Dicyclohexylcarbodiimide-binding proteins related to the short circuit of the proton-pumping activity of photosystem II. Identified as light-harvesting chlorophyll-a/b-binding proteins

Abstract

In photosynthesis of higher plants, photosystem II drives electron transfer from the water-oxidizing manganese centre at the lumenal side to bound plastoquinone at the stromal side of the thylakoid membrane. Proton release into the lumen and proton uptake from the stroma, i.e. net proton pumping, follows as consequence of vectoral electron transport. The proton pumping activity can be short circuited by covalent modification with N,N'-dicyclohexylcarbodiimide (cHxN)2C of certain proteins in the 20-28-kDa range. After modification, protons from water oxidation are no longer released into the thylakoid lumen, but instead transferred through the photosystem complex to protonate the photoreduced bound quinone at the other side of the membrane [Jahns, P., Polle, A. & Junge, W. (1988) EMBO J. 7, 589-594]. Here we identify the pertinent (cHxN)2C-binding proteins by amino acid sequence analysis and localize (cHxN)2C-binding sites within their primary structure. The proteins that are associated with the proton short circuit are light-harvesting chlorophyll-a/b-binding proteins. Our results imply that in addition to acting as antennae they may serve another function: the funneling into the thylakoid lumen of protons, which are liberated in the water-oxidizing Mn centre.