Three extrinsic proteins of 33 kDa, 12 kDa, and Cyt c550 encoded by psbO, psbU and psbV genes are located at the lumenal side. Among these extrinsic proteins, the 33 kDa protein is attached to the lumenal surface at the D2/CP47 side. It has a characteristic cylindrical shape (Figure 4.2c) similar to those of outer membrane proteins family [25-27]. Cyt c550 is attached to the lumenal surface at the CP43/D1 side, with its overall structure being similar to other c-type cytochromes . The 12 kDa protein is composed of mainly a-helices, and is located between the 33 kDa protein and Cyt c550. Additionally, it is approximately 30 A away from the lumenal surface of the membrane and may be in contact with some of the loop regions of D2, CP47, and CP43 that protrude from the lumenal surface (Figure 4.1a). The contact of the 12 kDa protein with these membrane proteins, however, is not strong enough to hold it onto PSII since binding of the 12 kDa protein to PSII requires the presence of both the 33 kDa protein and Cyt c550 [29, 30]. These three extrinsic proteins together with the large extra-membrane loop regions of D1, D2, CP47, and CP43, form a large hydrophilic domain at the lumenal side that covers the Mn4 Ca cluster located close to the membrane at the lumenal surface. This structural organization suggests that both the 33 kDa protein and Cyt c550 function to stabilize the Mn4Ca cluster. Indeed, release of either the 33 kDa protein or Cyt c550 has been found to result in a destabilized Mn.Ca cluster which was deactivated rapidly in the dark [ 31, 32]. Absence of either of these two subunits even leads to a higher efficiency of photoactivation, a process of lightinduced reassembly of the Mn4Ca cluster [31, 32]. This suggests that Mn and/or Ca could access to the Mn4Ca cluster more freely in the absence of these subunits.
Two genes, homologous to psbP and psbQ, of higher plants that encode the extrinsic 23 kDa and 17 kDa proteins of green algal and higher plant PSII have been found in the cyanobacterial genome; the products of these genes have been confirmed in PSII purified from Synechocystis sp. PCC 6803 with a His-tag attached . The presence of PsbP and PsbQ proteins were also confirmed in the thylakoids of thermophilic cyanobacteria . In the present structure, however, the PsbP and PsbQ proteins were not found, and they were also absent in the purified PSII of thermophilic cyanobacteria. These proteins are thus released during the purification process, suggesting that they are weakly bound to PSII during assembly.
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