A plant split-ubiquitin system and TAP-tagging to study in vivo protein interactions in the chloroplast protein import machinery

Most chloroplast proteins are synthesized as precursor proteins in the cytosol. The import of these precursor proteins is mediated by molecular complexes located at the outer and inner membrane of the chloroplast. These complexes are called Toc (translocon at the outer envelope membrane) and Tic (translocon at the inner envelope membrane) respectively. In <i>Arabidopsis</i>, the Toc complex consists of three principle components: two homologous receptor GTPases, atToc159 and atToc33 and a protein-import channel: atToc75. During import, the two GTPases undergo complex interactions with precursor proteins and amongst themselves although precise mechanisms remain unknown. <i>In vitro</i> studies revealed that Toc159 and Toc33 interact with each other via the dimerization of their GTP-binding domain (G-domain). Moreover, the crystal structure of the pea Toc33 ortholog, psToc34 indicates that it can stably homodimerize via its G-domain. However, neither Toc159/Toc33 heterodimers nor Toc33 homodimerization have been demonstrated <i>in planta</i>. To get new insight into the in vivo interactions of Toc GTPases, we have developed a plant split-ubiquitin system. This method, originally developed for yeast, was adapted to study interactions between the Toc GTPases atToc159 and atToc33 in <i>Arabidopsis</i> and tobacco protoplasts. We also demonstrated that the peroxisomal membrane protein atPex11e, used initially as a model membrane protein in our system, self-interacts as does its yeast homolog. The plant split-ubiquitin system proves to be widely usable. Another approach of this thesis was to get more information on the import mechanism via the identification of interaction partners of the Toc GTPase atToc33. atToc33 and proteins associated were isolated from Arabidopsis plants, using the tandem affinity purification (TAP) tag. We proved that this technique is suitable to purify Toc33, which encourages us to purify Toc proteins and complexes at a larger scale.

Thèse de doctorat : Université de Neuchâtel, 2008 ; Th. 2079