A Chloroplast Protein Homologous to the Eubacterial Topological Specificity Factor MinE Plays a Role in Chloroplast Division

Abstract

We report the identification of a nucleus-encoded minE gene, designated AtMinE1, of Arabidopsis. The encoded AtMinE1 protein possesses both N-and C-terminal extensions, relative to the eubacterial and algal chloroplast-encoded MinE proteins. The N-terminal extension functioned as a chloroplast-targeting transit peptide, as revealed by a transient expression assay using an N terminus:green fluorescent protein fusion. Histochemical ␤-glucuronidase staining of transgenic Arabidopsis lines harboring an AtMinE1 promoter::uidA reporter fusion unveiled specific activation of the promoter in green tissues, especially at the shoot apex, which suggests a requirement for cell division-associated AtMinE1 expression for proplastid division in green tissues. In addition, we generated transgenic plants overexpressing a full-length AtMinE1 cDNA and examined the subcellular structures of those plants. Giant heteromorphic chloroplasts were observed in transgenic plants, with a reduced number per cell, whereas mitochondrial morphology remained similar to that of wild-type plants. Taken together, these observations suggest that MinE is the third conserved component involved in chloroplast division.Chloroplast division is one of the most critical cellular processes in plants because the plant cell is unable to synthesize this organelle de novo (Possingham and Lawrence, 1983). A series of nuclear recessive mutants of the higher plant Arabidopsis, in which the chloroplast number per cell is greater or fewer than that in the wild-type plant, have been well documented (Pyke and Leech, 1991, 1994 Pyke et al., 1994; Robertson et al., 1996;Marrison et al., 1999). Characterization of these mutants, referred to as accumulation and replication of chloroplasts (arc) mutants, indicates that chloroplast division is a complex process that involves multiple distinct steps, such as expansion, division site selection, division initiation, constriction, and scission of chloroplasts, which are controlled by different nuclear genes, although no ARC genes have yet been cloned. Intensive microscopic studies of the division process revealed that, upon chloroplast division, two concentric rings (plastid-dividing [PD] rings) appear on opposite sides of the chloroplast envelope at the constricted isthmus (Hashimoto, 1986;Kuroiwa et al., 1998). Recently, the outer cytosolic PD ring of the red alga Cyanidioschyzon merolae was observed as a bundle of novel 5-nm filaments, implying the innovation of eukaryote-specific organelle division machinery (Miyagishima et al., 2001). In addition, recent studies of transgenic land plants have shown that the same division machinery is conserved in chloroplasts and eubacterial cells. Strepp et al. (1998) and Osteryoung et al. (1998) demonstrated that a homolog(s) of the bacterial cell division protein FtsZ is required for chloroplast division, by generating knockout plants of the moss Physcomitrella patens or antisense transgenics of Arabidopsis, respectively. In bacteria, FtsZ is a cytoplasmic, tubulin-related GTPase, whi...

Reduced Expression of the Arabidopsis minE Gene Affects Size and Number of Chloroplasts.

“…Consequently, constriction is confined to the midcell in the presence of MinC, MinD, and MinE (de Boer et al 1989). Earlier work demonstrated that AtMinE 1 targeted chloroplasts, and that overexpression of the AtMinEl gene reduced chloroplast number, but resulted in chloroplast enlargement (Itoh et al 2001). These results demonstrated the critical role of AtMinEl in chloroplast division.…”

“…This fragment was digested with BamHI and SacI (sites indicated by underlines) and ligated in the antisense orientation into the same restriction sites of pBI121 (TOYOBO, Tokyo, Japan). This plasmid was used for stable transformation of Arabidopsis with Agrobacterium tumefaciens as described previously (Itoh et al 2001). …”

“…Complete genome sequencing of Arabidopsis thaliana (Arabidopsis Genome Initiative 2000) revealed that this species possesses 5 nuclear-encoded components for the conservative chloroplast division system: 3 ftsZ homologues, AtFtsZ1-1, AtFtsZ2-1, and AtFtsZ2-2 (Osteryoung et al 1998); one minD homologue, AtMinD1 (Colletti et al 2000, Kanamaru et al 2000; and one minE homologue, AtMinEl (Itoh et al 2001). MinE is a component of the eubacterial septum sitedetermining Min system, which serves to prevent the MinCD division inhibitory complex from blocking division at the proper midcell site, while permitting MinCD to prevent division at other potential division sites.…”

FtsZ and the division of prokaryotic cells and organelles

“…An additional homologue of FtsZ that is distinct from FtsZ1 and FtsZ2, but is still related to cyanobacterial FtsZs, is also present in plastids of primitive algae. Homologues of cyanobacterial MinD and MinE proteins are found in plants, and are required for the proper localization of plastid Z rings in Arabidopsis thaliana 108,109 , which indicates that the function of MinD and MinE in the spatial regulation of plastid Z rings has been conserved. Nevertheless, no homologues of MinC have been found in plants, so the factor that negatively regulates FtsZ assembly through MinD in plastids is not yet known.…”

An Arabidopsis Homolog of the Bacterial Cell Division Inhibitor SulA Is Involved in Plastid Division[W]

“…The first step seems to be the polymerization of AtFtsZ at the division site, forming the Z-ring , probably stabilized by ARC6, a DnaJ domain protein (Vitha et al, 2003). Like in bacteria (Justice et al, 2000), establishment of the proper division site is mediated by AtMinD and AtMinE (Colletti et al, 2000;Itoh et al, 2001). Then, sequential assembly of the inner and outer plastid division rings (PD ring) follows.…”