Preprotein import in chloroplast biogenesis: identification and characterisation of a strong dimerisation mutant of atToc159

The chloroplast is the hallmark organelle of plant having evolved from the endosymbiotic event. Most chloroplast proteins are synthesized as preproteins in the cytosol. The import of these preproteins 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 of the Chloroplast) and TIC (Translocon at the Inner envelope of the Chloroplast), respectively. In <i>Arabidopsis thaliana</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 dimerisation of their GTP-binding domain (G-domain). According to the crystal structure of pea Toc34 homodimer and based on the G-domain homology of the TOC GTPases it is likely that the process of dimerisation is a key step for the process of import of preprotein into the chloroplast. This thesis intends to identify a mutant of Toc159 with an increased dimerisation interaction towards Toc33. Thus, Toc159 D919V was identified and revealed itself to bind strongly to Toc33, hydrolyse GTP and complement the TOC159 null mutant. <br> This mutant is a promising candidate for crystallisation purposes and for the identification of interaction partners of TOC GTPases by TAP-tag purification.

Thèse de doctorat : Université de Neuchâtel, 2011 ; 2243