Protective effect of bicarbonate against extraction of the extrinsic proteins of the water-oxidizing complex from Photosystem II membrane fragments

A protective effect of bicarbonate (BC) against extraction of the extrinsic proteins, predominantly the Mn-stabilizing protein (PsbO protein), during treatment of Photosystem II (PS II) membrane fragment from pea with 2 M urea, and at low pH (using incubation in 0.2 M glycine-HCl buffer, pH 3.5 or 0.5 M citrate buffer, pH 4.0-4.5) was detected. It was shown that the extraction of the proteins with Mw 24 kDa (PsbP protein) and 18 kDa (PsbQ protein) by the use of highly concentrated solutions of NaCl does not depend on the presence of BC in the medium. An optimal concentration of BC at which it produces the maximum protecting effect was shown to be between 1 mM and 10 mM. The addition of formate did not influence the protein extraction but it reduced the stabilizing effect of BC. Independence of the stabilizing effect on the presence of the functionally active Mn within the water-oxidizing complex indicates that the protecting effect of BC is not related to its interaction with Mn ions. The fact that there is a preferable sensitivity of the PsbO protein to the absence of BC in the medium during all the treatments makes it possible to suggest that either BC interacts directly with the PsbO protein or it binds to some other sites within PS II and this binding facilitates the preservation of the native structure of this protein.