Identification of a proteolipid oligomer as a constituent part of CF₀, the proton channel of the chloroplast ATP synthase

We studied the action of the photophosphorylation inhibitor, N,N ′‐dicyclohexylcarbodiimide (DCCD) on the channel portion (CF 0 ) of the chloroplast ATP synthase (CF 0 CF 1 ). We found that the target for binding of [ 14 C]DCCD was an oligomer of the small proteolipid‐subunit (subunit III) of CF 0 . We treated thylakoids with low concentrations of DCCD, sufficient to inhibit photophosphorylation. The 14 C‐labelled inhibitor was found on polyacrylamide gels in a position corresponding to an apparent molecular mass of 50 kDa. This band comprised a homooligomer of proteolipid subunits of CF 0 . At higher concentrations of DCCD, it fell apart into proteolipid monomers. This dissociation was prevented by the presence of venturicidin, another CF 0 inhibitor acting on the proteolipid subunit, during the incubation with DCCD. The existence of such an oligomeric substructure in CF 0 is discussed in the light of diverging structural models for the proton channel of F 0 F 1 ‐type ATPases.