Quaternary structure of chloroplast F1‐ATPase in solution

Abstract

The hydrodynamic properties of isolated ATPases were studied via their rotational diffusion in buffer solution. Chloroplast F₁‐ATPase (CF₁) and Escherichia coli F₁‐ATPase (EF₁) were covalently labeled with eosin‐isothiocyanate and then investigated by polarized laser spectroscopy. The rotational correlation time in aqueous buffer of latent (five‐subunit) CF₁ was 390 ns. Four‐subunit (δ‐deficient) CF₁ showed the same correlation time, however, for three‐subunit (δ, ɛ‐deficient) CF₁ the rotational correlation time was more than eight times larger (3200 ns). The rotational correlation time of activated CF₁ was three times larger than the one of latent CF₁. These large changes in the rotational correlation times are directly related to changes in the quaternary structure of CF₁ upon activation. EF₁ was found to behave essentially as activated CF₁. Based on the observed rotational correlation times we concluded that the mass distributions of latent CF₁ and of δ‐deficient CF₁ resemble a dimeric arrangement. The structure of δ,ɛ‐deficient CF₁ more likely resembles a hexagon, the mass centers of the six main subunits lie in one plane. The structure of the activated forms of CF₁ can be described best as an intermediate between the dimeric arrangement of latent CF₁ and an octahedron. The large changes in the quaternary structure of isolated CF₁ are reversed when the activation of the enzyme is reversed.