Maturation of thylakoid lumen proteins proceeds post-translationally through an intermediate in vivo

Many thylakoid lumen proteins are synthesized outside the chloroplast as larger molecular weight precursors and then processed to their mature size during transport to the lumenal space. We have examined the post-translational processing of thylakoid lumen proteins in vivo by pulse-radiolabeling experiments with Chlamydomonas reinhardtii. Antibodies against the lumenal protein cytochrome c6 specifically immunoprecipitated three polypeptides from extracts of briefly pulse-radiolabeled cells. The molecular weights and kinetics of synthesis and turnover indicate that these three polypeptides are (i) the full-length cytochrome c6 precursor, (ii) a partially processed precursor (intermediate), and (iii) the completely processed mature protein. Identification of analogous forms of two other lumenal proteins, plastocyanin and the oxygen evolving enhancer 1 protein, indicates that the maturation of thylakoid lumen proteins occurs post-translationally in vivo and that the partially processed intermediate is a general feature of the pathway. The intermediate form of cytochrome c6 accumulated to a greater extent in cells incubated at 10 degrees C, relative to cells incubated at 22 degrees C, concomitantly with a decrease in the accumulation of the mature protein. The intermediate accumulating at 10 degrees C is quantitatively converted to the mature protein upon incubation at higher temperature, thus demonstrating a precursor-product relationship between the intermediate and mature forms of cytochrome c6. Our results prove the model [Smeekens, S., Bauerle, C., Hageman, J., Keegstra, K. & Weisbeek, P. (1986) Cell 46, 365-375] that precursors of lumenal proteins are post-translationally converted to their mature forms in two steps through a distinct intermediate.