of ammonium sulphate but not with mitochondrial soluble proteins. This increase by cytoplasmic proteins was pronounced in 200 mM sucrose but was negligible in 100 mM KCI where the spontaneous transfer was quite high. Cytoplasmic proteins stimulated the synthesis of cardiolipin and phosphatidylglycerol in mitochondria deprived of the outer membrane but not in intact mitochondria when phosphatidic acid was supplied either by microsomes or liposomes. It is suggested that the transfer of phosphatidic acid from the outer to the inner mitochondrial membrane is not mediated by transfer proteins but occurs either by direct contact of the membranes or as free diffusion through the aqueous phase.     

Cranial airways and the integration between the inner and outer facial skeleton in humans

The cranial airways are in the center of the human face. Therefore variation in the size and shape of these central craniofacial structures could have important consequences for the surrounding midfacial morphology during development and evolution. Yet such interactions are unclear because one school of thought, based on experimental and developmental evidence, suggests a relative independence (modularity) of these two facial compartments, whereas another one assumes tight morphological integration. This study uses geometric morphometrics of modern humans (N = 263) and 40 three‐dimensional‐landmarks of the skeletal nasopharynx and nasal cavity and outer midfacial skeleton to analyze these questions in terms of modularity. The sizes of all facial compartments were all strongly correlated. Shape integration was high between the cranial airways and the outer midfacial skeleton and between the latter and the anterior airway openings (skeletal regions close to and including piriform aperture). However, no shape integration was detected between outer midface and posterior airway openings (nasopharynx and choanae). Similarly, no integration was detected between posterior and anterior airway openings. This may reflect functional modularization of nasal cavity compartments related to respiratory physiology and differential developmental interactions with the face. Airway size likely relates to the energetics of the organism, whereas airways shape might be more indicative of respiratory physiology and climate. Although this hypothesis should be addressed in future steps, here we suggest that selection on morphofunctional characteristics of the cranial airways could have cascading effects for the variation, development, and evolution of the human face. Am J Phys Anthropol 152:287–293, 2013. © 2013 Wiley Periodicals, Inc.     

The inner and outer compartments of mitochondria are sites of distinct cAMP/PKA signaling dynamics

Cyclic AMP (cAMP)-dependent phosphorylation has been reported to exert biological effects in both the mitochondrial matrix and outer mitochondrial membrane (OMM). However, the kinetics, targets, and effectors of the cAMP cascade in these organellar domains remain largely undefined. Here we used sensitive FRET-based sensors to monitor cAMP and protein kinase A (PKA) activity in different mitochondrial compartments in real time. We found that cytosolic cAMP did not enter the matrix, except during mitochondrial permeability transition. Bicarbonate treatment (expected to activate matrix-bound soluble adenylyl cyclase) increased intramitochondrial cAMP, but along with membrane-permeant cAMP analogues, failed to induce measureable matrix PKA activity. In contrast, the OMM proved to be a domain of exceptionally persistent cAMP-dependent PKA activity. Although cAMP signaling events measured on the OMM mirrored those of the cytosol, PKA phosphorylation at the OMM endured longer as a consequence of diminished control by local phosphatases. Our findings demonstrate that mitochondria host segregated cAMP cascades with distinct functional and kinetic signatures.     

Trends and Carrying Capacity of Sea Otters in Southeast Alaska

Sea otter populations in Southeast Alaska, USA, have increased dramatically from just over 400 translocated animals in the late 1960s to >8,000 by 2003. The recovery of sea otters to ecosystems from which they had been absent has affected coastal food webs, including commercially important fisheries, and thus information on expected growth and equilibrium abundances can help inform resource management. We compile available survey data for Southeast Alaska and fit a Bayesian state-space model to estimate past trends and current abundance. Our model improves upon previous analyses by partitioning and quantifying sources of estimation error, accounting for over-dispersion of aerial count data, and providing realistic measurements of uncertainty around point estimates of abundance at multiple spatial scales. We also provide estimates of carrying capacity (K) for Southeast Alaska, at regional and sub-regional scales, and analyze growth rates, current population status and expected future trends. At the regional scale, the population increased from 13,221 otters in 2003 to 25,584 otters in 2011. The average annual growth rate in southern Southeast Alaska (7.8%) was higher than northern Southeast Alaska (2.7%); however, growth varied at the sub-regional scale and there was a negative relationship between growth rates and the number of years sea otters were present in an area. Local populations vary in terms of current densities and expected future growth; the mean estimated density at K was 4.2 ± 1.58 sea otters/km² of habitat (i.e., the sub-tidal benthos between 0 m and 40 m depth) and current densities correspond on average to 50% of projected equilibrium values (range = 1–97%) with the earliest-colonized sub-regions tending to be closer to K. Assuming a similar range of equilibrium densities for currently un-occupied habitats, the projected value of K for all of Southeast Alaska is 74,650 sea otters. Future analyses can improve upon the precision of K estimates by employing more frequent surveys at index sites and incorporating environmental covariates into the process model to generate more accurate, location-specific estimates of equilibrium density. © 2019 The Authors. The Journal of Wildlife Management Published by Wiley Periodicals, Inc.     

Forested Buffer Strips and Breeding Bird Communities in Southeast Alaska

Abstract: Forested buffer strips are used to mitigate fragmentation and habitat loss and are a common feature in management of riparian landscapes. Low-elevation, old-growth coastal forests are a rare riparian habitat that can benefit from similar conservation measures. We evaluated the effectiveness of postlogging, forested buffer strips for forest-dwelling birds in the coastal temperate rainforest of southeast Alaska, USA. Our objective was to compare bird composition and density among forested buffer strips of differing widths and controls at the stand and landscape scales. We applied a 2-stage sampling design stratified by forested buffer width and randomly selected 24 managed and 18 control sites to sample over 2 breeding seasons. We estimated abundance of birds using the paired-observer, variable-circular plot method. We modeled combined effects of buffer width and vegetation and landscape characteristics on bird density at 2 spatial scales. Species richness and diversity were greatest in the narrowest buffers, but species composition in the largest buffers (≥400 m) was most similar to that in control blocks. Abundance of 3 of 10 common species differed across forested buffer treatments and controls. Densities of red-breasted sapsucker (Sphyrapicus ruber) and Pacific-slope flycatcher (Empidonax difficilis) were positively related to buffer width, whereas density of ruby-crowned kinglet (Regulus calendula) was negatively associated with buffer width. Parameter estimates for buffer width effects at both spatial scales were similar within species. We found few habitat and landscape variables that clearly affected density of our focal species, and among species no predictor variables affected density in a similar fashion. We recommend retaining forested buffers ≥400 m to support composition and abundance of forest-dwelling birds, particularly those species that rely on interior forest conditions. (JOURNAL OF WILDLIFE MANAGEMENT 72(3):674-681; 2008)