The cyanelles of Cyanophora paradoxa and their relation
to the protoplastid
Löffelhardt, W.1, Steiner, J. M.1, Stirewalt, V. L.2, Michalowski, C. B.3, Bryant, D. A.2, Bohnert, H. J.3
1Institut für Biochemie und Molekulare Zellbiologie der Universität Wien und Ludwig-Botzmann-Forschungsstelle für Biochemie, A-1030 Vienna, Austria
2Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA 16802, USA
3Department of Biochemistry, University of Arizona, Tucson, AZ 85721, USA
Among the photosynthetic organelles only the cyanelles retained the peptidoglycan wall of their cyanobacterial ancestors. This is the main difference between them and the plastids of red algae that are otherwise comparable with respect to morphology, pigment composition, and genome organization. Cyanelles have been considered for long as a sideline (dead end) of plastid evolution or as resulting from a relatively recent endosymbiotic event, different from that (those) that led to the other plastid types. However, the comparison of 10 plastid genomes from evolutionarily different lines render a singular primary endosymbiotic event and a "protoplastid" that was ancestral to all recent plastids, very likely. During the formation of the protoplastid, most probably a long-lasting process, gene transfer to the nucleus and their functionalization therein went in parallel with the development of a protein import system in the envelope of the endosymbiont. Nucleus-encoded precursors of cyanelle proteins show stroma targeting sequences analogous to those from higher plants, green algae, and red algae. Homologous and heterologous in vitro import experiments were successful involving isolated cyanelles (or their thylakoid vesicles), isolated pea chloroplasts and precursors to stromal and lumenal polypeptides from C. paradoxa as well as from higher plants. This shows that not only "conservative sorting" across the thylakoid membrane is performed by a similar apparatus in cyanelles and chloroplasts, but also the import across the two envelope membranes. This implies that the protoplastid contained a peptidoglycan wall, which (as in the case of the extant cyanelles) did not constitute a barrier to protein translocation. Possible explanations for this
model will be discussed.
| LOCATION |
DATE |
TIME |
| Lecture Hall I |
Monday, April 6 |
05:10 pm |