Import of Plastid Precursor Proteins Into Pea Chloroplasts

Plastids are a heterogeneous family of organelles found ubiquitously in plant and algal cells (1). Most prominent are the chloroplasts, which carry out such essential processes as photosynthesis and the biosynthesis of fatty acids as well as of amino acids. As mitochondria, chloroplasts derived from a single endosymbiotic event (2). They are believed to have evolved from an ancient cyanobacterium, which had been engulfed by an early eukaryotic ancestor. During evolution the plastid genome has been greatly reduced, and most of the genes have been transferred to the host nucleus. Consequently, more than 98% of all plastid proteins are translated onto cytosolic ribosomes. They have to be posttranslationally targeted to and imported into the organelle. Targeting is assisted by cytosolic proteins, which interact with proteins destined for plastids and thereby keep them in an import-competent state. After reaching the target organelle, many proteins have to conquer the barrier of the chloroplast outer and inner envelopes. This process is mediated by complex molecular machines in the outer (Toc complex) and inner (Tic complex) envelope of chloroplasts, respectively (3). Most proteins destined for compartments inside the chloroplast contain a cleavable N-terminal transit peptide (4), whereas most of the outer envelope components insert into the membrane without such a targeting peptide (5).