Identification of the plastocyanin binding subunit of photosystem I

Plastocyanin is specifically cross‐linked by incubation with N‐ethyl‐3‐[3‐(dimethylamino)propyl]carbodiimide (EDC) to a subunit of photosystem I in stroma lamellae and in isolated photosystem I complex. SDS‐PAGE shows the disappearance of a 18.5 kDa subunit and the appearance of a new 31.5 kDa protein which was recognized by anti‐plastocyanin antibodies. The isolated subunit was identified by its N‐terminal amino acid sequence as the mature peptide coded by the nuclear gene psaF [Steppuhn et al. (1988) FEBS Lett. 237, 218–224]. P700+ was reduced by cross‐linked plastocyanin with the same halftime of 13 μs as found in the native complex. This is evidence that cross‐linking conserved the orientation of the complex and that the 18.5 kDa subunit provides the conformation of photosystem I necessary for the extremely rapid electron transfer from plastocyanin to P700+.