Identification of the Carboxyl-Terminal Processing Protease for the D1 Precursor Protein of the Photosystem II Reaction Center of Spinach

The enzyme involved in the carboxyl-terminal processing of the D1 precursor protein (pD1) of the photosystem II reaction center was purified from extracts of sonicated spinach thylakoids by a method that included chromatography on quaternary aminoethyl anion-exchange, hydroxylapatite, copper-chelating affinity and gel-filtration columns. The enzyme was identified, from its chromatographic behavior, to be a monomeric protein of about 45 kDa. The sequence of the amino-terminal 27 amino acids of this protein was determined directly, which exhibited low but appreciable (37%) homology to that deduced from a gene (ctpA) in Synechocystis sp. PCC 6803 that was proposed recently to encode the processing protease from results of genetic complementation analysis.