The assembly of chloroplast membranes

During the last five or six years there has been a shift in focus in the field of chloroplast protein transport, with greater emphasis being placed on understanding the sorting of proteins to the thylakoids and the envelope membranes. As a result, we have a much-improved understanding of the variety of important pathways that function during chloroplast biogenesis. It is now clear that a considerable number of distinct intraorganellar sorting mechanisms operate to direct imported proteins to their correct destinations. Some of the underlying mechanisms are also beginning to emerge, although it is accurate to say that we are still a long way from understanding in genuine detail how proteins are translocated across chloroplast membranes. However, the availability of such a range of efficient in vitro import assays should ensure that rapid progress is made in coming years. The major gaps in this field now concern the identities and roles of the elements of the important apparatus: Although at least two distinct translocation systems operate during chloroplast biogenesis, none of these components has been identified, purified, or cloned. This is primarily because these proteins are often difficult to assay individually, and they are usually of very low abundance. Nevertheless, it is essential that progress is made in this area soon in order to maintain the present momentum.