Formation of the b Subunit Dimer Is Necessary for Interaction with F ₁ -ATPase

In earlier work, we [McCormick, K. A., et al. (1993) J. Biol. Chem. 268, 24683-24691] observed that mutations at Ala-79 of the b subunit affect assembly of F1F0 ATP synthase. Polypeptides modeled on the soluble portion of the b subunit (bsol) with substitutions at the position corresponding to Ala-79 have been used to investigate secondary structure and dimerization of the b subunit. Circular dichroism spectra and chymotrypsin digestion experiments suggested that the recombinant polypeptides with Ala-79 substitutions assumed conformations similar to the bsol polypeptide. However, cross-linking studies of the Ala-79 substitution bsol polypeptides revealed defects in dimerization. The efficiency of dimer formation appeared to be related to the capacity of the altered bsol polypeptides for competing with F1-ATPase for binding to F1-depleted membrane vesicles. Ala-79 substitution polypeptides displaying limited dimerization, such as bsol Ala-79-->Leu, were shown to elute with F1-ATPase during size exclusion chromatography, suggesting a specific interaction. Sedimentation equilibrium studies indicated that 8% of the bsol Ala-79-->Leu polypeptide was in the form of a 30.6 kDa dimer and 92% a 15.3 kDa monomer. When the dimer concentration of bsol Ala-79-->Leu was normalized to the concentration of bsol, both had virtually identical capacities for competing with F1-depleted membrane vesicles for binding F1-ATPase. The result indicated that the amount of dimer formed is directly proportional to its ability to bind F1-ATPase. This suggests that formation of the b subunit dimer may be a necessary step preceding F1-ATPase binding in the assembly of the enzyme complex.