The origin of the chloroplast protein translocon
Heins, L., Bölter, B., Caliebe, A., Soll, J.
Universität Kiel, Institut für Botanik, Am Botanischen Garten 1-9, D-24118 Kiel, Germany
During the evolution of chloroplasts from a photosynthetic active, prokaryotic endosymbiont a complex organell has developed with a multiplicity of subcompartments. Particularly, the origin of the outer envelope membrane is not clear. Most of the genes of the endosymbiont were displaced to the host nucleus. Therefore, the biogenesis of chloroplasts requires a targeting and translocation mechanism to transport nuclear-encoded proteins across the chloroplast envelope membranes. Several components of both the protein translocon at the outer- and the inner-envelope-membranes have been characterized namely Toc86, Toc75, Toc34, Tic110 and Tic55. Toc86 probably functions as receptor for nuclear-encoded precursor proteins. Both Toc86 and Toc34 represent a novel type of GTP binding protein and are in close physical proximity to Toc75, which forms the translocation pore. Tic110 and Tic55 are prominent constituents of the translocon at the inner envelope membranes.
Recently, proteins homologous to several subunits of the chloroplast import machinery (Toc75, Toc34, Tic55) were identified in the cyanobacterium Synechocystis PCC6803. The bacterial homolgue of Toc75 was overexpressed and reconstituted in liposomes. In vitro synToc75 forms a voltage-dependent channel like Toc75 of pea. By cell-frationation and immunogold-decoration we are currently trying to localize the synechocystis homologues of Toc75, Toc34 and Tic55 to either the plasmamembrane or the outer membrane. These results will allow important insights into the origin of the chloroplast outer envelope membrane and in general, organell biogenesis. Probably the transport systems of the prokaryotic ancestor and the host organism have merged during development ot the outer envelope of chloroplasts.
| LOCATION |
DATE |
TIME |
| Lecture Hall I |
Monday, April 6 |
04:30 pm |