Sizes of Mn-binding sites in spinach thylakoids

The sizes of the Mn-binding sites in spinach thylakoids were estimated by target size analysis, assaying the membrane-bound Mn that was resistant to EDTA washing after radiation inactivation. The inactivation curve showed well the inactivation of two independent Mn-binding sites of different sizes: about two-thirds of the Mn coordinated to a binding site of 65 kDa, and the rest bound to a much smaller site of only about 3 kDa. In the large site, there was about 1 g atom of Mn/110 mol of chlorophyll in spinach thylakoids, which was constant in normally grown plants, although the Mn level in the small site depended on culture conditions. Thylakoids that had been incubated with hydroxylamine or in 0.8 M Tris lost Mn exclusively from the large binding site.