Protein targeting into secondary plastids of chlorarachniophytes

Most plastid proteins are encoded by the nuclear genome, and consequently, need to be transported into plastids across multiple envelope membranes. In diverse organisms possessing secondary plastids, nuclear-encoded plastid precursor proteins (preproteins) commonly have an N-terminal extension that consists of an endoplasmic reticulum (ER)-targeting signal peptide and a transit peptide-like sequence (TPL). This bipartite targeting peptide is believed to be necessary for targeting the preproteins into the secondary plastids. Here, we newly demonstrate the function of the bipartite targeting peptides of an algal group, chlorarachniophytes, and characterize the functional domains of the TPL in the precursor of a plastid protein, ATP synthase delta subunit (AtpD), using a GFP as a reporter molecule. We show that the C-terminal portion of the TPL is important for targeting the AtpD preprotein from the ER into the chlorarachniophyte plastids, and several positively charged amino acids in the TPL are also necessary for transporting the preprotein across the 2 innermost plastid membranes. Compared with other groups with secondary plastids, the TPL functional domains of the chlorarachniophytes are unique, which might be caused by independent acquisition of their plastids.