Colletti K S, Tattersall E A, Pyke K A, Froelich J E, Stokes K D, Osteryoung K W
Department of Biochemistry, University of Nevada, Reno, 89557, USA.
Curr Biol. 2000 May 4;10(9):507-16. doi: 10.1016/s0960-9822(00)00466-8.
Chloroplast division in plant cells occurs by binary fission, yielding two daughter plastids of equal size. Previously, we reported that two Arabidopsis homologues of FtsZ, a bacterial protein that forms a cytokinetic ring during cell division, are essential for plastid division in plants, and may be involved in the formation of plastid-dividing rings on both the stromal and cytosolic surfaces of the chloroplast envelope membranes. In bacteria, positioning of the FtsZ ring at the center of the cell is mediated in part by the protein MinD. Here, we identified AtMinD1, an Arabidopsis homologue of MinD, and investigated whether positioning of the plastid-division apparatus at the plastid midpoint might involve a mechanism similar to that in bacteria.
Sequence analysis and in vitro chloroplast import experiments indicated that AtMinD1 contains a transit peptide that targets it to the chloroplast. Transgenic Arabidopsis plants with reduced AtMinD1 expression exhibited variability in chloroplast size and number and asymmetrically constricted chloroplasts, strongly suggesting that the plastid-division machinery is misplaced. Overexpression of AtMinD1 inhibited chloroplast division. These phenotypes resemble those of bacterial mutants with altered minD expression.
Placement of the plastid-division machinery at the organelle midpoint requires a plastid-targeted form of MinD. The results are consistent with a model whereby assembly of the division apparatus is initiated inside the chloroplast by the plastidic form of FtsZ, and suggest that positioning of the cytosolic components of the apparatus is specified by the position of the plastidic components.
植物细胞中的叶绿体分裂通过二分裂进行，产生两个大小相等的子质体。此前，我们报道过，FtsZ（一种在细胞分裂过程中形成细胞分裂环的细菌蛋白）的两个拟南芥同源物对植物中的质体分裂至关重要，并且可能参与叶绿体被膜的基质和胞质表面上质体分裂环的形成。在细菌中，FtsZ环在细胞中心的定位部分由MinD蛋白介导。在此，我们鉴定出了AtMinD1（MinD的拟南芥同源物），并研究了质体分裂装置在质体中点的定位是否可能涉及与细菌中类似的机制。
序列分析和体外叶绿体导入实验表明，AtMinD1含有一个将其靶向叶绿体的转运肽。AtMinD1表达降低的转基因拟南芥植物表现出叶绿体大小和数量的变异性以及叶绿体不对称缢缩，这强烈表明质体分裂机制定位错误。AtMinD1的过表达抑制叶绿体分裂。这些表型类似于MinD表达改变的细菌突变体的表型。
质体分裂机制在细胞器中点的定位需要一种靶向质体的MinD形式。这些结果与一种模型一致，即分裂装置的组装由质体形式的FtsZ在叶绿体内启动，并表明该装置的胞质成分的定位由质体成分的位置决定。
