A thylakoid processing protease is required for complete maturation of the lumen protein plastocyanin

Plastocyanin, a photosynthetic electron carrier functional in the chloroplast lumen, is synthesized in the cytosol as a precursor (preplastocyanin) with an amino-terminal transit sequence¹. This transit peptide contains the information specifying import into and routing within the chloroplasts and is removed in at least two steps². An intermediate is observed in the stroma after the transport of preplastocyanin through the chloroplast envelope; mature plastocyanin is present in the lumen, after transport over the thylakoid membrane. We show here that the stromal processing protease³ is not responsible for both processing events. It cleaves the precursor protein only to the intermediate size and a novel protease located in the thylakoids processes this intermediate to the mature protein. This second protease recognizes the processing intermediate but not the precursor. Thus plastocyanin import involves cleavage at the intermediate processing site mediated by the stromal protease and then cleavage at the mature processing site mediated by the thylakoid protease.