Chloroplast quest: a journey from the cytosol into the chloroplast and beyond.

Chloroplasts are characteristic organelles of plants and algae and the site of oxygenic photosynthesis. They are surrounded by a double membrane and possess an internal membrane system, the thylakoids, on which the photosynthetic machinery is located. They originated more than 1.2 billion years ago from an endosymbiotic event between an already photosynthetic ancestor of present day cyanobacteria and a mitochondriate host cell. During the transformation of the internalized cyanobacterium into a cell organelle most of the genetic information of the endosymbiot got lost or was transferred into the nucleus of the host. Chloroplast proteins encoded by nuclear genes are synthesized on cytoplasmic ribosomes and have to be relocated into the organelle. This is achieved by a proteinaceous import machinery in the outer and inner envelope of the chloroplasts. Proteins destined for the thylakoid membrane and the thylakoid lumen are further translocated by several different pathways into or across this membrane. The subject of this review is the quest of nuclear encoded chloroplast proteins into the organelle and to their final suborganellar location.