We have investigated the question of the presence of a cap structure located at the top of the F1 α3β3 hexamer of the yeast mitochondrial F1F0-ATP synthase complex. Specifically, we sought to determine whether the putative cap has a rigid structure and occludes the central shaft space formed by the α3β3 hexamer or alternatively whether the cap is more flexible permitting access to the central shaft space under certain conditions. Thus, we sought to establish whether subunit γ, an essential component of the F1 central stalk housed within the central shaft space and whose N and C termini would both lie beneath a putative cap, could be fused at its C terminus to green fluorescent protein (GFP) without loss of enzyme function. The GFP moiety serves to report on the integrity and location of fusion proteins containing different length polypeptide linkers between GFP and subunit γ, as well as being a potential occluding structure in itself. Functional incorporation of subunit γ-GFP fusions into ATP synthase of yeast cells lacking native subunit γ was demonstrated by the ability of intact complexes to hydrolyze ATP and retain sensitivity to oligomycin. Our conclusion is that the putative cap structure cannot be an inflexible structure, but must be of a more flexible nature consistent with the accommodation of subunit γ-GFP fusions within functional ATP synthase complexes.