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

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Abstract
We studied the action of the photophosphorylation inhibitor, N,N′‐dicyclohexylcarbodiimide (DCCD) on the channel portion (CF0) of the chloroplast ATP synthase (CF0CF1). We found that the target for binding of [14C]DCCD was an oligomer of the small proteolipid‐subunit (subunit III) of CF0. We treated thylakoids with low concentrations of DCCD, sufficient to inhibit photophosphorylation. The 14C‐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 CF0. At higher concentrations of DCCD, it fell apart into proteolipid monomers. This dissociation was prevented by the presence of venturicidin, another CF0 inhibitor acting on the proteolipid subunit, during the incubation with DCCD. The existence of such an oligomeric substructure in CF0 is discussed in the light of diverging structural models for the proton channel of F0F1‐type ATPases.