Photosystem rebinding

Cammarata and Cheniae found that purified photosystem II of spinach contains two per reaction center. Ono and Inoue found that treating the thylakoids with citrate at pH 3 selectively releases only one ofthe two Ca in the PS-II reaction center. The low-pH treatment also removes the 23- and 17-kDa polypeptides, but they rebind to the thylakoid when the pH is raised to 6.5, after washing to remove the free Ca. Even with all three extrinsic polypeptides intact, oxygen evolution in the citrate-treated thyla-... 

Leuschner C, Bricker TM. Interaction of the 33 kDa extrinsic protein with photosystem II Rebinding of the 33 kDa extrinsic protein to photosystem II membranes that contain four, two or zero manganese per photosystem II reaction center. Biochemistry 1996 35 4551-4557. 

Rebinding of the Reconstituted-F IFB Polypeptide to the Photosystem I Core Protein with Tetemke Mehari and Kevin Parrett [refs. 12,17]... 

Fig. 6. Flash-induced absorption changes in the Photosystem I core protein after rebinding of the Fa/Fb polypeptide. (A) Absorption transient in the Photosystem I core protein after rebinding of the native-Fa/Fb polypeptide. (B) Absorption transient in the Photosystem I core protein after attempted rebinding of the Fa/Fb apoprotein. (C) Absorption transient in the Photosystem I core protein after rebinding of the reconstituted-pA/pB polypeptide. All measurements were performed at 5 ig Chl/ml in 50 mM Tris buffer, pH 8.3, containing 1.7 mM ascorbate and 0.033 mM DCPIP.

The ESR spectra of the native Fa/Fb protein, the Fa/Fb apoprotein, and the reconstituted Fa/Fb protein after rebinding to the Photosystem I core protein are shown in Fig. 7. When the native-pA/pB polypeptide is rebound to the Photosystem I core protein... 

When the native-pA/pB polypeptide is rebound to the Photosystem I core protein, frozen in darkness, and illuminated at 16 K, Fa is 85% photoreduced (g values of 2.05, 1.94 and 1.86), and Fb is about 15% photoreduced (g values of 2.07, 1.92 and 1.89). Note that this differs significantly from the earlier reported rebinding of the spinach Fa/Fb protein to the Synechococcus Photosystem I core protein [9], where 50% of Fa and 50% of Fb were photoreduced. When the Fa/Fb apoprotein is reconstituted with inorganic iron and sulfide in the presence of 6-mercaptoethanol and rebound to the Photosystem I core protein, the photoreduction of Fa and Fb occur in the same amounts as the control (data not shown). Clearly, the differences in the amount of Fa and Fb photoreduced in these two experiments are not a function of the cross-species reconstitution, but instead, may be related to a time-dependent reorientation of the Fa/Fb polypeptide on the Photosystem I core protein. Experiments are now in progress to test this proposal. 

The reconstitution protocol outlined here allows complete experimental control over iron-sulfur cluster denaturation and reconstitution in the Fx and Fa/Fb polypeptides, and rebinding of the reconstituted Fa/Fb polypeptide to the Photosystem I core protein to yield the intact Photosystem I complex. Studies which include chemical or genetic modification of the Fx and Fa/Fb apoproteins followed by cluster reconstitution can now be performed prior to in vitro reassembly of the Photosystem I complex. 

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