Cyanobacterial and chloroplast F₁-ATPases: cross-reconstitution of photophosphorylation and subunit immunological relationships

The photosynthetic F₁-ATPase from the cyanobacterium Spirulina platensis was recently purified in a five-subunit, reconstitutively active form (Hicks, D.B. and Yocum, C.F. (1986) Arch. Biochem. Biophys. 245, 220-229). Here we report on the similarities of the cyanobacterial F₁ to the higher plant chloroplast F₁ (CF₁), as judged by two distinct methods. The ability of each coupling factor to reconstitute photophosphorylation in photosynthetic membranes depleted of F₁ content by 2 M NaBr treatment was tested. Addition of either the homologous enzyme (e.g., Spirulina F₁, Spirulina membranes) or the heterologous enzyme (e.g., spinach CF₁ Spirulina membranes) to depleted membranes increased the rate of phenazine methosulfate-dependent cyclic photophosphorylation from nearly zero to up to 70 μmol ATP/h per mg Chl. Antibodies against four subunits of CF₁ (α, γ, δ and ε) and against β of Escherichia coli F₁ were reacted with the Spirulina enzyme by protein blotting. The α, β and γ subunits of Spirulina F₁ cross-reacted with antibodies against the corresponding subunits from spinach. The cross-reactivity of the γ subunit correlated with previous observations that Spirulina membrane ATPase activity can be modulated by light and dithiothreitol, in a similar fashion to their effect on the enzyme from spinach chloroplasts. The ability of cyanobacterial and chloroplast enzymes to restore activity to heterologous membranes in the absence of immunological similarities between their respective δ and ε subunits may suggest that structural features other than particular amino acid sequences of these subunits are paramount in their roles in binding F₁ to the membrane and in sealing proton leaks.