Structural Analysis of the PsbQ Protein of Photosystem II by Fourier Transform Infrared and Circular Dichroic Spectroscopy and by Bioinformatic Methods

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Abstract
The structure of PsbQ, one of the three main extrinsic proteins associated with the oxygen-evolving complex (OEC) of higher plants and green algae, is examined by Fourier transform infrared (FTIR) and circular dichroic (CD) spectroscopy and by computational structural prediction methods. This protein, together with two other lumenally bound extrinsic proteins, PsbO and PsbP, is essential for the stability and full activity of the OEC in plants. The FTIR spectra obtained in both H2O and D2O suggest a mainly α-helix structure on the basis of the relative areas of the constituents of the amide I and I‘ bands. The FTIR quantitative analyses indicate that PsbQ contains about 53% α-helix, 7% turns, 14% nonordered structure, and 24% β-strand plus other β-type extended structures. CD analyses indicate that PsbQ is a mainly α-helix protein (about 64%), presenting a small percentage assigned to β-strand (≈7%) and a larger amount assigned to turns and nonregular structures (≈29%). Independent of the spectroscopic analyses, computational methods for protein structure prediction of PsbQ were utilized. First, a multiple alignment of 12 sequences of PsbQ was obtained after an extensive search in the public databases for protein and EST sequences. Based on this alignment, computational prediction of the secondary structure and the solvent accessibility suggest the presence of two different structural domains in PsbQ: a major C-terminal domain containing four α-helices and a minor N-terminal domain with a poorly defined secondary structure enriched in proline and glycine residues. The search for PsbQ analogues by fold recognition methods, not based on the secondary structure, also indicates that PsbQ is a four α-helix protein, most probably folding as an up−down bundle. The results obtained by both the spectroscopic and computational methods are in agreement, all indicating that PsbQ is mainly an α protein, and show the value of using both methodologies for protein structure investigation.