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Chloroplast division is driven by a macromolecular complex containing components that are positioned on the cytosolic surface of the outer envelope, the stromal surface of the inner envelope, and in the intermembrane space. The only constituents of the division apparatus identified thus far are the tubulin-like proteins FtsZ1 and FtsZ2, which colocalize to rings at the plastid division site. However, the precise positioning of these rings relative to the envelope membranes and to each other has not been previously defined. Using newly isolated cDNAs with open reading frames longer than those reported previously, we demonstrate here that both FtsZ2 proteins in Arabidopsis, like FtsZ1 proteins, contain cleavable transit peptides that target them across the outer envelope membrane. To determine their topological arrangement, protease protection experiments designed to distinguish between stromal and intermembrane space localization were performed on both in vitro imported and endogenous forms of FtsZ1 and FtsZ2. Both proteins were shown to reside in the stromal compartment of the chloroplast, indicating that the FtsZ1- and FtsZ2-containing rings have similar topologies and may physically interact. Consistent with this hypothesis, double immunofluorescence labeling of various plastid division mutants revealed precise colocalization of FtsZ1 and FtsZ2, even when their levels and assembly patterns were perturbed. Overexpression of FtsZ2 in transgenic Arabidopsis inhibited plastid division in a dose-dependent manner, suggesting that the stoichiometry between FtsZ1 and FtsZ2 is an important aspect of their function. These studies raise new questions concerning the functional and evolutionary significance of two distinct but colocalized forms of FtsZ in plants and establish a revised framework within which to understand the molecular architecture of the plastid division apparatus in higher plants.

In higher plants, two nuclear gene families, FtsZ1 and FtsZ2, encode homologs of the bacterial protein FtsZ, a key component of the prokaryotic cell division machinery. We previously demonstrated that members of both gene families are essential for plastid division, but are functionally distinct. To further explore differences between FtsZ1 and FtsZ2 proteins we investigated the phenotypes of transgenic plants overexpressing AtFtsZ1-1 or AtFtsZ2-1, Arabidopsis members of the FtsZ1 and FtsZ2 families, respectively. Increasing the level of AtFtsZ1-1 protein as little as 3-fold inhibited chloroplast division. Plants with the most severe plastid division defects had 13- to 26-fold increases in AtFtsZ1-1 levels over wild type, and some of these also exhibited a novel chloroplast morphology. Quantitative immunoblotting revealed a correlation between the degree of plastid division inhibition and the extent to which the AtFtsZ1-1 protein level was elevated. In contrast, expression of an AtFtsZ2-1 sense transgene had no obvious effect on plastid division or morphology, though AtFtsZ2-1 protein levels were elevated only slightly over wild-type levels. This may indicate that AtFtsZ2-1 accumulation is more tightly regulated than that of AtFtsZ1-1. Plants expressing the AtFtsZ2-1 transgene did accumulate a form of the protein smaller than those detected in wild-type plants. AtFtsZ2-1 levels were unaffected by increased or decreased accumulation of AtFtsZ1-1 and vice versa, suggesting that the levels of these two plastid division proteins are regulated independently. Taken together, our results provide additional evidence for the functional divergence of the FtsZ1 and FtsZ2 plant gene families.