The "additional subunit" CF0II of the photosynthetic ATP-synthase and the thylakoid polypeptide, binding ferredoxin NADP reductase: are they different?

Evidence is presented to support the notion that the 16 kDa thylakoid polypeptide, called CF0II, is an essential subunit of the photosynthetic ATP-synthase complex CF0CF1: It is co-isolated with the other subunits of CF0CF1 in preparations either using octylglucoside/cholate or Triton X-100. It is co-precipitated by antibodies together with the other CF0CF1 subunits. It is immunochemically not related to thylakoid polypeptides of higher molecular weight nor to some thylakoid polypeptides with similar apparent molecular weight between 16 and 18 kDa: CF1 epsilon, CF0I, subunit IV of the b6f complex, the 16.5 kDa peripheral polypeptide of the oxygen evolving complex of PS II, and the intrinsic ferredoxin NADP reductase binding protein. The N-terminal amino acid sequences of CF0II and the reductase binding protein is determined by Edman degradation and compared: The two sequences are different and not identical to other characterized thylakoid polypeptides. Monospecific antibodies against CF0II inhibit rebinding of CF1 to EDTA treated thylakoid membranes, H+ efflux from EDTA treated membranes and cyclic photophosphorylation. Thus the additional polypeptide CF0II qualifies for a functional subunit of the photosynthetic ATP-synthase.