Novel mechanisms for the targeting of proteins into and across chloroplastmembranes

The assembly of the photosynthetic apparatus requires the translocation of numerous proteins from the cytosol, initially into the stroma and thereafte r into or across the thylakoid membrane. Recent studies have shown that pro teins are transported into this membrane by a variety of mechanisms, some o f which are derived from a cyanobacterial-type ancestor, whereas others hav e evolved in response to the more complex transport pathway used by cytosol ically synthesized chloroplast proteins. It is now apparent that some of th e targeting pathways are used exclusively by hydrophobic thylakoid membrane proteins; here we review recent progress in our understanding of the bioge nesis of this important class of protein.