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Using a combination of cyanogen bromide cleavage and endoproteinase digestion we have shown that the putative epitope for the monoclonal antibody FAC2 lies in the region 360Pro-391Ser on the apoprotein of CPa-1. This region lies entirely within the large extrinsic loop of this protein. We have shown previously that the epitope of FAC2 becomes exposed in oxygen-evolving membranes upon treatment with alkaline Tris which releases all four of the manganese associated with the oxygen-evolving site of photosystem II. The epitope is not exposed, however, after CaCl2 treatment and exposure to low concentrations of chloride, conditions which lead to the release of two of the four manganeses associated with the oxygen-evolving site [(1987) Arch. Biochem. Biophys. 256, 295-301]. These results suggest that, upon release of the chloride-insensitive manganese from photosystem II membranes, a conformational change occurs which leads to the exposure of 360Pro-391Ser on CPa-1 to the monoclonal antibody FAC2. © 1989.

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FEBS Letters

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