Trypsin-Mediated Removal of Herbicide Binding Sites within the Photosystem II Complex

Abstract

Trypsin treatment of isolated chloroplast thylakoids resulted in a step-wise modification of surface exposed membrane polypeptides. Early effects of the protease action resulted in a decrease in inhibitory activity of atrazine, diuron, pyrazon, and bromacil, but an initial increase in the activity of bromnitrothymol and dinoseb. Direct measurements of atrazine binding demonstrated that decreased inhibitory activity corresponded to a decreased binding affinity in the treated membranes. Longer term effects of trypsin caused removal of atrazine binding sites and a concomitant block of electron transport chains. The data are consistent with a concept that the traizine receptor protein is a component of the electron transport chain which is successively degraded in two or more steps by protease attack. Polyacrylamide gel electrophoresis of trypsin-treated membranes and sub-membrane fragments derived from these membranes revealed that several polypeptides are membrane surface exposed. The involvement of a 32000 dalton polypeptide in creating the atrazine binding site is discussed.