Figure 18. (a) EM image of a predominately α-helical trimer (Copied from Wang et al. [41]) and (b-e) a Type 1 α-β barrel trimer model. This model resembles the tetramer of Fig. 5 except that there is one less subunit and all subunits are oriented in the same direction. (b) wedge representation, (c) side view schematic of helices, (d) side view of two subunits of the outer Nt helices, (e) flattened representation of two subunits of a core 6-stranded antiparallel β-barrel formed by three NAC β-hairpins.
Type 1A trimer
The α-β barrel trimer may transition to a concentric β-barrel structure in which the Nt domains form an outer 15-stranded antiparallel β-barrel that surrounds the core NAC β-barrel (Fig. 19). We examined this in greater detail by developing an atomic-scale model. The small size and apolar nature of the side-chains inside the core barrel and between the two barrels allows the gap distance between the two barrels to be small (~ 0.8 nm) without disrupting backbone H-bonding within each barrel. The putative15-stranded Ct β-barrel structure is highly tentative; this domain is likely to be relatively disordered.