The Role of the Tic Machinery in Chloroplast Protein Import

Plastids are a heterogeneous family of organelles found ubiquitously in plant cells. Most prominent are the chloroplasts which carry out essential functions such as photosynthesis, the biosynthesis of fatty acids, as well as amino acids. Like mitochondria, chloroplasts are derived from an endosymbiotic event. They are believed to have evolved from an ancient cyanobacterium which has been engulfed by an early eukaryotic mitochondriate ancestor. During evolution, the plastid genome has been greatly reduced, and most of the genes have been transferred to the host nucleus. Consequently, most plastid proteins are translated in the cytosol and must be posttranslationally imported into the organelle. Thus, chloroplast development is dependent on the coordinated expression of both plastid and nuclear-encoded genes. Especially import into the chloroplast, which is mediated by complex molecular machines in the outer (Toc complex) and inner (Tic complex) envelope of chloroplasts, respectively, and assembly into functional complexes have to be tightly regulated. This chapter deals with the translocon of the inner envelope membrane (IEM), the Tic complex. Seven components have been annotated so far to exert a function in protein translocation across the inner envelope. Here, we summarize what is known today about the single Tic constituents, take a glimpse at evolutionary aspects and discuss a possible regulation mechanism of Tic.