Poster: Protein Targeting & Vesicular Trafficking

Abs # 1193: Protein targeting to “complex” plastids acquired by secondary endosymbiogenesis

Presenter:	Chaal, Balbir K, chaal@mail.botany.ubc.ca
Authors	Chaal, Balbir K (A)   Ishida, Ken-ichiro  (B)   Green, Beverley R (A)
Affiliations:	(A): University of British Columbia (B): Kanazawa University

A number of algal groups, such as brown algae, diatoms, and dinoflagellates, are believed to have obtained their chloroplasts by secondary endosymbiosis, where a non-photosynthetic eukaryote engulfed a red algal endosymbiont and retained its chloroplast. As a result, nuclear-encoded proteins have to cross two additional membranes to be imported into the chloroplast. The outermost membrane of the chloroplast is continuous with ER membranes, indicating that the chloroplast lies within the ER lumen. To understand some of the steps in protein import, we are studying processing of precursor proteins using the model heterokont chlorophyll a/c alga Heterosigma akashiwo, a unicellular, wall-less marine alga. Precursors of the nuclear-encoded thylakoid lumen protein, PsbO, from Heterosigma and other chlorophyll a/c algae possess a presequence composed of a typical ER signal peptide followed by putative stromal and thylakoid targeting domains. A processing enzyme associated with Heterosigma thylakoids cleaves the presequence in a single step, giving a product of the size of the mature protein. Its sensitivity to a penem inhibitor suggests that it is a member of the Type I signal peptidase family, and it appears to have similar substrate specificity to the plant thylakoidal processing peptidase. We were also able to detect a stromal processing peptidase activity, which cleaves off the stromal targeting domain of the PsbO precursor without the ER signal peptide. However, comparison of its inhibitor and processing specificities revealed that it has a different reaction and substrate specificity to the plant stromal processing peptidase.