Backbone assignment and secondary structure of the PsbQ protein from Photosystem II

PsbQ is one of the extrinsic proteins situated on the lumenal surface of Photosystem II (PSII) in the higher plants and green algae. Its three-dimensional structure was determined by X-ray crystallography apart from the residues 14–33. To obtain further details about its structure and potentially its dynamics, we approached the problem by NMR. In this paper we report 1H, 15N, and 13C NMR assignments for the PsbQ protein. The very challenging poly-proline stretches could be assigned using 13C-detected NMR experiments that enabled the assignments of twelve out of the thirteen proline residues of PsbQ. The identification of PsbQ secondary structure elements on the basis of our NMR data was accomplished with the programs TALOS+, web server CS23D and CS-Rosetta. To obtain additional secondary structure information, three-bond HN-Hα J-coupling constants and deviation of experimental 13Cα and 13Cβ chemical shifts from random coil values were determined. The resulting “consensus” secondary structure of PsbQ compares very well with the resolved regions of the published X-Ray structure and gives a first estimate of the structure of the “missing link” (i.e. residues 14–33), which will serve as the basis for the further investigation of the structure, dynamics and interactions by NMR.