Proteins involved in maturation pathways of plant mitochondrial and plastid c-type cytochromes.

c-type cytochromes are characterized by the presence of two covalent bonds linking heme to apocytochrome and by the heme attachment motif in the apoprotein. Several molecular systems for the maturation of c-type cytochromes have evolved in different organisms. The best characterized are three of them: system I, system II and system III. Heme is synthesized in bacterial cytoplasm, in plastids, and in animal and fungal mitochondria. Therefore the maturation of bacterial and plastid c-type cytochromes involves the transport of heme and apocytochrome from the n-side to the p-side of the respective biological membranes and the formation of the covalent bond at the p-side. It should be underlined that the site of the c-type apocytochrome synthesis is also distinct from the site of its functioning. The aim of this review is to present the current state of knowledge concerning the structure and function of two systems - system I and system II - in the maturation of plant mitochondrial and plastid c-type cytochromes, respectively.