Targeting of proteins into and across the thylakoid membrane

The assembly of the photosynthetic apparatus utilizes component proteins that are synthesized by two genomes and then targeted into and across the thylakoid membrane. The emerging picture is one of a remarkably complex system of protein trafficking, in which at least four distinct pathways operate within the chloroplast - two for lumenal proteins and two for integral membrane proteins. Some of the pathways can be traced back to the prokaryotic ancestor of the chloroplast, whereas others appear to have arisen more recently - one in response to the transfer of genes to the plant nucleus and another, possibly, in response to the acquisition of new photosynthetic proteins. Remarkably, proteins in three of these pathways are synthesized with cleavable signal type peptides that are almost identical in overall structure, yet that execute entirely different functions. Recent studies have begun to reconcile the function of these targeting signals with the nature of the protein being targeted.