Solute channels of the outer membrane: from bacteria to chloroplasts

Chloroplasts, unique organelles of plants, originated from endosymbiosis of an ancestor of today's cyanobacteria with a mitochondria-containing host cell. It is assumed that the outer envelope membrane, which delimits the chloroplast from the surrounding cytosol, was thus inherited from its Gram-negative bacterial ancestor. This plastid-specific membrane is thus equipped with elements of prokaryotic and eukaryotic origin. In particular, the membrane-intrinsic outer envelope proteins (OEPs) form solute channels with properties reminiscent of porins and channels in the bacterial outer membrane. OEP channels are characterised by distinct specificities for metabolites and a quite peculiar expression pattern in specialised plant organs and plastids, thus disproving the assumption that the outer envelope is a non-specific molecular sieve. The same is true for the outer membrane of Gram-negative bacteria, which functions as a permeability barrier in addition to the cytoplasmic membrane, and embeds different classes of channel pores. The channels of these prokaryotic prototype proteins, ranging from unspecific porins to specific channels to ligand-gated receptors, are exclusively built of P-barrels. Although most of the OEP channels are formed by P-strands as well, phylogeny based on sequence homology alone is not feasible. Thus, the comparison of structural and functional properties of chloroplast outer envelope and bacterial outer membrane channels is required to pinpoint the ancestral OEP `portrait gallery'

Unique components of the plant mitochondrial protein import apparatus

AbstractThe basic mitochondrial protein import apparatus was established in the earliest eukaryotes. Over the subsequent course of evolution and the divergence of the plant, animal and fungal lineages, this basic import apparatus has been modified and expanded in order to meet the specific needs of protein import in each kingdom. In the plant kingdom, the arrival of the plastid complicated the process of protein trafficking and is thought to have given rise to the evolution of a number of unique components that allow specific and efficient targeting of mitochondrial proteins from their site of synthesis in the cytosol, to their final location in the organelle. This includes the evolution of two unique outer membrane import receptors, plant Translocase of outer membrane 20 kDa subunit (TOM20) and Outer membrane protein of 64 kDa (OM64), the loss of a receptor domain from an ancestral import component, Translocase of outer membrane 22 kDa subunit (TOM22), evolution of unique features in the disulfide relay system of the inter membrane space, and the addition of an extra membrane spanning domain to another ancestral component of the inner membrane, Translocase of inner membrane 17 kDa subunit (TIM17). Notably, many of these components are encoded by multi-gene families and exhibit differential sub-cellular localisation and functional specialisation. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids

An in silico analysis of the mitochondrial protein import apparatus of plants

<p>Abstract</p> <p>Background</p> <p>An <it>in silico </it>analysis of the mitochondrial protein import apparatus from a variety of species; including <it>Chlamydomonas reinhardtii</it>, <it>Chlorella variabilis, Ectocarpus siliculosus</it>, <it>Cyanidioschyzon merolae</it>, <it>Physcomitrella patens</it>, <it>Selaginella moellendorffii, Picea glauca</it>, <it>Oryza sativa </it>and <it>Arabidopsis thaliana </it>was undertaken to determine if components differed within and between plant and non-plant species.</p> <p>Results</p> <p>The channel forming subunits of the outer membrane components Tom40 and Sam50 are conserved between plant groups and other eukaryotes. In contrast, the receptor component(s) in green plants, particularly Tom20, (<it>C. reinhardtii, C. variabilis, P. patens, S. moellendorffii, P. glauca, O. sativa and A. thaliana</it>) are specific to this lineage. Red algae contain a Tom22 receptor that is orthologous to yeast Tom22. Furthermore, plant mitochondrial receptors display differences between various plant lineages. These are evidenced by distinctive motifs in all plant Metaxins, which are absent in red algae, and the presence of the outer membrane receptor OM64 in Angiosperms (rice and Arabidopsis), but not in lycophytes (<it>S. moellendorffii</it>) and gymnosperms (<it>P. glauca</it>). Furthermore, although the intermembrane space receptor Mia40 is conserved across a wide phylogenetic range, its function differs between lineages. In all plant lineages, Tim17 contains a C-terminal extension, which may act as a receptor component for the import of nucleic acids into plant mitochondria.</p> <p>Conclusions</p> <p>It is proposed that the observed functional divergences are due to the selective pressure to sort proteins between mitochondria and chloroplasts, resulting in differences in protein receptor components between plant groups and other organisms. Additionally, diversity of receptor components is observed within the plant kingdom. Even when receptor components are orthologous across plant and non-plant species, it appears that the functions of these have expanded or diverged in a lineage specific manner.</p

TGD1, -2, and -3 Proteins Involved in Lipid Trafficking Form ATP-binding Cassette (ABC) Transporter with Multiple Substrate-binding Proteins

Background: ATP-binding cassette transporters exist in all life forms and usually require only one substrate-binding domain. Results: The TRIGALACTOSYLDIACYLGLYCEROL (TGD) complex contains 8–12 substrate-binding proteins. Conclusion: Multiple substrate-binding proteins may be needed by the TGD complex to enhance its putative lipid transport activity. Significance: Knowing the subunit stoichiometry of the TGD complex furthers understanding of lipid transfer between chloroplast membranes

TGD1, -2, and -3 Proteins Involved in Lipid Trafficking Form ATP-binding Cassette (ABC) Transporter with Multiple Substrate-binding Proteins

Background: ATP-binding cassette transporters exist in all life forms and usually require only one substrate-binding domain. Results: The TRIGALACTOSYLDIACYLGLYCEROL (TGD) complex contains 8–12 substrate-binding proteins. Conclusion: Multiple substrate-binding proteins may be needed by the TGD complex to enhance its putative lipid transport activity. Significance: Knowing the subunit stoichiometry of the TGD complex furthers understanding of lipid transfer between chloroplast membranes

The dual targeting ability of type II NAD(P)H dehydrogenases arose early in land plant evolution

Background: Type II NAD(PH) dehydrogenases are located on the inner mitochondrial membrane of plants, fungi, protists and some primitive animals. However, recent observations have been made which identify several Arabidopsis type II dehydrogenases as dual targeted proteins. Targeting either mitochondria and peroxisomes or mitochondria and chloroplasts. Results: Members of the ND protein family were identified in various plant species. Phylogenetic analyses and subcellular targeting predictions were carried out for all proteins. All ND proteins from three model plant species Arabidopsis, rice and Physcomitrella were cloned as N- and C-terminal GFP fusions and subcellular localisations were determined. Dual targeting of plant type II dehydrogenases was observed to have evolved early in plant evolution and to be widespread throughout different plant species. In all three species tested dual targeting to both mitochondria and peroxisomes was found for at least one NDA and NDB type protein. In addition two NDB type proteins from Physcomitrella were also found to target chloroplasts. The dual targeting of NDC type proteins was found to have evolved later in plant evolution. Conclusions: The functions of type II dehydrogenases within plant cells will have to be re-evaluated in light of this newly identified subcellular targeting information

Mitochondrial Biogenesis: Cell-Cycle-Dependent Investment in Making Mitochondria

SummaryMitochondria cannot be made de novo, so pre-existing mitochondria must be inherited at each cell division. A new study demonstrates cell-cycle-dependent regulation of the activity of the TOM translocase complex to induce mitochondrial biogenesis during the M phase of the cell cycle

Imaging and analysis of mitochondrial dynamics in living cells.

One of the most striking features of plant mitochondria when visualized in living tissue is their dynamism. The beauty of cytoplasmic streaming, driving, and being driven by the motility of mitochondria and other small organelles belies the complexity of the process. Equally, capturing that dynamism and investigating the genes, proteins, and mechanisms underpinning the processes using molecular cell biology and bioimaging is a complex process. It requires the generation of fluorescent protein constructs, stable transgenic plants sometimes expressing multiple fusions, and generation of mutants, even before one is ready for analytical experimentation. Here, we describe some of the key tools and methods necessary to investigate plant mitochondrial dynamics

MSL1 is a mechanosensitive ion channel that dissipates mitochondrial membrane potential and maintains redox homeostasis in mitochondria during abiotic stress

Mitochondria must maintain tight control over the electrochemical gradient across their inner membrane to allow ATP synthesis while maintaining a redox-balanced electron transport chain and avoiding excessive reactive oxygen species production. However, there is a scarcity of knowledge about the ion transporters in the inner mitochondrial membrane that contribute to control of membrane potential. We show that loss of MSL1, a member of a family of mechanosensitive ion channels related to the bacterial channel MscS, leads to increased membrane potential of Arabidopsis mitochondria under specific bioenergetic states. We demonstrate that MSL1 localises to the inner mitochondrial membrane. When expressed in Escherichia coli, MSL1 forms a stretch-activated ion channel with a slight preference for anions and provides protection against hypo-osmotic shock. In contrast, loss of MSL1 in Arabidopsis did not prevent swelling of isolated mitochondria in hypo-osmotic conditions. Instead, our data suggest that ion transport by MSL1 leads to dissipation of mitochondrial membrane potential when it becomes too high. The importance of MSL1 function was demonstrated by the observation of a higher oxidation state of the mitochondrial glutathione pool in msl1-1 mutants under moderate heat- and heavy-metal-stress. Furthermore, we show that MSL1 function is not directly implicated in mitochondrial membrane potential pulsing, but is complementary and appears to be important under similar conditions