Recognition signal for the C-terminal processing protease of D1 precursor protein in the photosystem II reaction center An analysis using synthetic oligopeptides

Synthetic oligopeptides of different chain lengths of 11 to 38 amino acids, corresponding to the carboxyl-terminal sequence of D1 precursor protein of the photosystem II reaction center, were subjected to a proteolytic cleavage by a processing enzyme isolated from spinach, in order to analyze the recognition signal. Practically the same K_m and V_max values were obtained for the oligopeptides consisting of more than 19 amino acids; a decrease in affinity, without affecting the V_max value, was observed for the peptide consisting of 16 amino acids; no detectable activity was found for the peptide with 11 amino acids. When Asp-342 (12th residue from C-terminus) was replaced with Asn, for the peptide consisting of 16 amino acids, the enzymatic activity was completely abolished. In contrast, replacing Asp-342 with Glu had little effect. The efficiency of these oligopeptides as a substrate mentioned above, together with their effectiveness as an inhibitor, clearly demonstrated that the negative charge on Asp-342 plays a crucial role in the recognition, i.e. binding and cleavage, of the substrate by the processing enzyme, and suggested that the carboxyl-terminal extension consisting of 9 amino acids, by itself is not important in the binding.