Identification and characterization of a new conserved motif within the presequence of proteins targeted into complex diatom plastids

Several groups of algae evolved by secondary endocytobiosis, which is defined as the uptake of a eukaryotic alga into a eukaryotic host cell and the subsequent transformation of the endosymbiont into an organelle. Due to this explicit evolutionary history such algae possess plastids that are surrounded by either three or four membranes. Protein targeting into plastids of these organisms depends on N-terminal bipartite presequences consisting of a signal and a transit peptide domain. This suggests that different protein targeting systems may have been combined during establishment of secondary endocytobiosis to enable the transport of proteins into the plastids. Here we demonstrate the presence of an apparently new type of transport into diatom plastids. We analyzed protein targeting into the plastids of diatoms and identified a conserved amino acid sequence motif within plastid preprotein targeting sequences. We expressed several diatom plastid presequence:GFP fusion proteins with or without modifications within that motif in the diatom Phaeodactylum tricornutum and found that a single conserved phenylalanine is crucial for protein transport into the diatom plastids in vivo, thus indicating the presence of a so far unknown new type of targeting signal. We also provide experimental data about the minimal requirements of a diatom plastid targeting presequence and demonstrate that the signal peptides of plastid preproteins and of endoplasmic reticulum-targeted preproteins in diatoms are functionally equivalent. Furthermore we show that treatment of the cells with Brefeldin A arrests protein transport into the diatom plastids suggesting that a vesicular transport step within the plastid membranes may occur.