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Kessler, Félix

Nom
Kessler, Félix
Affiliation principale
Fonction
Professeur ordinaire
Email
felix.kessler@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/320

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  • Publication
    Accès libre
    Identification and characterization of putative Toc159 interacting partners
    (2015)
    Montandon, Cyrille
    ;
    Kessler, Félix 
    Le chloroplaste est l’organelle qui caractérise les plantes terrestres ainsi que les autres eucaryotes photosynthétiques. Il remplit diverses fonctions métaboliques, mais la photosynthèse est son activité principale, sa structure et sa composition protéique en témoignent. Le chloroplaste est le résultat d’une endosymbiose entre un eucaryote ancestral et une cyanobactérie photosynthétique. Le génome du chloroplaste, vestige de son ancienne autonomie, encode environ une centaine de protéines. Les 2000-3000 autres protéines présentes dans le chloroplaste sont codées dans le noyau et traduites dans le cytosol et doivent donc être importées dans le chloroplaste. Le « transit peptide » (peptide de transit), une courte séquence à la terminaison aminée des protéines destinées au chloroplaste, est suffisant et nécessaire pour l’import spécifique dans le chloroplaste. La voie Toc/Tic (Translocon at the Outer/Inner membrane of the chloroplast envelope: « Translocon à la membrane externe/interne de l’enveloppe du chloroplaste ») reconnait et import ces protéines en présence d’ATP et de GTP. Le noyau du complexe Toc est composé de Toc159 et Toc34/33, 2 récepteurs possédant un domaine GTPase, et de Toc75 qui constitue le pore du complexe. Toc159 est formé de 3 domaines, un domaine ancrant la protéine dans la membrane à l’extrémité carboxyle (domaine M), un domaine GTPase (domaine G) et un domaine acide (domaine A). Le taux d’import des protéines et sa spécificité envers les protéines clientes varient en fonction du développement de la plante, du tissu ou du type de plaste. Les différents homologues de Toc159 et de Toc34/33 chez A. thaliana forment des complexes distincts. Ils montrent une spécificité d’import différente et leur expression varie en fonction du stade de développement ou de la partie de la plante. Le domaine A de Toc159 et de ses homologues détermine partiellement la spécificité envers les différents types de protéines clientes. Le domaine A est hyper-phosphorylé et existe sous une forme soluble, certainement le résultat d’un clivage spécifique. Ces modifications post-traductionnelles pourraient influencer la spécificité de Toc159 envers les protéines clientes. Le contrôle de l’activité GTPase de Toc159 ou de Toc34/33 pourrait également influencer l’activité d’import. Le but de ce travail de thèse était la caractérisation de protéines co-précipitées avec une version taguée de Toc159 et identifiées par spectrométrie de masse, qui pourraient être potentiellement responsable des modifications mentionnées ci-dessus. L’effort principal a été fait sur Emb2004/AT1G10510, une protéine LRR (Leucine Rich Repeat) et des contributions ont été faites à la caractérisation d’une protéine kinase potentielle/AT4G32250 et de Tic56/AT5G01590., The chloroplast is the distinctive organelle of land plants and other photosynthetic eukaryotes. It carries out a variety of metabolic process, but photosynthesis is its main task and this is reflected in its structures and protein composition. Chloroplasts evolved from an endosymbiosis between a photosynthetic cyanobacterium and an ancestral eukaryote. Reminiscent of this autonomous origin, the chloroplast genome encodes approximately 100 proteins. The majority of the remaining 2000-3000 proteins identified in the chloroplast are encoded in the nucleus and translated in the cytosol and must be imported into the chloroplast. The transit peptide, a small sequence at the N-terminus of the proteins destined for the chloroplast, is necessary and sufficient for specific import into the chloroplast. The Toc/Tic (Translocon at the Outer/Inner membrane of the chloroplast envelope) pathway mediates the recognition and the translocation of these proteins in an ATP- and GTP-dependent way. The Toc core complex is constituted of two receptors with a GTPase domain, Toc159 and Toc34/33, and a channel, Toc75. Toc159 is formed of three domains, a C-terminal membrane anchoring domain (M domain), a GTPase domain (G domain) and an acidic domain (A-domain). The protein import rate and specificity toward client proteins vary depending on the developmental stage, tissue or plastid type. The different A. thaliana homologues of Toc159 and Toc34/33 form distinct complexes. They have different import specificities and their expression level depends on the developmental stage and the anatomical part of the plant. The A-domain of Toc159 and its homologues partially determine the specificity toward the different types of client proteins. The A-domain is hyper-phosphorylated and exists as a soluble form, likely the result of a specific cleavage. These post-translational modifications of the A-domain might play a role in determining the affinity of Toc159 toward client proteins. The control of the GTPase activity of Toc159 or Toc34/33 might also have an influence on the import. The aim of this thesis was the characterization of proteins potentially responsible for these modifications and that were co-purified with a tagged Toc159 and identified by mass-spectrometry. The main effort was made on Emb2004/AT1G10510, a LRR (Leucine Rich Repeat) protein and contributions were made for the characterization of a predicted protein kinase (AT4G32250) and Tic56 (AT5G01590).
  • Publication
    Accès libre
    Protein transport in organelles: The Toc complex way of preprotein import
    (2009)
    Agne, Birgit
    ;
    Kessler, Félix 
    Most of the estimated 1000 or so chloroplast proteins are synthesized as cytosolic preproteins with N-terminal cleavable targeting sequences (transit peptide). Translocon complexes at the outer (Toc) and inner chloroplast envelope membrane (Tic) concertedly facilitate post-translational import of preproteins into the chloroplast. Three components, the Toc34 and Toc159 GTPases together with the Toc75 channel, form the core of the Toc complex. The two GTPases act as GTP-dependent receptors at the chloroplast surface and promote insertion of the preprotein across the Toc75 channel. Additional factors guide preproteins to the Toc complex or support their stable ATP-dependent binding to the chloroplast. This minireview describes the components of the Toc complex and their function during the initial steps of preprotein translocation across the chloroplast envelope.
  • Publication
    Accès libre
    The Function and Diversity of Plastid Protein Import Pathways : A Multilane GTPase Highway into Plastids
    (2006)
    Kessler, Félix 
    ;
    Schnell, Danny J.
    The photosynthetic chloroplast is the hallmark organelle of green plants. During the endosymbiotic evolution of chloroplasts, the vast majority of genes from the original cyanobacterial endosymbiont were transferred to the host cell nucleus. Chloroplast biogenesis therefore requires the import of nucleus-encoded proteins from their site of synthesis in the cytosol. The majority of proteins are imported by the activity of Toc and Tic complexes located within the chloroplast envelope. In addition to chloroplasts, plants have evolved additional, non-photosynthetic plastid types that are essential components of all cells. Recent studies indicate that the biogenesis of various plastid types relies on distinct but homologous Toc–Tic import pathways that have specialized in the import of specific classes of substrates. These different import pathways appear to be necessary to balance the essential physiological role of plastids in cellular metabolism with the demands of cellular differentiation and plant development.
  • Publication
    Accès libre
    AtToc90, a New GTP-Binding Component of the Arabidopsis Chloroplast Protein Import Machinery
    (2004)
    Hiltbrunner, Andreas
    ;
    Grünig, Kathrin
    ;
    Alvarez-Huerta, Mayte
    ;
    Infanger, Sibylle
    ;
    Bauer, Jörg
    ;
    Kessler, Félix 
    AtToc159 is a GTP-binding chloroplast protein import receptor. In vivo, atToc159 is required for massive accumulation of photosynthetic proteins during chloroplast biogenesis. Yet, in mutants lacking atToc159 photosynthetic proteins still accumulate, but at strongly reduced levels whereas non-photosynthetic proteins are imported normally: This suggests a role for the homologues of atToc159 (atToc132, -120 and -90). Here, we show that atToc90 supports accumulation of photosynthetic proteins in plastids, but is not required for import of several constitutive proteins. Part of atToc90 associates with the chloroplast surface in vivo and with the Toc-complex core components (atToc75 and atToc33) in vitro suggesting a function in chloroplast protein import similar to that of atToc159. As both proteins specifically contribute to the accumulation of photosynthetic proteins in chloroplasts they may be components of the same import pathway.