SUBUNIT-DELTA OF H+-ATPASES - AT THE INTERFACE BETWEEN PROTON FLOW AND ATP SYNTHESIS

The ATP synthases in photophosphorylation and respiration are of the F-type with a membrane-bound proton channel, F0, and an extrinsic catalytic portion, F1. The properties of one particular subunit, δ (in chloroplasts and Escherichia coli) and OSCP (in mitochondria), are reviewed and the role of this subunit at the interface between F0 and F1 is discussed. δ and OSCP from the three sources have in common the molecular mass (≈ 20 kDa), an elongated shape (axial ratio in solution about 3:1), one high-affinity binding site to F1 (Kd ≈ 100 nM) plus probably one or two further low-affinity sites. When isolated δ is added to CF1-depleted thylakoid membranes, it can block proton flow through exposed CF0 channels, as do CF1 or CF1(-δ) + δ. This identifies δ as part of the proton conductor or, alternatively, conformational energy transducer between F0 (proton flow) and F1 (ATP). Hybrid constructs as CF1(-δ) + E. coli δ and EF1(-δ) + chloroplast δ diminish proton flow through CF0. CF1(-δ) + E. coli δ does the same on EF0. Impairment of proton leaks either through CF0 or through EF0 cause 'structural reconstitution' of ATP synthesis by remaining intact F0F1. Functional reconstitution (ATP synthesis by fully reconstructed F0F1), however, is absolutely dependent on the presence of subunit δ and is therefore observed only with CF1 or CF1(-δ) + chloroplast δ on CF0 and EF1 or EF1(-δ) + E. coli δ on EF0. The effect of hybrid constructs on F0 channels is surprising in view of the limited sequence homology between chloroplast and E. coli δ (36% conserved residues including conservative replacements). An analysis of the distribution of the conserved residues at present does not allow us to discriminate between the postulated conformational or proton-conductive roles of subunit δ. © 1990.