Threading structural model of the manganese-stabilizing protein PsbO reveals presence of two possible beta-sandwich domains

The manganese-stabilizing protein (PsbO) is an essential component of photo system II (PSII) and is present in all oxyphotosynthetic organisms. PsbO al lows correct water splitting and oxygen evolution by stabilizing the reacti ons driven by the manganese cluster. Despite its important role, its struct ure and detailed functional mechanism are still unknown. In this article we propose a structural model based on fold recognition and molecular modelin g. This model has additional support from a study of the distribution of ch aracteristics of the PsbO sequence family, such as the distribution of cons erved, apolar, tree-determinants, and correlated positions. Our threading r esults consistently showed PsbO as an all-beta (beta) protein, with two hom ologous beta domains of approximately 120 amino acids linked by a flexible Proline-Glycine-Glycine (PGG) motif. These features are compatible with a g eneral elongated and flexible architecture, in which the two domains form a sandwich-type structure with Greek key topology. The first domain is predi cted to include 8 to 9 beta -strands, the second domain 6 to 7 beta -strand s. An Ig-like beta -sandwich structure was selected as a template to build the 3-D model. The second domain has, between the strands, long-loops rich in Pro and Gly that are difficult to model. One of these long loops include s a highly conserved region (between P148 and P174) and a short a-helix (be tween E181 and N188)). These regions are characteristic parts of PsbO and s how that the second domain is not so similar to the template. Overall, the model was able to account for much of the experimental data reported by sev eral authors, and it would allow the detection of key residues and regions that are proposed in this article as essential for the structure and functi on of PsbO. (C) 2001 Wiley-Liss, Inc.