Plastid ultrastructure defines the protein import pathway in dinoflagellates

Eukaryotic cells contain a variety of different compartments that are distinguished by their own particular function and characteristic set of proteins. Protein targeting mechanisms to organelles have an additional layer of complexity in algae, where plastids may be surrounded by three or four membranes instead of two as in higher plants. The mechanism of protein import into dinoflagellates plastids, however, has not been previously described despite the importance of plastid targeting in a group of algae responsible for roughly half the ocean's net primary production. Here, we show how nuclear-encoded proteins enter the triple membrane-bound plastids of the dinoglafellate Gonyaulax. These proteins all contain an N-terminal leader sequence with two distinct hydrophobic regions flanking a region rich in hydroxylated amino acids (S/T). We demonstrate that plastid proteins transit through the Golgi in vivo, that the first hydrophobic region in the leader acts as a typical signal peptide in vitro, and that the S/T-rich region acts as a typical plastid transit sequence in transgenic plants. We also show that the second hydrophobic region acts as a stop transfer sequence so that plastid proteins in Golgi-derived vesicles are integral membrane proteins with a predominant cytoplasmic component. The dinoflagellate mechanism is thus different from that used by the phylogenetically related apicomplexans, and instead, is similar to that of the phylogenetically distant Euglena, whose plastids are also bound by three membranes. We conclude that the protein import mechanism is dictated by plastid ultrastructure rather than by the evolutionary history of the cell.