The 1, 4, and 8 tyrosine (Tyr) residues on the PSII extrinsic 23 kDa protein were modified with 5, 10 or 40 mM N-acetylimidazole (NAI) respectively. The amount of rebound NAI-modified extrinsic 23 kDa protein was 98%, 80%, and 5% of that in the unmodified protein, respectively. These results indicate that the Tyr residues are absolutely essential to reconstitution ability. Further, the fluorescence and circular dichroism spectra among native and NAI-modified extrinsic 23 kDa proteins were similar, suggesting that the modification by NAI did not markedly influence the basic secondary structure of the native conformation. Thus, we have concluded that the tyrosine residues in the extrinsic 23 kDa protein are important for interaction with PSII membranes. In addition, we found that the structure of the extrinsic 23 kDa protein is stable in suspension (pH 4-9 or Tm 25-55°C).