Figure 19. Concentric β-barrel models for Type 1A trimer. (a) Wedge
schematic of a cross-section of the barrels. (b & c) Flattened
representations of the inner and outer barrels of the trimer. The dashed
line indicates the location where Sy5 and Ct1 of the outer barrels link
to Sy6 and Sy8 of the inner barrel. Green and red backgrounds as in Fig.
5. (d) Ribbon representation of an atomic-scale model as viewed from the
side. Ribbons are colored by the rainbow spectrum beginning with red at
the N-termini and ending with magenta at the C-termini.
We suggest that if and/or when either of these structures interact with
the membrane, the NAC domains traverses the alkyl phase as a β-barrel
with K80 of the β-turn region interacting with negatively charged lipid
head-groups of the trans bilayer while the Nt domains remain on the
surface of the cis bilayer as partially-occupied amphipathic α-helices.
These types of trimers may serve as precursors for larger transmembrane
channels.