ENDOMEMBRANE ULTRASTRUCTURE AND POSSIBLE CHLOROPLAST PROTEIN IMPORT PATHWAY IN HETEROSIGMA AKASHIWO (RAPHIDOPHYCEAE)

Chloroplasts in heterokont algae probably originated from a red algal endosymbiont which was engulfed and retained by a eukaryotic host, and are surrounded by four envelope membranes. The outermost of these membranes is called chloroplast ER (CER) and usually connects with the nuclear envelope. This information, however, is based mainly on studies on single‐plastid heterokont algae. In multi‐plastid heterokont algae, it is still unclear whether CER is continuous with the nuclear envelope. Since nuclear‐encoded chloroplast proteins are synthesized by ribosomes on the ER membrane, clarifying the ER‐CER structure in the heterokont algae is important in order to know the targeting pathway of those proteins. We did a detailed ultrastructural observation of endomembrane systems in a multi‐plastid heterokont alga: Heterosigma akashiwo , and confirmed that the CER membrane was continuous with the ER membrane. However, unlike the CER membranes in other heterokont algae, it seemed to have very few ribosome attached. We also performed experiments for protein targeting into canine microsomes using a precursor for a nuclear‐encoded chloroplast protein, a fucoxanthin‐chlorophyll protein (FCP), of H. akashiwo , to see if the protein is targeted to the ER. It demonstrated that the precursor has a functional signal sequence for ER targeting, and is co‐translationally translocated into the microsomes. Based on these data, we propose a hypothesis that, in H. akashiwo , nuclear‐encoded chloroplast protein precursors that have been co‐translationally inserted into the ER lumen are sorted in the ER and transported to the chloroplasts through the ER.