The Intermembrane Space Loop of Subunit b (4) Is a Major Determinant of the Stability of Yeast Oligomeric ATP Synthases

The involvement of the b-subunit, subunit 4 in yeast, a component of the peripheral stalk of the ATP synthase, in the dimerization/oligomerization process of this enzyme was investigated. Increasing deletions were introduced by site-directed mutagenesis in the loop located in the mitochondrial intermembrane space and linking the two transmembrane (TM) segments of subunit 4. The resulting strains were still able to grow on nonfermentable media, but defects were observed in ATP synthase dimerization/oligomerization along with concomitant mitochondrial morphology alterations. Surprisingly, such defects, already depicted in the absence of the so-called dimer-specific subunits e and g, were found in a mutant harboring a full amount of subunit g associated to the monomeric form of the ATP synthase. Deletion of the intermembrane space loop of subunit 4 modified the profile of cross-linking products involving cysteine residues belonging to subunits 4, g, 6, and e. This suggests that this loop of subunit 4 participates in the organization of surrounding hydrophobic membranous components (including the two TM domains of subunit 4) and thus is involved in the stability of supramolecular species of yeast ATP synthase in the mitochondrial membrane.