[125I]Azido-Ioxynil Labels Val249 of the Photosystem II D -1 Reaction Center Protein

Azido-ioxynil (3,5-diiodo-2-azido-4-hydroxy-benzonitrile) is a potent photosystem II inhibitor (pI -value 7.38) and as effective as the parent compound ioxynil itself. [ I]azido-ioxynil exhibits specific binding to isolated thylakoids with a binding constant K = 7.14. Upon UV -illumination it binds covalently to thylakoids or photosystem II particles. It labels predominantly the 32 kDa D-1 photosystem II reaction center protein . A 41 kDa protein is only tagged in trace amounts. After proteolytic treatment of labeled D -1 protein with Staphylococcus aureaus V8 -protease two major and two minor fragments are obtained. Automated gas phase sequencing of a 7 kDa cleavage peptide revealed that Val is the primary target of azido-ioxynil binding. Compared to urea type herbicides, this places the ioxynil binding site in a different environment of the D -1 photosystem II protein.