The 60-kDa precursor to the dithiothreitol-sensitive tetrameric protease of spinach thylakoids: structural similarities between the protease and polyphenol oxidase.

The 60-kDa precursor to the 39-kDa dithiothreitol-sensitive protease was purified from photosystem II membranes of spinach. When partially purified 60-kDa protein was stored at 4 degrees C, the protein was degraded to fragments of 39 and 21 kDa. The 39-kDa fragment was suggested to be identical to the 39-kDa protease from effects of dithiothreitol on these polypeptides. The N-terminal amino acid sequences of the 60-kDa protein and the 39-kDa protease were the same, APILPDVEK-, suggesting that the latter was derived from the N-terminal portion of the former. Immunostaining with polyclonal antibodies against the 60-kDa protein indicated that the 60-kDa protein represents the species that occurs in the native thylakoids. These and other structural properties suggest that the protein might be identical to polyphenol oxidase.