Double-membrane-bounded organelles: Recent findings regarding division, inheritance, structure, and evolution of the nucleus, mitochondria, and chloroplasts

The nucleus of Cyanidioschyzon merolae has quite a limited set of components. The core and linker histones, H2A.Z, and a centromeric histone variant CenH3 are preserved, but H2A variants including H2A.X, H2A.M, and H2A.W are absent. Some proteins of the nuclear pore complex and nucleoli are also preserved, but almost all homologs of nuclear lamina and nuclear envelope proteins are lacking. The chloroplast of this species is divided by a ring, which is composed of two inner rings, the FtsZ ring and inner plastid-dividing (PD) ring, and two outer rings, the outer PD ring and dynamin ring. Identification of the PD ring components in C. merolae enabled us to elucidate the molecular mechanism behind the PD machinery. For example, a glycosyltransferase protein, plastid-dividing ring 1, acts in establishing the PD ring by forming ring-shaped polyglucan filaments. The division of mitochondria is performed by the mitochondrion-dividing (MD), FtsZ, and dynamin rings. Mda1 functions as a mediator for stably linking a dynamin. In addition, an FtsZ-associated protein, ZED, is involved in completing the inner rings of the MD machinery. Analyses of the mechanisms of mitochondrial and chloroplast division in C. merolae contribute to our understanding of the endosymbiotic process of double-membrane-bounded organelles.