The QB site modulates the conformation of the photosystem II reaction center polypeptides.

The sensitivity of the D-1 and D-2 polypeptide subunits of photosystem II towards trypsin treatment of the thylakoid membrane has been probed with specific antibodies. As long known, electron flow from water to ferricyanide becomes inhibitor insensitive after this trypsin treatment. We show that under these conditions the D-2 polypeptide is cut by trypsin at arg 234. Also the D-1 polypeptide is cut, probably at arg 238. When short time trypsination of the membrane is done in the presence of inhibitors, electron flow also becomes inhibitor insensitive and the D-2 polypeptide is still cut, but the D-1 polypeptide is cut only under certain conditions. A protection of the D-1 polypeptide is possible with inhibitors of photosystem II of the DCMU/triazine-type and with an artificial acceptor quinone, but not with inhibitors of the phenol-type. In hexane extracted membranes plastoquinone has been removed from the QB site. Both the D-1 and D-2 polypeptides are more trypsin sensitive in such preparations. The D-1, but not the D-2 polypeptide is protected when plastoquinone has been readded to the membrane before the trypsin digestion.The results show that plastoquinone, artificial quinones and inhibitors of photosystem II at the QB site, but also carotene to a lesser extent, have an effect on the conformation of both the D-1 and D-2 polypeptide. it is postulated that the amino acid sequence around arginine 238 of the D-1 polypeptide is part of the QB binding niche. Furthermore this sequence is modified or its conformation is changed if the QB site is occupied by either plastoquinone or a DCMU-type inhibitor because under these conditions arginine 238 is less accessible to the trypsin. If the QB site, however, is empty, the amino acid sequence with arg 238 is very trypsin sensitive. This property of modulation or the conformation of the amino acid sequence of the D-1 polypeptide by the state of the QB site is likely to be relevant also for the events in the rapid turnover of the D-1 polypeptide.