Identification of a primary in vivo degradation product of the rapidly-turning-over 32 kd protein of photosystem II.

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Abstract
The 32 kd photosystem II protein of plant chloroplasts is rapidly turned over in the light. The initial events in the degradation of the 32 kd protein were studied. A 23.5 kd breakdown product was identified in Spirodela oligorrhiza membranes using immunological analysis. The 23.5 kd polypeptide was shown to be derived from the amino‐terminal portion of the 32 kd protein using partial proteolytic fingerprinting. An in vivo precursor–product relationship between the 32 kd protein and the 23.5 kd polypeptide was kinetically demonstrated by radiolabeling and pulse‐chase experiments. The cleavage site yielding the 23.5 kd polypeptide was localized to a functionally active region (between helices IV and V) of the 32 kd protein. We propose that an alpha‐helix‐destabilizing ‘degradation’ sequence, bordered by arginine residues 225 and 238, is involved in the formation of the 23.5 kd polypeptide.