tülakoidide membraani luumenipoolsel küljel paiknev vaske sisaldav väike 10,5 kDa valk, mis transpordib fotosünteesi elektrontransportahelas elektrone tsütokroom b6f kompleksilt P700-le, Copper-containing protein that transports electrons from cytochrome b6f complex to P700. Located in the lumen side of thylacoid membranes.