From protein transport to organelle development

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Johannes Herrmann, Tom Lutz, Stephan Meier, Marc Preuss, Gregor Szyrach, Frank Baumann Protein insertion into the inner membrane of mitochondria The inner membrane of mitochondria contains a large number and variety of proteins. Most of these proteins are synthesized in the cytoplasm and imported into mitochondria. Inner membrane proteins are inserted on different sorting pathways. Monotopic proteins can be inserted following a translocation arrest at the level of the inner membrane. An insertion route from the intermembrane space is also used by polytopic proteins which have not evolved from bacterial homologues. In contrast, we found that polytopic proteins derived from the endosymbiotic ancestors of mitochondria are first completely imported into the mitochondrial matrix and subsequently inserted into the membrane. This insertion process depends on the membrane potential and resembles the insertion of membrane proteins in bacteria in several respects. The same insertion route is used by inner membrane proteins that are synthesized on mitochondrial ribosomes. Membrane integration of these proteins depends on the Oxa1 protein, a component related to bacterial YidC. Oxa1 forms an oligomeric complex in the inner membrane and contains a matrix-exposed domain which has the ability to bind translating mitochondrial ribosomes and is required for efficient insertion of nascent polypeptides. This suggests that cotranslational membrane insertion of mitochondrial translation products is achieved by a physical contact of translation complexes with the OXA translocase in the inner membrane. contact: Dr. Johannes Herrmann Universität München Institut für Physiologische Chemie hannes.herrmann@bio.med.uni-muenchen.de Butenandtstrasse 5 81377 München (Germany)
Ralf Bernd Klösgen, Bo Hou The mechanism of deltapH/TAT-dependent protein translocation across the thylakoid membrane Protein translocation into or across the thylakoid membrane is accomplished by at least four distinct pathways: the Sec-dependent pathway, the SRP-dependent pathway, the pH/TAT-dependent pathway and the spontaneous protein insertion pathway. The pH/TAT-dependent protein translocation pathway is the sole system that is capable of transporting both folded and unfolded proteins across the thylakoid membrane. Translocation of precursors by this pathway does not need stromal factors or nucleoside triphosphates, but is strictly dependent on the thylakoidal pH. In order to analyze the mechanism of pH/TAT-dependent protein translocation, we have constructed a chimeric 16/23 protein, which consists of the transit peptide of the 16 kDa protein and the mature part of the 23 kDa protein. Like the two corresponding authentic precursor proteins (pre-16 kDa and pre-23 kDa, respectively), the chimeric 16/23 is targeted by pH/TAT-dependent pathway. During thylakoid import, the chimeric protein is retarded in the pH/TAT-dependent translocation machinery, resulting in transmembrane translocation intermediates. Two distinct steps of the transport process have been identified, which are characterized by different topologies of the translocation intermediates. Taking advantage of this finding, we could make detailed studies on the mechanism of each processing step. Data on the differences between these two steps, e.g. considering their dependence upon pH/TAT-translocase or the energy requirement, will be presented. contact: Master Bo Hou Martin-Luther-Universität Halle-Wittenberg Institut für Pflazenphysiologie hou@pflanzenphys.uni-halle.de Weibergweg 10 06120 Halle (Saale) (Germany)
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