Master of Science (MSc)
Faculty of Science
Most chloroplast proteins are encoded in the nucleus and translated in the cytosol with an N-terminal cleavable transit peptide, which can be recognized by the translocon at the outer envelope membrane of chloroplasts (Toc complex). The core Toc complex is composed of two GTPases, Toc159 and Toc34, and the Toc75 channel. atToc159, atToc132 and atToc120 are homologues of the Toc159 family in Arabidopsis, and have been shown to assemble into structurally and functionally distinct Toc complexes. Targeting of atToc159 to the chloroplast outer membrane is mediated by its GTPase (G-) domain. However, the role of the acidic (A-) domain in targeting of the receptor to chloroplasts is still unclear. The members of the Toc159 family are most variable in the A-domain suggesting that this domain might play a role in the functional specificity. The overall goal of this study is to clarify the role of the A-domain in the targeting of Toc159 to chloroplasts and in the assembly of distinct Toc complexes. In order to reach this goal, in vitro chloroplast targeting and solid-phase binding assays focusing on members of the Toc159 family were used. The data suggest that atToc132 interacts with both members of the Toc34 family in Arabidopsis (atToc33 and atToc34), but that the A-domain affects the targeting and assembly of atToc132 into specific Toc complexes by restricting the binding of this receptor parimarily to atToc34. In vitro chloroplast targeting assays show that competition with atToc159G also inhibits the targeting of atToc132, indicating that atToc132 may not exclusively bind with a specific member of the Toc34 family. The A-domain inhibits the targeting of atToc132GM (an A-domain deletion mutant of atToc132) to both wild-type and ppi3 (an atToc34 knockout mutant) chloroplasts ii much more effectively than its targeting to ppi1 (an atToc33 knockout mutant) chloroplasts. In addition, in vitro solid-phase binding assays indicate that the A-domain deletion fragment of atToc132 increases its affinity for 33G. These results support the contention that the A-domain influences the targeting of atToc132 to chloroplasts and the assembly of atToc132-containing Toc complexes by inhibiting its interaction with atToc33, thereby promoting an interaction with atToc34.
Chen, Yi, "The role of the acidic domain of the Toc159 protein import receptor family in Toc complex assembly in Arabidopsis thaliana" (2011). Theses and Dissertations (Comprehensive). 833.