Three-dimensional (3D) structural model and neutron scattering length densities of the tetradecamer of proteasomal α7 subunit computed from the 3D structural model. (A) Top and side views of the double-ring structure of the α7 tetradecamer derived from the SANS data in conjunction with the crystal structure of the 20S core particle (19,23). Each α7 heptameric ring with sevenfold symmetry is drawn in green or cyan. The diameter and thickness are ∼120 Å and 50 Å, respectively. (B) Neutron scattering length densities of tetradecamers of proteasomal α7 composed of different numbers of h-α7 and d-α7 subunits computed from its 3D structural model, shown with bars having color gradient. Blue and red broken lines express the scattering length densities of h-α7 (ρh) and d-α7 (ρd) tetradecamers, respectively, whereas cyan, orange, and purple solid lines express the scattering length densities of H2O, D2O, and 81% D2O, respectively. The arrows indicate the scattering contrasts of tetradecamers in 81% D2O solution.