Structural Stability of the PsbQ Protein of Higher Plant Photosystem II

We have characterized the stability and folding behavior of the isolated extrinsic PsbQ protein of photosystem II (PSII) from a higher plant, Spinacia oleracea, using intrinsic protein fluorescence emission and near- and far-UV circular dichroism (CD) spectroscopy in combination with differential scanning calorimetry (DSC). Experimental results reveal that both chemical denaturation using guanidine hydrochloride (GdnHCl) and thermal unfolding of PsbQ proceed as a two-state reversible process. The denaturation free-energy changes (GD) at 20 C extrapolated from GdnHCl (4.0 ± 0.6 kcal mol-1) or thermal unfolding (4.4 ± 0.8 kcal mol-1) are very close. Moreover, the far-UV CD spectra of the denatured PsbQ registered at 90 C in the absence and presence of 6.0 M GdnHCl superimpose, leading us to conclude that both denatured states of PsbQ are structurally and energetically similar. The thermal unfolding of PsbQ has been also characterized by CD and DSC over a wide pH range. The stability of PsbQ is at its maximum at pH comprised between 5 and 8, being wider than the optimal pH for oxygen evolution in the lumen of thylakoid membranes. In addition, no significant structural changes were detected in PsbQ between 50 and 55 C in the pH range of 3-8, suggesting that PsbQ behaves as a soluble and stable particle in the lumen when it detaches from PSII under physiological stress conditions such as high temperature (45-50 C) or low pH (<5.0). Sedimentation experiments showed that, in solution at 20 C, the PsbQ protein is a monomer with an elongated shape.Spanish Minitry of Science and Technology (PB1998-0480 and AGL2004-00045)This work was funded by the Spanish Ministry of Science and Technology (project references PB1998-0480 and AGL2003-0045). M.B. holds a fellowship from the Spanish Ministry of Science and Technology.Peer reviewe

Structural Stability of the PsbQ Protein of Higher Plant Photosystem II

We have characterized the stability and folding behavior of the isolated extrinsic PsbQ protein of photosystem II (PSII) from a higher plant, Spinacia oleracea, using intrinsic protein fluorescence emission and near- and far-UV circular dichroism (CD) spectroscopy in combination with differential scanning calorimetry (DSC). Experimental results reveal that both chemical denaturation using guanidine hydrochloride (GdnHCl) and thermal unfolding of PsbQ proceed as a two-state reversible process. The denaturation free-energy changes (GD) at 20 C extrapolated from GdnHCl (4.0 ± 0.6 kcal mol-1) or thermal unfolding (4.4 ± 0.8 kcal mol-1) are very close. Moreover, the far-UV CD spectra of the denatured PsbQ registered at 90 C in the absence and presence of 6.0 M GdnHCl superimpose, leading us to conclude that both denatured states of PsbQ are structurally and energetically similar. The thermal unfolding of PsbQ has been also characterized by CD and DSC over a wide pH range. The stability of PsbQ is at its maximum at pH comprised between 5 and 8, being wider than the optimal pH for oxygen evolution in the lumen of thylakoid membranes. In addition, no significant structural changes were detected in PsbQ between 50 and 55 C in the pH range of 3-8, suggesting that PsbQ behaves as a soluble and stable particle in the lumen when it detaches from PSII under physiological stress conditions such as high temperature (45-50 C) or low pH (<5.0). Sedimentation experiments showed that, in solution at 20 C, the PsbQ protein is a monomer with an elongated shape.Spanish Minitry of Science and Technology (PB1998-0480 and AGL2004-00045)This work was funded by the Spanish Ministry of Science and Technology (project references PB1998-0480 and AGL2003-0045). M.B. holds a fellowship from the Spanish Ministry of Science and Technology.Peer reviewe