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.