THE ALPHA-1-BETA-1 HETERODIMER, THE UNIT OF ATP SYNTHASE

The alpha-3-beta-3 hexamer (M(r) 319582) was reconstituted from the alpha and beta subunits of the TF1 portion of ATP synthase of thermophilic bacterium PS3 (Kagawa, Y. et al. (1989) FEBS Lett. 249, 67-69). The radius of gyration (R(g)) of the alpha-3-beta-3 hexamer determined by small-angle X-ray scattering was 4.64 +/- 0.03 nm. However, in the presence of A(D)P-Mg, R(g) of the alpha-beta complex was markedly reduced to 3.47 +/- 0.02 nm, indicating the dissociation of the hexamer. The heterodimer was isolated by gel-permeation chromatography of the alpha-3-beta-3 hexamer in the presence of AT(D)P-Mg. Judging from the apparent molecular weight by the gel permeation chromatography, the dissociation product was the alpha-1-beta-1 heterodimer (M(r) = 106 524). On gel electrophoresis, both the dimer and hexamer gave bands of material with ATPase activity (relative mobilities: TF1:alpha-3-beta-3:alpha:alpha-1-beta-1:beta = 1:1.3:2.1:2.9:3.6, in 7.5% polyacrylamide gel at pH 8.8). The dissociation of the hexamer was induced by IT(D)P, but not by unyhydrolyzable ATP analogues - Mg, P(i)-Mg and Mg. During gel-permeation column chromatography in the presence of ATP-Mg, the ATPase activity appeared before the peak of the heterodimer (about 100 kDa). This observation strongly suggests the interconversion of the alpha-1-beta-1 dimer to the alpha-3-beta-3 hexamer during catalysis.