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Envelope Membranes (envelope + membrane)
Selected AbstractsThe most C-terminal tri-glycine segment within the polyglycine stretch of the pea Toc75 transit peptide plays a critical role for targeting the protein to the chloroplast outer envelope membraneFEBS JOURNAL, Issue 7 2006Amy J. Baldwin The protein translocation channel at the outer envelope membrane of chloroplasts (Toc75) is synthesized as a larger precursor with an N-terminal transit peptide. Within the transit peptide of the pea Toc75, a major portion of the 10 amino acid long stretch that contains nine glycine residues was shown to be necessary for directing the protein to the chloroplast outer membrane in vitro[Inoue K & Keegstra K (2003) Plant J34, 661,669]. In order to get insights into the mechanism by which the polyglycine stretch mediates correct targeting, we divided it into three tri-glycine segments and examined the importance of each domain in targeting specificity in vitro. Replacement of the most C-terminal segment with alanine residues resulted in mistargeting the protein to the stroma, while exchange of either of the other two tri-glycine regions had no effect on correct targeting. Furthermore, simultaneous replacement of the N-terminal and middle tri-glycine segments with alanine repeats did not cause mistargeting of the protein as much as those of the N- and C-terminal, or the middle and C-terminal segments. These results indicate that the most C-terminal tri-glycine segment is important for correct targeting. Exchanging this portion with a repeat of leucine or glutamic acid also caused missorting of Toc75 to the stroma. By contrast, its replacement with repeats of asparagine, aspartic acid, serine, and proline did not largely affect correct targeting. These data suggest that relatively compact and nonhydrophobic side chains in this particular region play a crucial role in correct sorting of Toc75. [source] A HYPOTHESIS FOR IMPORT OF THE NUCLEAR-ENCODED PsaE PROTEIN OF PAULINELLA CHROMATOPHORA (CERCOZOA, RHIZARIA) INTO ITS CYANOBACTERIAL ENDOSYMBIONTS/PLASTIDS VIA THE ENDOMEMBRANE SYSTEM,JOURNAL OF PHYCOLOGY, Issue 5 2010Mackiewicz The cyanobacterial endosymbionts of Paulinella chromatophora can shed new light on the process of plastid acquisition. Their genome is devoid of many essential genes, suggesting gene transfer to the host nucleus and protein import back into the endosymbionts/plastids. Strong evidence for such gene transfer is provided by the psaE gene, which encodes a PSI component that was efficiently transferred to the Paulinella nucleus. It remains unclear, however, how this protein is imported into the endosymbionts/plastids. We reanalyzed the sequence of Paulinella psaE and identified four potential non-AUG translation initiation codons upstream of the previously proposed start codon. Interestingly, the longest polypeptide, starting from the first UUG, contains a clearly identifiable signal peptide with very high (90%) predictability. We also found several downstream hairpin structures that could enhance translation initiation from the alternative codon. These results strongly suggest that the PsaE protein is targeted to the outer membrane of Paulinella endosymbionts/plastids via the endomembrane system. On the basis of presence of respective bacterial homologs in the Paulinella endosymbiont/plastid genome, we discuss further trafficking of PsaE through the peptidoglycan wall and the inner envelope membrane. It is possible that other nuclear-encoded proteins of P. chromatophora also carry signal peptides, but, alternatively, some may be equipped with transit peptides. If this is true, Paulinella endosymbionts/plastids would possess two distinct targeting systems, one cotranslational and the second posttranslational, as has been found in higher plant plastids. Considering the endomembrane system-mediated import pathway, we also discuss homology of the membranes surrounding Paulinella endosymbionts/plastids. [source] EVIDENCE FOR A SPECIALIZED LOCALIZATION OF THE CHLOROPLAST ATP-SYNTHASE SUBUNITS ,, ,, AND , IN THE EYESPOT APPARATUS OF CHLAMYDOMONAS REINHARDTII (CHLOROPHYCEAE),JOURNAL OF PHYCOLOGY, Issue 2 2007Melanie Schmidt The eyespot apparatus (EA) of Chlamydomonas reinhardtii P. A. Dang. consists of two layers of carotenoid-rich lipid globules subtended by thylakoids. The outermost globule layer is additionally associated with the chloroplast envelope membranes and the plasma membrane. In a recent proteomic approach, we identified 202 proteins from isolated EAs of C. reinhardtii via at least two peptides, including, for example, structural components, signalling-related proteins, and photosynthetic-related membrane proteins. Here, we have analyzed the proteins of the EA with regard to their topological distribution using thermolysin to find out whether the arrangement of globules and membranes provides protection mechanisms for some of them. From about 230 protein spots separated on two-dimensional gels, the majority were degraded by thermolysin. Five major protein spots were protected against the action of this protease. These proteins and some that were degradable were identified by mass spectrometry. Surprisingly, the thermolysin-resistant proteins represented the , and , subunits of the soluble CF1 complex of the chloroplast ATP synthase. Degradable proteins included typical membrane proteins like LHCs, demonstrating that thermolysin is not in general sterically prevented by the EA structure from reaching membrane-associated proteins. A control experiment showed that the CF1 complex of thylakoids is efficiently degraded by thermolysin. Blue native PAGE of thermolysin-treated EAs followed by SDS-PAGE revealed that the , and , subunits are present in conjunction with the , subunit in a thermolysin-resistant complex. These results provide strong evidence that a significant proportion of these ATP-synthase subunits have a specialized localization and function within the EA of C. reinhardtii. [source] DO PLASTID-RELATED CHARACTERS SUPPORT THE CHROMALVEOLATE HYPOTHESIS?,JOURNAL OF PHYCOLOGY, Issue 3 2005Andrzej Body According to the idea of secondary endosymbiosis, plastids with three and four envelope membranes have evolved from either red or green algal endosymbionts engulfed by phagotrophic protozoans. Although this hypothesis is nowadays commonly accepted, the number of secondary endosymbioses still remains controversial. One of the models, known as the "chromalveolate" hypothesis, postulates that the 4 membrane-bound plastids of Chromista and the 3 or 4 membrane-bound plastids of Alveolata result from a single secondary endosymbiosis involving a rhodophyte as the endosymbiont. Although this model has found many followers, a variety of data clearly contradict it. The ideas that became the direct inspiration for formulation of the "chromalveolate" hypothesis are also now questionable. In this comment, I discuss all these problems in the light of the most recent phylogenetic, cytological, and genomic data. [source] ENDOMEMBRANE STRUCTURE AND THE CHLOROPLAST PROTEIN TARGETING PATHWAY IN HETEROSIGMA AKASHIWO (RAPHIDOPHYCEAE, CHROMISTA)JOURNAL OF PHYCOLOGY, Issue 6 2000Ken-ichiro Ishida Chloroplasts in heterokont algae are surrounded by four membranes and probably originated from a red algal endosymbiont that was engulfed and retained by eukaryotic host. Understanding how nuclear-encoded chloroplast proteins are translocated from the cytoplasm into the chloroplast across these membranes could give us some insights about how the endosymbiont was integrated into the host cell in the process of secondary symbiogenesis. In multiplastid heterokont algae such as raphidophytes, it has been unclear if the outermost of the four membranes surrounding the chloroplast (the chloroplast endoplasmic reticulum [CER] membrane) is continuous with the nuclear envelope and rough endoplasmic reticulum (ER). Here, we report detailed ultrastructural observations of the raphidophyte Heterosigma akashiwo (Hada) Hada ex Y. Hara et Chihara that show that the CER membranes were continuous with ER membranes that had attached ribosomes, implying that the chloroplast with three envelope membranes is located within the ER lumen, that is, topologically the same structure as that of monoplastid heterokont algae. However, the CER membrane of H. akashiwo had very few, if any, ribosomes attached, unlike the CER membranes in other heterokont algae. To verify that proteins are first targeted to the ER, we assayed protein import into canine microsomes using a precursor for a nuclear-encoded chloroplast protein, the fucoxanthin-chlorophyll a/c protein of H. akashiwo. This demonstrated that the precursor has a functional signal sequence for ER targeting and is cotranslationally translocated into the ER, where a signal sequence of about 17 amino acids is removed. Based on these data, we hypothesize that in H. akashiwo, nuclear-encoded chloroplast protein precursors that have been cotranslationally transported into the ER lumen are sorted in the ER and transported to the chloroplasts through the ER lumen. [source] Evolutionary Origin of a Preprotein Translocase in the Periplastid Membrane of Complex Plastids: a HypothesisPLANT BIOLOGY, Issue 5 2004A. Body Abstract: Plastids with four envelope membranes have evolved from red and green algae engulfed by phagotrophic protozoans. It is assumed that the Sec translocon resides in their outermost membrane, while in the two innermost membranes the Toc-Tic supercomplex is embedded. However, such a single Sec/single Toc-Tic model cannot explain the passage of proteins across the second (or periplastid) membrane which represents the endosymbiont plasmalemma. One of the most recent models postulates that this membrane contains the Toc75 channel which was relocated here from the endosymbiont plastid. Unfortunately, the precursor of this protein carries a bipartite presequence, which means that its insertion into the new membrane would require relocation and/or modification of two different processing peptidases. I suggest that these obstacles can be easily bypassed by the assumption that the mitochondrial Tim23 channel was inserted into the endosymbiont plasmalemma. In contrast to Toc75, this protein has an internal, uncleavable targeting signal and its insertion into the new membrane would require neither relocation nor modification of additional proteins. Besides, such a relocated Tim23 channel could import not only plastid, but also mitochondrial proteins. I hypothesize that from the latter proteins, initially directed to the endosymbiont mitochondrion, periplastid proteins have evolved which are now targeted to the former cytosol and/or nucleus of the eukaryotic algal endosymbiont. [source] Evolution of Protein Targeting into "Complex" Plastids: The "Secretory Transport Hypothesis"PLANT BIOLOGY, Issue 4 2003O. Kilian Abstract: In algae different types of plastids are known, which vary in pigment content and ultrastructure, providing an opportunity to study their evolutionary origin. One interesting feature is the number of envelope membranes surrounding the plastids. Red algae, green algae and glaucophytes have plastids with two membranes. They are thought to originate from a primary endocytobiosis event, a process in which a prokaryotic cyanobacterium was engulfed by a eukaryotic host cell and transformed into a plastid. Several other algal groups, like euglenophytes and heterokont algae (diatoms, brown algae, etc.), have plastids with three or four surrounding membranes, respectively, probably reflecting the evolution of these organisms by so-called secondary endocytobiosis, which is the uptake of a eukaryotic alga by a eukaryotic host cell. A prerequisite for the successful establishment of primary or secondary endocytobiosis must be the development of suitable protein targeting machineries to allow the transport of nucleus-encoded plastid proteins across the various plastid envelope membranes. Here, we discuss the possible evolution of such protein transport systems. We propose that the secretory system of the respective host cell might have been the essential tool to establish protein transport into primary as well as into secondary plastids. [source] Stromules: Mobile Protrusions and Interconnections Between PlastidsPLANT BIOLOGY, Issue 3 2001J. C. Gray Abstract: Stroma-filled tubules, recently named stromules, extend from the surface of plastids in most cell types and plant species examined. Stromules are highly dynamic structures, continuously and rapidly changing shape. They have been shown to interconnect plastids and permit the exchange of green fluorescent protein (GFP) between plastids. Stromules are enclosed by the inner and outer plastid envelope membranes and are 0.4 - 0.8 ,m in diameter and up to 65 ,m long. Movement of stromules is dependent on the actin cytoskeleton and the ATPase activity of myosin. Stromules are more abundant in cells containing a relatively small plastid volume and provide a means of enormously increasing the plastid surface area. Many important questions on the structure, function and mobility of stromules remain unanswered. [source] Homologous protein import machineries in chloroplasts and cyanelles,THE PLANT JOURNAL, Issue 4 2005Jürgen M. Steiner Summary The cyanelles of the glaucocystophyte alga Cyanophora paradoxa resemble endosymbiotic cyanobacteria, especially in the presence of a peptidoglycan wall between the inner and outer envelope membranes. However, it is now clear that cyanelles are in fact primitive plastids. Phylogenetic analyses of plastid, nuclear and mitochondrial genes support a single primary endosymbiotic event. In this scenario, cyanelles and all other plastid types are derived from an ancestral photosynthetic organelle combining the high gene content of rhodoplasts and the peptidoglycan wall of cyanelles. This means that the import apparatuses of all primary plastids, i.e. those from glaucocystophytes, red algae, green algae and higher plants, should be homologous. If this is the case, then transit sequences should be similar and heterologous import experiments feasible. Thus far, heterologous in vitro import has been shown in one direction only: precursors from C. paradoxa were imported into isolated pea or spinach chloroplasts. Cyanelle transit sequences differ from chloroplast stroma targeting peptides in containing in their N-terminal domain an invariant phenylalanine residue which is shown here to be crucial for import. In addition, we now demonstrate that heterologous precursors are readily imported into isolated cyanelles, provided that the essential phenylalanine residue is engineered into the N-terminal part of chloroplast transit peptides. The cyanelle and likely also the rhodoplast import apparatus can be envisaged as prototypes with a single receptor/channel showing this requirement for N-terminal phenylalanine. In chloroplasts, multiple receptors with overlapping and less stringent specificities have evolved, explaining the efficient heterologous import of native precursors from C. paradoxa. [source] Localization and targeting of the VP14 epoxy-carotenoid dioxygenase to chloroplast membranesTHE PLANT JOURNAL, Issue 5 2001Bao-Cai Tan Summary Abscisic acid (ABA) is a key regulator of seed dormancy and plant responses to environmental challenges. ABA is synthesized via an oxidative cleavage of 9- cis epoxy-carotenoids, the first committed and key regulatory step in the ABA biosynthetic pathway. Vp14 of maize encodes an epoxy-carotenoid dioxygenase that is soluble when expressed in E. coli. An important goal has been to determine how the soluble VP14 protein is targeted to epoxy-carotenoid substrates that are located in the thylakoid and envelope membranes of chloroplasts and other plastids. Using an in vitro chloroplast import assay, we have shown that VP14 is imported into chloroplasts with cleavage of a short stroma-targeting domain. The mature VP14 exists in two forms, one which is soluble in stroma and the other bound to thylakoid membranes. Analysis of a series of truncated VP14 mutants mapped the membrane targeting signal to the 160 amino acid N-terminal sequence. A putative amphipathic ,-helix within this region is essential, but not sufficient, for the membrane targeting. Either deletion of or insertion of helix breaking residues into this region abolished the membrane binding, whereas a chimeric protein carrying just the amphipathic region fused with bacterial glutathione S -transferase failed to associate with the thylakoid membrane. The membrane-bound VP14 was partially resistant to chaotropic washes such as 0.1 m Na2CO3 (pH 11.5) and 6 m urea. Unlabelled recombinant VP14 inhibited the tight binding of imported VP14, suggesting that VP14 is associated with specific components of the thylakoid membrane. [source] | ||||||||||