Chloroplasts are the plant organelles carrying out photosynthesis. To do this chloroplasts must import most of their proteins. These are synthesized as preproteins with N-terminal targeting sequences known as transit peptides. The transit peptides are cleaved upon arrival inside the chloroplast. Translocon complexes at the outer (Toc) and inner (Tic) envelope membranes facilitate the preprotein import into the chloroplast. The Toc-complex consists of a core of three proteins: The Toc75 translocation channel and the two GTP-binding proteins Toc33 and Toc159. Toc33 and Toc159 function as preprotein receptors at the chloroplast surface. In addition to the GTP-binding (G-) domain Toc159 has a C-terminal membrane insertion (M-) domain and an extensive N-terminal acidic (A-domain). Both the A- and G-domains are located in the cytosol. The G- and M-domains are essential for the receptor function of Toc159. In contrast, the A-domain is not essential for the function of Toc159 but current research demonstrated that the A-domain is highly phosphorylated and that it may be cleaved by proteases. Our research will therefore focus on the identification of the kinases and proteases involved in the regulation at the A-domain. These are excellent candidates for new factors involved in chloroplast protein import and organelle biogenesis. These factors contribute to one of the key processes in plant development and may lead to exciting new applications in the future.