Mutational analysis of the stability of Psb27 from Synechocystis sp. PCC 6803: implications for models of Psb27 structure and binding to CP43.

Psb27 associates with the CP43 subunit of photosystem II during biogenesis of the photosystem. Several models have been proposed for the interaction between Psb27 and CP43. The utility of predictions and hypotheses arising from these models depends on the accuracy of the Psb27 structure used in the model. Two of the Psb27 structures used to model the Psb27-CP43 interaction place residue E98 on the surface of Psb27 and D14 in a position to form hydrogen bonds that stabilise the fold of the protein; however, a third structure questions the surface exposure of E98 and does not identify significant interactions of D14. Here we present evidence that D14 contributes to the thermal stability of Psb27 and that E98 is located on the surface. A D14A mutation was shown to reduce the apparent midpoint of unfolding of Psb27 by 16 °C. Four highly conserved surface residues and E98 were subject to charge-reversal mutations (R54E, R94E, E98R, E103R, R108E). The stabilities of the charge-reversal variants and the unmodified control were similar, suggesting E98 is a surface residue. Placing E98 in the correct, surface position will support more reliable models of the interaction of Psb27 with CP43.