Novel Dynamics of FtsZ Ring Before Plastid Abscission

Evolutionary significance of the ring-like plastid nucleus in the primitive red alga Cyanidioschyzon merolae as revealed by drying

Primary plastids originated from a free-living cyanobacterial ancestor and possess their own genomes-probably a few DNA copies. These genomes, which are organized in centrally located plastid nuclei (CN-type pt-nuclei), are produced from preexisting plastids by binary division. Ancestral algae with a CN-type pt-nucleus diverged and evolved into two basal eukaryotic lineages: red algae with circular (CL-type) pt-nuclei and green algae with scattered small (SN-type) pt-nuclei. Although the molecular dynamics of pt-nuclei in green algae and plants are now being analyzed, the process of the conversion of the original algae with a CN-type pt-nucleus to red algae with a CL-type one has not been studied. Here, we show that the CN-type pt-nucleus in the primitive red alga Cyanidioschyzon merolae can be changed to the CL-type by application of drying to produce slight cell swelling. This result implies that CN-type pt-nuclei are produced by compact packing of CL-type ones, which suggests that a C. merolae-like alga was the original progenitor of the red algal lineage. We also observed that the CL-type pt-nucleus has a chain-linked bead-like structure. Each bead is most likely a small unit of DNA, similar to CL-type pt-nuclei in brown algae. Our results thus suggest a C. merolae-like alga as the candidate for the secondary endosymbiont of brown algae.

The bacterial ZapA-like protein ZED is required for mitochondrial division

Bacterial cell division systems that include FtsZ are found throughout prokaryotes. Mitochondria arose from an endosymbiotic alpha-proteobacterial ancestor and proliferate by division. However, how the mitochondrial division system was established from bacterial division is not clear. Here, we have isolated intact mitochondrial division (MD) machineries from the primitive red alga Cyanidioschyzon merolae and identified a bacterial ZapA-like protein, ZED, that constricts the basal structure of MD machinery with FtsZ. ZED contains a predicted mitochondrial transit signal and two coiled-coil regions and has partial homology with the bacterial division protein ZapA. Cytological studies revealed that ZED accumulates to form a ring structure that colocalizes with FtsZ beneath the inner membrane. ZED proteins are expressed just before mitochondrial division. The short-form ZED (S-ZED) then appears at the mitochondrial constriction phase. Protein-protein interaction analysis and transient expression of antisense against ZED showed that S-ZED interacts with FtsZ1 to constitute the basal structure of the MD machinery and is required for mitochondrial division. We also demonstrate compelling functional similarity between bacterial ZapA and mitochondrial ZED, suggesting that the bacterial cell division system was incorporated into the MD machinery with remodeling of bacterial division proteins during evolution.