Translocation of Proteins into Complex Plastids with Three Envelope Membranes

The unrelated euglenophytes and peridinin dinoflagellates have secondary plastids with three envelope membranes. All other secondary plastids have four envelope membranes including Apicomplexans, which are close relatives of dinoflagellates. The endosymbiont that gave rise to the plastid in Euglena was a green alga while the peridinin dinoflagellate endosymbiont was an alga with a red alga-derived plastid. Despite this, the euglenophytes and dinoflagellates share a number of unique features that are not present in organisms with four envelope membrane complex plastids. Proteins are targeted to all complex plastids regardless of the number of envelope membranes by a bipartite presequence composed of a signal peptide domain targeting the precursor to an ER-like compartment and a chloroplast targeting transit peptide domain. Unique to some Euglena and dinoflagellate bipartite targeting sequences is a hydrophobic domain, the stop-transfer membrane anchor sequence, which stops translocation into the ER lumen anchoring the protein in the ER membrane with the N-terminal transit peptide in the ER lumen and most of the protein in the cytoplasm. In vivo and in vitro studies have shown that Euglena and dinoflagellateDinoflagellates plastidPlastid proteins are transported in vesiclesVesicles from the ER to the Golgi apparatus to the plastid. Upon vesicleVesicles fusion with the outer envelope membrane, proteins with a presequence containing a stop-transfer membrane anchor sequence become integral envelope membrane proteins, while those without a stop-transfer membrane anchor sequence are soluble proteins in the intermembrane space between the outer and middle envelope membrane. During protein transit through the ER and Golgi apparatus proteins fold, they can be glycosylated, and disulfide bonds are formed. Import of these post-translationally modified proteins through the middle and inner envelope membrane requires translocationTranslocation machinery that can transport bulky glycosylated or folded proteins, remove integral membrane proteins from the outer envelope membrane, and translocate them and intermembrane soluble proteins across the middle and inner envelope membranes into the stroma. It is unlikely that the typical translocons at the outer envelope membrane of chloroplasts (TOC) and at the inner envelope membrane of chloroplasts (TIC) can perform these functions. Genome, transcriptome, and proteome studies have not found homologues of these translocons in Euglena plastids. In this review, we will discuss the similarities between the unrelated euglenophyte and peridinin dinoflagellateDinoflagellates plastid protein presequences, plastid protein importProtein import pathways, and suggest selection pressures that may have led to modifications of the plastid glycoproteinGlycoproteins import pathway rather than the conventional TOC/TICTOC/TIC pathway for translocationTranslocation of proteins into their complex plastidsComplex plastids.