Reactivation of the chloroplast CF₁-ATPase β subunit by trace amounts of the CF₁ α subunit suggests a chaperonin-like activity for CF(1α)

Incubation of tobacco and lettuce thylakoids with 2 M LiCl in the presence of MgATP removes the β subunit from their CF₁-ATPase (CF₁β) together with varying amounts of the CF₁ α subunit (CF₁α). These 2 M LiCl extracts, as with the one obtained from spinach thylakoids (Avital, S., and Gromet-Elhanan, Z. (1991) J. Biol. Chem. 266, 7067-7072), could form active hybrid ATPases when reconstituted into inactive β-less Rhodospirillum rubrum chromatophores. Pure CF₁β fractions that have been isolated from these extracts could not form such active hybrids by themselves, but could do so when supplemented with trace amounts (less than 5%) of CF₁α. A mitochondrial F₁-ATPase α subunit was recently reported to be a heat-shock protein, having two amino acid sequences that show a highly conserved identity with sequences found in molecular chaperones (Luis, A.M., Alconada, A., and Cuezva, J.M. (1990) J. Biol. Chem. 265, 7713-7716). These sequences are also conserved in CF₁α isolated from various plants, but not in F₁β subunits. The above described reactivation of CF₁β by trace amounts of CF₁α could thus be due to a chaperonin-like function of CF₁α, which involves the correct, active folding of isolated pure CF₁β.