Chloroplasts arose from the endosymbiosis of an ancient cyanobacterium by a primitive heterotrophic eukaryote. Among their diverse functions, they are best known for the capture and conversion of light energy into chemical energy through the process of photosynthesis. Therefore, their propagation as plant cells grow and divide is critical. Chloroplasts cannot be made de novo but maintain normal accumulation through the process of binary fission, where a constriction forms at the mid-point and squeezes until separation of two daughter chloroplasts. As chloroplasts are descended from bacteria, many of their division proteins are homologs of bacterial cell division factors.Chloroplast division is driven by the assembly and constriction of multiple ring structures at the chloroplast midpoint. The central structure is the FtsZ ring (Z ring), which likely provides at least some of the constrictive force driving division. Unlike bacterial Z rings, which are homopolymers composed of a single FtsZ, those of plants and green algae are composed of two FtsZ isoforms, FtsZ1 and FtsZ2. FtsZ1 and FtsZ2 are each critical for division and perform distinct functions. However, the functional relationship between the FtsZ families prior to this study was poorly understood. In this work, I present my research analyzing the distinct behaviors and functionality of FtsZ1 and FtsZ2 using Schizosaccharomyces pombe as a heterologous expression system for analyzing the inherent assembly and dynamic properties of chloroplast FtsZs.FtsZ1 and FtsZ2 from Arabidopsis assembled filaments with differing morphologies when expressed alone, and coassembled into FtsZ2-like filaments when coexpressed. FRAP experiments showed that FtsZ1 filaments are more dynamic than FtsZ2 filaments, and that FtsZ1 enhances FtsZ2 turnover in coassembled hetero- polymers. Additionally, GTPase activity is essential for FtsZ2 filament turnover but is not solely responsible for turnover of FtsZ1 filaments. These data suggest that FtsZ2 is the structural determinant of the Z ring while FtsZ1 facilitates Z-ring remodeling.In subsequent studies, I analyzed the conservation of distinct assembly and dynamic properties of FtsZ pairs from phylogenetically diverse photosynthetic organisms and the effect the FtsZ N- and C-termini have on FtsZ1 and FtsZ2 filament assembly and dynamics. FtsZ1/FtsZB and FtsZ2/FtsZA proteins from diverse photosynthetic eukaryotes displayed filament assembly and dynamic properties consistent with those of Arabidopsis FtsZ1 and FtsZ2, respectively, while FtsZ from a cyanobacterium showed filament properties more consistent with those of FtsZ2. In other experiments, FtsZ1 and FtsZ2 proteins lacking their N- and/or C-termini assembled filaments with reduced bundling and turnover properties. These data suggest that the distinct properties of the FtsZ1 and FtsZ2 families are well conserved across the various photosynthetic lineages and that FtsZ2 is likely the direct homolog of the ancient cyanobacterial FtsZ. They also show that the N- and C-termini of FtsZ1 and FtsZ2 promote filament bundling and turnover and also contribute to the distinct behaviors of the chloroplast FtsZ families, while not being solely responsible for them. This work has substantially improved our understanding of FtsZ functionality and provides many hypotheses for testing in future analyses.
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