Protein targeting to the chloroplasts of photosynthetic eukaryotes: getting there is half the fun

The plastids of many algae are surrounded by three or four membranes, thought to be a consequence of their evolutionary origin through secondary endosymbiosis between photosynthetic and non-photosynthetic eukaryotes. Each membrane constitutes a barrier to the passage of proteins, so protein targeting in these complex plastids has an extra level of difficulty when compared to higher plants. In the latter, protein translocation across the two membranes uses multi-protein complexes that together import proteins possessing an N-terminal leader sequence rich in serine and threonine (S/T). In contrast, while targeting to most complex plastids also involves an S/T-rich region, this region is preceded by an N-terminal hydrophobic signal peptide. This arrangement of peptide sequences suggests that proteins directed to complex plastids pass through the ER, as do other proteins with hydrophobic signal peptides. However, this simplistic view is not always easy to reconcile with what is known about the different secondary plastids. In the first group, with plastids bounded by three membranes, plastid-directed proteins do indeed arrive in Golgi-derived vesicles, but a second hydrophobic region follows the S/T-rich region in all leaders. In the second group, where four membranes completely surround the plastids, it is still not known how the proteins arrive at the plastids, and in addition, one member of this group uses a targeting signal rich in asparagine and lysine in place of the S/T-rich region. In the third group, the fourth bounding membrane is contiguous with the ER, but it is not clear what distinguishes plastid membranes from others in the endomembrane system. Knowing what to expect is important, as genomic sequencing programs may soon be turning up some of the missing pieces in these translocation puzzles.