PROTEIN-TRANSPORT INTO COMPLEX DIATOM PLASTIDS UTILIZES 2 DIFFERENT TARGETING SIGNALS

The plastids found in diatoms and other chromophytic algae are complet ely enclosed by four membranes in contrast to chloroplasts of higher p lants, which are surrounded by only two membranes. The bipartite targe ting sequence of diatom nuclear-encoded plastid proteins contains an e ndoplasmic 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 gamma subunit of the chloropla st 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 th rough endoplasmic reticulum membranes and that there is an additional targeting step which is similar to the import of precursor proteins in to chloroplasts of higher plants and green algae indicating that the t ransit peptide-like domain of the diatom precursor is functionally equ ivalent to the respective targeting signal of higher plants. Our resul ts suggest that the transit peptide depending targeting mechanism in p lastids has apparently remained relatively unchanged over the course o f evolution, with only the peptidase cleavage site significantly modif ied.