A protein that plays a key role in the division of chloroplasts within nucleated or eukaryotic plant cells is derived from those involved in the separation of whole dividing cells, Japanese molecular biologists from RIKEN and the University of Tsukuba have found. They suggest the close relationship between the two allows chloroplast division to be synchronized with cell division, so that chloroplasts—the membranous organelles inside cells where photosynthesis takes place—can be allocated to both daughter cells.
Cellular assistance
Chloroplasts are thought to have evolved from blue-green or cyano-bacteria that were engulfed by an ancient plant cell. After more than a billion years within cells, chloroplasts still multiply by division. They retain only a portion of their original genetic material, however. The rest has migrated to the nucleus of the cell, so chloroplasts cannot divide without assistance from the cell itself.
During division, protein-based ring structures form inside and outside the chloroplast’s double membrane. These rings tighten and pinch the organelle into two. The structure of the inner ring is based on FtsZ protein and the outer on a member of the dynamin protein family (Fig. 1).
Earlier work by other researchers unraveled details of the evolutionary history of the FtsZ protein, which is of cyanobacterial origin. Yet little was known of the origin of the dynamin proteins involved in this process before Shin-ya Miyagishima from the RIKEN Advanced Science Institute in Wako and his colleagues published details of their work in the Proceedings of the National Academy of Sciences1.
The role of these dynamin proteins turns out to be critical to how chloroplasts lost their independence and how they are regulated by the cell—hence, to the development and control of photosynthesis itself. And the research may also be relevant to mitochondria—the energy-production organelles of eukaryotic cells—which are thought to have a similar evolutionary history to chloroplasts.
Determining the roles of dynamin proteins
Dynamin proteins are widespread in eukaryotic cells, including those in plants and algae, and even in organisms which have no chloroplasts because they split off the evolutionary line before their acquisition. Each member of the dynamin family has specialized to perform fission or fusion functions in specific membranes.
Miyagishima and colleagues began by analyzing the differences in the amino acid sequences of dynamin family members to determine the evolutionary links between them. They found that the dynamins involved in chloroplast division were most closely related to those crucial to whole cell division in plants and algae, and also in amoebas and slime moulds that lack chloroplasts.
The researchers then investigated the functions of three of these closely related dynamin proteins in an organism without chloroplasts, the amoeba Dictyostelium discoideum. They generated non-functional mutants of each of the proteins and found these mutants led to cells with multiple nuclei (Fig. 2). This suggests the cells had duplicated their nuclei ready for division, but had never undergone cytokinesis, when the body of the cell splits into two. Knocking out each of the dynamin proteins in the amoeba had disrupted the process of cytokinesis.
Next, the researchers tracked the synthesis and localization of one of the amoeba dynamin proteins. They found the protein appeared in the cell cycle phase just before cytokinesis, and concentrated along the cleavage furrow that forms when the cells divide. The dynamin was directly involved in cytokinesis.
Again with fluorescent tagging, the team studied a pair of closely related dynamin proteins in the model organism for plant genetics, Arabidopsis thaliana. One of the proteins, DRP5B, had previously been shown to be involved in chloroplast division. Because of its similarity, the other protein, DRP5A, was also assumed to play a role in chloroplast division. An investigation of where DRP5A was synthesized in the plant, however, showed it occurred only where cells were actively dividing in the meristem tissue in the root and shoot tips. There was no necessary association with dividing chloroplasts.
Miyagishima and colleagues also grew mutant plants in which either DRP5A or DRP5B protein did not function. In the former, the size and number of chloroplasts in cells were normal, but the meristem tissues were twisted and disrupted; in the latter, the meristem tissues were normal, but there were fewer and larger chloroplasts in cells. These results, say the researchers, suggest that DRP5A is involved in whole cell division whereas the closely related DRP5B is associated with chloroplast division. Both are likely to have evolved from a common ancestor most likely involved in the cytokinesis of pre-chloroplast eukaryotic cells.
Equal inheritance
This close relationship between the dynamin proteins involved in chloroplast division and in whole cell cytokinesis throws up the possibility of synchronization of the two processes, according to the team. And this could lead to the establishment of permanent chloroplasts, because it provides a mechanism to ensure that where there is only one chloroplast per cell—such as in many algal species—it will divide at the same time as the whole cell, allowing one chloroplast to pass into each of the daughter cells.
Like chloroplasts, mitochondria are thought to have evolved from engulfed primitive bacteria, and like chloroplasts they divide within eukaryotic cells forming protein ring structures in the process. So, as the researchers continue to investigate chloroplast division, they now want to broaden their studies to include mitochondria.
“We want to identify more of the proteins involved in chloroplast division and its regulation,” says Miyagishima. “We are also studying how dynamin proteins are involved in mitochondrial division.”
Reference
1. Miyagishima, S., Kuwayama, H., Urushihara, H. & Nakanishi, H. Evolutionary linkage between eukaryotic cytokinesis and chloroplast division by dynamin proteins. Proceedings of the National Academy of Sciences USA 105, 15202–15207 (2008).
The corresponding author for this highlight is based at the RIKEN Miyagishima Initiative Research Unit
About the Author
Shin-ya Miyagishima
Shin-ya Miyagishima was born in Shizuoka, Japan, in 1975. After graduating with a B.Sc. from Department of Biological Sciences, the University of Tokyo, in 1997, he began studies on the mechanism of mitochondrion and chloroplast division using algae and obtained a PhD in 2002 from the same university. From 2002 to 2003, he continued research as postdoctoral researcher at the University of Tokyo and the Department of Life Science, Rikkyo University. In August 2003, he moved to the Department of Plant Biology, Michigan State University, USA, and continued his research as a postdoctoral researcher, using blue-green algae and terrestrial plants. His research focuses on the mechanism, origin, and evolution of organelle division machinery and evolution of eukaryotic cells. Miyagishima's primary research goal is to better understand how free-living bacterial cells have been enslaved as mitochondria and chloroplasts in eukaryotic cells through the endosymbiotic relationships.
