A protein that plays a key role in the division of chloroplasts in nucleated or eukaryotic plant cells is derived from those involved in the overall separation of dividing cells, Japanese molecular biologists 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 where photosynthesis takes place—can be allocated to both daughter cells.
Chloroplasts are thought to have evolved from blue-green bacteria that were engulfed by an ancient plant cell. After more than a billion years inside cells, the chloroplasts retain genetic material, but not enough to be able to divide without assistance from the cell itself. During chloroplast division, protein-based ring structures form inside and outside its double membrane (Fig. 1). The inner ring is based on the FtsZ protein made in the chloroplast, and the outer ring on a member of the dynamin protein family produced in the body of the cell.
Although earlier work by other researchers has unraveled details of the evolutionary history of the FtsZ protein, little was known of the origin of the dynamin proteins. Yet the role of these cellular proteins is critical to how chloroplasts lost their independence and to how they are regulated by the cell—therefore, their role is critical to photosynthesis. Studies on this topic by researchers from RIKEN’s Advanced Science Institute in Wako and the University of Tsukuba were recently published in the Proceedings of the National Academy of Sciences (1).
Dynamin proteins have been present in all eukaryotic cells since before the acquisition of chloroplasts. Members of the family have specialized to perform fission or fusion functions in specific membranes. The researchers analyzed the differences in the amino acid sequences of dynamin family members to determine the evolutionary links between them. They found those involved in chloroplast division were most closely related to dynamin proteins crucial to cell division in plants, and also in amoebas that lack chloroplasts.
Also, in the model organism for plant genetics, Arabidopsis thaliana, the researchers found two closely related dynamin proteins, one of which plays a role in chloroplast division and the other in general cell division.
“We now want to identify more of the proteins involved in chloroplast division and its regulation,” says first author Shin-ya Miyagishima. “We are also studying mitochondria, which evolved in the same way. Dynamin proteins are involved in their division.”
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