tetramers
The Type 2A tetramer model of Fig. 7 resembles the α-β-barrel of Fig. 4 except that the Nt domains have transitioned from helical hairpins to 5-stranded antiparallel β-sheets. The NAC β-hairpins still form an 8-stranded antiparallel core β-barrel, but now it is surrounded by a 20-stranded antiparallel β-barrel formed by Nt. Most side-chains of the central interior of the inner NAC barrel are small and apolar, alanines and glycines. The interacting faces of the pleats are composed primarily of small side-chains (G, A, V, and T for the inner barrel and G, A, V, and S for the outer barrel), facilitating relative tight packing between the barrels (gap distance ~ 0.95 nm). The major advantages of this model are its simplicity, the facts that the most hydrophobic segments form its core and most polar side-chains are on the surfaces, the negatively charged glutamate side-chains of the signature loops can all salt-bridge to positively charged lysines, and the core NAC β-barrel is the same as proposed for the α-β-barrel tetramer model.