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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
    Métadonnées seulement
    The function and diversity of plastid protein import pathways: A multilane GTPase highway into plastids
    (2006)
    Kessler, Félix 
    ;
    Schnell, Danny
    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
    Métadonnées seulement
    AtToc90, a new GTP-binding component of the Arabidopsis chloroplast protein import machinery
    (2004)
    Hiltbrunner, Andreas
    ;
    Grunig, 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.
  • Publication
    Métadonnées seulement
    AtTic110 functions as a scaffold for coordinating the stromal events of protein import into chloroplasts
    (2003)
    Inaba, Takehito
    ;
    Li, Ming
    ;
    Alvarez-Huerta, Mayte
    ;
    Kessler, Félix 
    ;
    Schnell, Danny
    The translocon of the inner envelope membrane of chloroplasts ( Tic) mediates the late events in the translocation of nucleus-encoded preproteins into chloroplasts. Tic110 is a major integral membrane component of active Tic complexes and has been proposed to function as a docking site for translocation-associated stromal factors and as a component of the protein-conducting channel. To investigate the various proposed functions of Tic110, we have investigated the structure, topology, and activities of a 97.5-kDa fragment of Arabidopsis Tic110 ( atTic110) lacking only the amino-terminal transmembrane segments. The protein was expressed both in Escherichia coli and Arabidopsis as a stable, soluble protein with a high alpha-helical content. Binding studies demonstrate that a region of the at-Tic110-soluble domain selectively associates with chloroplast preproteins at the late stages of membrane translocation. These data support the hypothesis that the bulk of Tic110 extends into the chloroplast stroma and suggest that the domain forms a docking site for preproteins as they emerge from the Tic translocon.
  • Publication
    Métadonnées seulement
    Dimerization of Toc-GTPases at the chloroplast protein import machinery
    (2003)
    Weibel, Petra 
    ;
    Hiltbrunner, Andreas
    ;
    Brand, Lukas
    ;
    Kessler, Félix 
    Import of chloroplast precursor proteins is controlled by the coordinate action of two homologous GTPases, Toc159 and Toc33, located at the cytosol-outer membrane interface. Recent studies in Arabidopsis showed that the cytosolic form of the precursor binding protein Toc159 is targeted to its receptor at the import machinery, Toc33, via heterodimerization of their GTP-binding domains. Toc33 may also form GDP-bound homodimers, as suggested by the crystal structure of its pea ortholog. Moreover, the structural data suggested that arginine 130 ( Arg(130)) of Arabidopsis Toc33 may function as a GTPase-activating "arginine-finger" at the other monomer in the Toc33 dimer. Here, we demonstrate that Arg(130) of Toc33 does not function as an Arginine-finger. A mutant, Toc33-R130A, binds and hydrolyzes GTP like the wild type. However, we demonstrate that Arg(130) is involved in both homodimerization of Toc33 and in heterodimerization with the GTP-binding domain of Toc159. The dependence of Toc33 homodimerization on Arg(130) is mutual, requiring the presence of Arg(130) at both monomers. As the GTPase is not activated by dimerization, it may be activated independently at either monomer, possibly even before dimerization. Independent regulation of GTPase activity may serve to coordinate the interactions of the GTPases during the import of proteins into the chloroplast.
  • Publication
    Métadonnées seulement
    The targeting of the atToc159 preprotein receptor to the chloroplast outer membrane is mediated by its GTPase domain and is regulated by GTP
    (2002)
    Smith, Matthew
    ;
    Hiltbrunner, Andreas
    ;
    Kessler, Félix 
    ;
    Schnell, Danny
    The multimeric translocon at the outer envelope membrane of chloroplasts (Toc) initiates the recognition and import of nuclear-encoded preproteins into chloroplasts. Two Toc GTPases, Toc159 and Toc33/34, mediate preprotein recognition and regulate preprotein translocation. Although these two proteins account for the requirement of GTP hydrolysis for import, the functional significance of GTP binding and hydrolysis by either GTPase has not been defined. A recent study indicates that Toc159 is equally distributed between a soluble cytoplasmic form and a membrane-inserted form, raising the possibility that it might cycle between the cytoplasm and chloroplast as a soluble preprotein receptor. In the present study, we examined the mechanism of targeting and insertion of the Arabidopsis thaliana orthologue of Toc159, atToc159, to chloroplasts. Targeting of atToc159 to the outer envelope membrane is strictly dependent only on guanine nucleotides. Although GTP is not required for initial binding, the productive insertion and assembly of atToc159 into the Toc complex requires its intrinsic GTPase activity. Targeting is mediated by direct binding between the GTPase domain of atToc159 and the homologous GTPase domain of atToc33, the Arabidopsis Toc33/34 orthologue. Our findings demonstrate a role for the coordinate action of the Toc GTPases in assembly of the functional Toc complex at the chloroplast outer envelope membrane.
  • Publication
    Métadonnées seulement
    A GTPase gate for protein import into chloroplasts
    (2002)
    Kessler, Félix 
    ;
    Schnell, Danny
    Protein import into chloroplasts is regulated by the binding and hydrolysis of GTP at two homologous GTPases, Toc34 and Toc159. The crystal structure of the Toc34 GTP-binding domain suggests that GTP-regulated dimerization of the Toc GTPase domains controls the targeting and translocation of preproteins at the chloroplast envelope.
  • Publication
    Métadonnées seulement
    Essential role of the G-domain in targeting of the protein import receptor atToc159 to the chloroplast outer membrane
    (2002)
    Bauer, Jörg
    ;
    Hiltbrunner, Andreas
    ;
    Weibel, Petra 
    ;
    Vidi, Pierre-Alexandre
    ;
    Alvarez-Huerta, Mayte
    ;
    Smith, Matthew
    ;
    Schnell, Danny
    ;
    Kessler, Félix 
    Two homologous GTP-binding proteins, atToc33 and atToc159, control access of cytosolic precursor proteins to the chloroplast. atToc33 is a constitutive outer chloroplast membrane protein, whereas the precursor receptor atToc159 also exists in a soluble, cytosolic form. This suggests that atToc159 may be able to switch between a soluble and an integral membrane form. By transient expression of GFP fusion proteins, mutant analysis, and biochemical experimentation, we demonstrate that the GTP-binding domain regulates the targeting of cytosolic atToc159 to the chloroplast and mediates the switch between cytosolic and integral membrane forms. Mutant atToc159, unable to bind GTP, does not reinstate a green phenotype in an albino mutant (ppi2) lacking endogenous atToc159, remaining trapped in the cytosol. Thus, the function of atToc159 in chloroplast biogenesis is dependent on an intrinsic GTP-regulated switch that controls localization of the receptor to the chloroplast envelope.
  • Publication
    Métadonnées seulement
    Targeting of an abundant cytosolic form of the protein import receptor at Toc159 to the outer chloroplast membrane
    (2001)
    Hiltbrunner, Andreas
    ;
    Bauer, Jörg
    ;
    Vidi, Pierre-Alexandre
    ;
    Infanger, Sibylle
    ;
    Weibel, Petra 
    ;
    Hohwy, Morten
    ;
    Kessler, Félix 
    Chloroplast biogenesis requires the large-scale import of cytosolically synthesized precursor proteins. A trimeric translocon (Toc complex) containing two homologous, GTP-binding proteins (atToc33 and atToc159) and a channel protein (atToc75) facilitates protein translocation across the outer envelope membrane. The mechanisms governing function and assembly of the Toc complex are not yet understood. This study demonstrates that atToc159 and its pea orthologue exist in an abundant, previously unrecognized soluble form, and partition between cytosol-containing soluble fractions and the chloroplast outer membrane. We show that soluble atToc159 binds directly to the cytosolic domain of atToc33 in a homotypic interaction, contributing to the integration of atToc159 into the chloroplast outer membrane. The data suggest that the function of the Toc complex involves switching of at Toc159 between a soluble and an integral membrane form.