Herbicide binding proteins discussion

Still the identity, nature, and size of the herbicide-binding protein remained a puzzle. This putative protein was likely very hydrophobic. A methodology for isolating such membrane proteins was not at that time available. Even more difficult to comprehend was the orientation of such hydrophobic proteins in a membrane that had been a matter of discussion for many decades. The first X-ray structure of a membrane protein complex in 1985 suddenly solved this orientation question, as will be discussed below. 

Herbicides that inhibit photosynthetic electron flow prevent reduction of plastoquinone by the photosystem II acceptor complex. The properties of the photosystem II herbicide receptor proteins have been investigated by binding and displacement studies with radiolabeled herbicides. The herbicide receptor proteins have been identified with herbicide-derived photoaffinity labels. Herbicides, similar in their mode of action to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) bind to a 34 kDa protein, whereas phenolic herbicides bind to the 43-51 kDa photosystem II reaction center proteins. At these receptor proteins, plastoquinone/herbicide interactions and plastoquinone binding sites have been studied, the latter by means of a plastoquinone-deriv-ed photoaffinity label. For the 34 kDa herbicide binding protein, whose amino acid sequence is known, herbicide and plastoquinone binding are discussed at the molecular level. 

The present paper is a discussion of the photosystem II herbicides and their mechanisms of action. Among the topics covered are the green plant photosystems, photochemistry and electron transfers within photosystem II, requirements for herbicidal activity, mechanisms of action, herbicide selectivity and resistance, herbicide-binding proteins, and theoretical studies of herbicidebinding site interactions. 

The other major functional area of PSII is that of the antenna proteins. Current ideas suggest that there are several hundred chlorophyll molecules associated with each reaction center, and they can be separated into distinct apoproteins with molecular weights of 29, 25-28, 27 or 31, and 21 kDa. As these proteins do not appear to be associated with herbicide binding, they will not be discussed further. 

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