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.