Crystal structure of CyanoQ from the thermophilic cyanobacterium Thermosynechococcus elongatus and detection in isolated photosystem II complexes.

The PsbQ-like protein, termed CyanoQ, found in the cyanobacterium Synechocystis sp. PCC 6803 is thought to bind to the lumenal surface of photosystem II (PSII), helping to shield the Mn4CaO5 oxygen-evolving cluster. CyanoQ is, however, absent from the crystal structures of PSII isolated from thermophilic cyanobacteria raising the possibility that the association of CyanoQ with PSII might not be a conserved feature. Here, we show that CyanoQ (encoded by tll2057) is indeed expressed in the thermophilic cyanobacterium Thermosynechococcus elongatus and provide evidence in support of its assignment as a lipoprotein. Using an immunochemical approach, we show that CyanoQ co-purifies with PSII and is actually present in highly pure PSII samples used to generate PSII crystals. The absence of CyanoQ in the final crystal structure is possibly due to detachment of CyanoQ during crystallisation or its presence in sub-stoichiometric amounts. In contrast, the PsbP homologue, CyanoP, is severely depleted in isolated PSII complexes. We have also determined the crystal structure of CyanoQ from T. elongatus to a resolution of 1.6 Å. It lacks bound metal ions and contains a four-helix up-down bundle similar to the ones found in Synechocystis CyanoQ and spinach PsbQ. However, the N-terminal region and extensive lysine patch that are thought to be important for binding of PsbQ to PSII are not conserved in T. elongatus CyanoQ.

Crystal structure of the Psb27 assembly factor at 1.6 Å: implications for binding to Photosystem II.

The biogenesis and oxygen-evolving activity of cyanobacterial Photosystem II (PSII) is dependent on a number of accessory proteins not found in the crystallised dimeric complex. These include Psb27, a small lipoprotein attached to the lumenal side of PSII, which has been assigned a role in regulating the assembly of the Mn(4)Ca cluster catalysing water oxidation. To gain a better understanding of Psb27, we have determined in this study the crystal structure of the soluble domain of Psb27 from Thermosynechococcus elongatus to a resolution of 1.6 Å. The structure is a four-helix bundle, similar to the recently published solution structures of Psb27 from Synechocystis PCC 6803 obtained by nuclear magnetic resonance (NMR) spectroscopy. Importantly, the crystal structure presented here helps us resolve the differences between the NMR-derived structural models. Potential binding sites for Psb27 within PSII are discussed in light of recent biochemical data in the literature.

Structure of CyanoP at 2.8 A: implications for the evolution and function of the PsbP subunit of photosystem II .

We present here the crystal structure of CyanoP (Tlr2075) from Thermosynechococcus elongatus at 2.8 A. CyanoP is a substoichiometric component of the isolated cyanobacterial Photosystem II (PSII) complex, distantly related to the PsbP extrinsic subunit of the oxygen-evolving PSII complex in higher plants and green algae. Despite the relatively low degree of sequence similarity, we have found that CyanoP adopts the same beta-sandwich fold as higher-plant PsbP and contains a well-conserved metal (zinc)-binding site that is also present in plant PsbP. Our results support the idea that CyanoP represents the basal structural fold of the PsbP superfamily.

Structural and functional studies on Ycf12 (Psb30) and PsbZ-deletion mutants from a thermophilic cyanobacterium.

Ycf12 (Psb30) and PsbZ are two low molecular weight subunits of photosystem II (PSII), with one and two trans-membrane helices, respectively. In order to study the functions of these two subunits from a structural point of view, we constructed deletion mutants lacking either Ycf12 or PsbZ from Thermosynechococcus elongatus, and purified, crystallized and analyzed the structure of PSII dimer from the two mutants. Our results showed that Ycf12 is located in the periphery of PSII, close to PsbK, PsbZ and PsbJ, and corresponded to the unassigned helix X1 reported previously, in agreement with the recent structure at 2.9 A resolution (A. Guskov, J. Kern, A. Gabdulkhakov, M. Broser, A. Zouni, W. Saenger, Cyanobacterial photosystem II at 2.9 A resolution: role of quinones, lipids, channels and chloride, Nat. Struct. Mol. Biol. 16 (2009) 334-342). On the other hand, crystals of PsbZ-deleted PSII showed a remarkably different unit cell constants from those of wild-type PSII, indicating a role of PsbZ in the interactions between PSII dimers within the crystal. This is the first example for a different arrangement of PSII dimers within the cyanobacterial PSII crystals. PSII dimers had a lower oxygen-evolving activity from both mutants than that from the wild type. In consistent with this, the relative content of PSII in the thylakoid membranes was lower in the two mutants than that in the wild type. These results suggested that deletion of both subunits affected the PSII activity, thereby destabilized PSII, leading to a decrease in the PSII content in vivo. While PsbZ was present in PSII purified from the Ycf12-deletion mutant, Ycf12 was present in crude PSII but absent in the finally purified PSII from the PsbZ-deletion mutant, indicating a preferential, stabilizing role of PsbZ for the binding of Ycf12 to PSII. These results were discussed in terms of the PSII crystal structure currently available.