Protein Transport into “Complex” Diatom Plastids Utilizes Two Different Targeting Signals

Abstract

The plastids found in diatoms and other chromophytic algae are completely enclosed by four membranes in contrast to chloroplasts of higher plants, which are surrounded by only two membranes. The bipartite targeting sequence of diatom nuclear-encoded plastid proteins contains an endoplasmic reticulum signal sequence and, based on sequence comparison, a transit peptide-like domain similar to that which targets proteins into the plastids of higher plants. By performing heterologous import experiments using the precursor of the ␥ subunit of the chloroplast ATPase from the diatom Odontella sinensis we were able to show that protein import into diatom plastids is at least a two-step event. We demonstrate that the first step involves co-translational transport through endoplasmic reticulum membranes and that there is an additional targeting step which is similar to the import of precursor proteins into chloroplasts of higher plants and green algae indicating that the transit peptide-like domain of the diatom precursor is functionally equivalent to the respective targeting signal of higher plants. Our results suggest that the transit peptide depending targeting mechanism in plastids has apparently remained relatively unchanged over the course of evolution, with only the peptidase cleavage site significantly modified.

Subcellular localization of acetyl-CoA carboxylase in the apicomplexan parasite Toxoplasma gondii

“…A distinct bipartite signal, identified at the N terminus of all apicoplast-targeted proteins to date, is sufficient to transport reporter proteins to the apicoplast (1,5,6). This transport is similar to the plastid-targeting mechanism found for nuclear-encoded proteins imported into three-or fourmembrane plastids in other species (7)(8)(9). In plants, protein import into plastids is mediated by a 25-to 125-aa N-terminal transit peptide with no clear sequence consensus among the many proteins imported.…”

Mining thePlasmodiumgenome database to define organellar function: what does the apicoplast do?

“…This second domain then mediates translocation across the remaining membranes into the apicoplast lumen (where processing to the mature protein occurs (He et al 2001b). Similar pathways have been proposed for targeting to other secondary endosymbiotic plastids (Apt et al 1995;Bodyl 1997;Lang et al 1998;Schwartzbach et al 1998), although the lack of transfection systems for these algae precludes direct experimental confirmation. Further cut-and-paste molecular genetic experiments indicate that it may be possible to distinguish subdomains within the plastid transit peptide, dissociating microdomains involved in targeting to the apicoplast, membrane translocation, and protein processing.…”

Second- and third-hand chloroplasts in dinoflagellates: Phylogeny of oxygen-evolving enhancer 1 (PsbO) protein reveals replacement of a nuclear-encoded plastid gene by that of a haptophyte tertiary endosymbiont

“…28; http:͞͞www.cbs.dtu.dk͞ser vices͞SignalP͞). The stromatargeting domain (STD) was predicted to be the region between the ER-and TTDs, taking into consideration the hydropathy plot, the few known cleavage sites for heterokont proteins (29), and the results from the CHLOROP program (ref. 30; http:͞͞ www.cbs.dtu.dk͞services͞ChloroP).…”

“…The STDs of Chl-c-containing algae are not well characterized and do not seem to be processed by higher plant-processing peptidases (ref. 29 and B. K. Chaal and B.R.G., unpublished data). Processing sites predicted by the CHLOROP program (30), which is trained on green plant sequences, may therefore not be valid.…”