Dynamin: the endosymbiosis ring of power?

O ne thing plant cells can’t afford to get wrong is chloroplast division. The successful splitting and passing on of chloroplasts to daughter cells is vital to plant cell survival. Indeed, plants (and their progenitors the green algae) have been dividing and passing down their endosymbiotic chloroplasts successfully from generation to generation since the establishment of the chloroplast many hundred of millions of years ago (1). Three new papers now open a new chapter in our understanding of this division (2–4). In this issue of PNAS, Hongbo Gao and colleagues from Katherine Osteryoung’s laboratory describe a plant dynamin with a pivotal role in chloroplast division (2). Dyamins are mechanochemical proteins with a range of roles including the pinching off of vesicles (5). Previous work, much of it also from the lab of Osteryoung, had already demonstrated that chloroplasts retain a lot of the division machinery they originally possessed as free-living cyanobacteria. Key bacterial division proteins such as FtsZ, MinE, and MinD also play vital roles in the fission of chloroplasts (6–9). Indeed, bacterial division has served as an excellent model for chloroplast division (10), but this new work forces us to add new components to the model, components derived from the host. Chloroplasts are no longer free-living, and clearly the host cell controls when fission occurs. A key factor in this control is the fact that the host has confiscated many of the endosymbiont’s genes for prokaryotic division proteins. Thus, FtsZ in plant cells is encoded by the nucleus, synthesised on cytoplasmic ribosomes then dispatched into chloroplasts. But, as Gao et al. show, the host exerts control over chloroplast division in other ways. In addition to holding many of the genes for the machinery that orchestrates division from within the chloroplast, it has now been shown that the host has added external machinery in the form of a dynamin-like protein known as ARC5 (2). arc5 (accumulation and replication of chloroplasts) is one of a panel of chloroplast division mutants generated by Kevin Pyke (11). Chloroplasts of arc5 plants initiate the typical mid-region constriction but often fail to complete the pinching in two, resulting in populations of dumbbell-shaped chloroplasts. Cells of arc5 plants have between 3 and 15 of these half-divided chloroplasts, whereas a normal wild-type plant cell contains as many as 120 chloroplasts, most of which are spherical to ovoid (11). Gao et al. used a clever positional cloning strategy to recover the gene responsible (2). Having tracked the arc5 locus to a single fragment of Arabidopsis genomic DNA cloned into a bacterial artificial chromosome, Gao et al. then cut the bacterial artificial chromosome into pieces by using restriction enzymes. Ordinarily, one would try to complement the mutant by introducing these