Sphingomylein-γ-Syn lipoproteins form elongateded assemblies (called
filaments here) rather than stacked discs [19]. Most of these
filaments are isolated; however some occur as parallel pairs. These
pairs helped us propose that two sizes of filaments occur with diameters
of ~ 7 - 8 and 12 nm (Fig. 16). Unfortunately, it is
more difficult to assertain the dimensions and periodicity of these
filaments than for the stacked-discs.
Figure 16. Negatively stained EM images of γ-Syn filament pairs and
schematics of our models. (a) Smaller filaments. Image on the far left
is averaged from 51 pairs and that on the right is an individual
filament pair. The yellow rectangles represent models of Υ-Syn octamers
stacked end-on. (b) Averaged and an individual image for larger
filaments. Red rectangles represent models of hexadecamers stacked
end-on. (c). Flattened represention for two subunits of the octamer
inner barrel formed by Sy2,3,8. (d) Flattened representation for two
subunits of outer 32-stranded β-barrels formed by Sy1,3,5,6 and by the
Sy7 β-hairpin connecting Sy6 to Sy8, and four short β-strands of the Ct
domain. The last two Ct strands could be from the same octamers as
illustrated or from adjacent octamers. These barrels would stack end-on
to form the outer walls of the filaments. (e) Wedge representation of a
central cross-section of the two concentric β-barrels. The central
region would be filled with lipid alkyl chains. The β-barrel parameters
of a hexadecamer would be N = 48, S/N = 1.0, D = 9.2 nm, P = 21 nm and N
= 64, S/N = 0.75, D = 11.1 nm, P = 19 nm.
The negatively charged Ct domain of γ-Syn is substantially shorter (most
of Ct1 and Ct2 are missing) than those of α- and β-Syn and the sequences
are difficult to align unambiguously. In this model four short segments
of the Ct-domains interact with Sy7 β-hairpins and portions of Sy5 and
Sy6 to form a negatively charged region of the pore that interacts with
positively charged components of sphingomylein headgroups. The location
of the last two Ct strands could depend upon interactions with other
octamers; if there is no interaction they could have the positions
indicated in the figure; if there is end-to-end interaction they could
interact with Sy7 hairpins of the adjacent octamer. This region of the
structure has some positively charged residues, but their side-chains
are all oriented outwardly. As in the other models this Ct barrel is in
series with Nt-NAC concentric β-barrels proposed to surround the lipid
alkyl chains and acidic headgroup moieties. This model is consistent
with sphingolipids binding inside the β-barrels but not between the
β-barrels.
The γ-Syn lipoproteins also produce some substantially larger filaments
(Fig. 16b). The diameter of the larger filaments is consistent with that
predicted for assemblies with twice as many monomers as in the smaller
filaments; i.e. , with a hexadecamer. S/N = 1.0 for the inner and
Ct barrels in both models, however, the strands of the outer barrel are
more tilted in the octamer than in the hexadecamer, making it more
consistent with pleats-into-pleats packing.