A thylakoidal processing peptidase from the heterokont alga Heterosigma akashiwo

Heterokont algae such as diatoms, brown seaweeds and the raphidophyte Heterosigma akashiwo acquired their chloroplasts via a secondary endosymbiosis involving a red algal endosymbiont and a eukaryote host, resulting in chloroplasts surrounded by four membranes rather than two. The precursor of a nuclear-encoded thylakoid lumen protein, PsbO, from Heterosigma has a presequence composed of a typical ER signal peptide followed by putative stromal and thylakoid targeting domains. A processing enzyme associated with Heterosigma thylakoids cleaved the presequence (with or without the ER signal sequence) in a single step, giving a product of the size of the mature protein. Its sensitivity to a penem inhibitor and insensitivity to other protease inhibitors suggest that it is a member of the Type I signal peptidase family. Furthermore the Heterosigma enzyme appeared to have similar substrate specificity to the pea thylakoidal processing peptidase.