Figure 14. Images and models of α-Syn and β-Syn stacked lipoprotein
discs. (a) Two sizes of α-Syn stacked pairs obtained by image averaging.
(b) Individual images of relatively long stacks for both α- and β-Syn.
(Images modified from original EM images of Eichmann et al. [19]).
(c) Side views of schematics of our models. The darkly shaded regions in
the middle of and between the cylinders represent lipid-filled regions.
The gray cylinders represent concentric β-barrels formed by the Nt and
NAC domains from two discs; the magenta cylinders represent 32-stranded
β-barrels formed by interlocking Ct domains from adjacent stacks. (d) A
schematic model for the central stack illustrating variations of the
numbers of octamers in adjacent discs of the stack above. (e) Wedge
cross-sections representations of the Nt-NAC barrel regions of the
octamers within a disc. The checkered regions inside and between the
octamers represent lipid alkyl chains.
We considered several models for the basic components of the α- and
β-Syn discs. The Type 2P octamer model illustrated in Figs. 15 and
Appendix A1 were the most consistent with dimensions obtained from the
EM images. Sy2,4,6 form an inner 24-stranded β-barrel and Sy1,3,5,8 form
a 32-stranded outer β-barrel. Four-stranded Ct β-sheets from adjacent
octamers interlock to form a 32-stranded antiparallel β-barrel that
connects octamers. The pore through the inner β-barrel is lined with
apolar side-chains and is ~ 3 nm in diameter. This
region is proposed to surround sphingolipid alkyl chains. The entrances
to this pore are surrounded by positively charged lysines of the
signature connectors. These may interact with negatively charged
phosphate moieties of the lipid head-groups. The negatively-charged Ct
β-barrel is proposed to surround the positively charged portions of the
sphingolipid head-groups. The calculated distance between discs based on
these model is ~ 7.6 nm, about the same as observed in
the micrographs.
The next challenge was to account for the lightly-shaded vertical
columns that are ~3.5 nm apart in the image-averaged
discs (Fig. 14a). The Type 2P octamer structure has four-fold radial
symmetry and 2-fold perpendicular symmetry. Adjacent octamers within the
same disc likely form a square lattice in which all contacts are
identical and occur at regions of 2-fold symmetry (Fig. 14 e). In an
aqueous environment these contacts would likely occur in the most
hydrophobic regions; i.e., between antiparallel pairs of Sy8 strands.
However, in a hydrophobic environment the contacts would likely occur
between more polar segments; i.e., at antiparallel pairs of Sy1 strands
and adjacent Sy3 strands. The latter case would occur if lipids occupy
the regions between the octamers, as illustrated in Fig. 14e. Positively
charged lysines at the ends of the Sy5-8 strands could interact with
negatively charged components of inter-octamer lipid head groups, and
negatively charged side-chains on the exterior of the Ct β-barrels could
interact with positively charged components of these lipids. This
arrangement creates two types of lipid columns (intra-octamer and
inter-octamer). If the diameter of the octamers is ~7
nm, these columns would be ~ 3.5 nm apart.