Fig. 5.

Ca²⁺ disrupts the Syt1^C2AB–SNARE organization on the lipid membrane. Cryo-EM micrograph of the Syt1^C2AB-SNARE decorated lipid tube in the presence of 1 mM Ca²⁺ (top left) shows that proteins heavily decorate the LNT surface without forming any pronounced organized structures. Correspondingly, Fourier transform of the vertically oriented tube (top right) shows smeared diffraction peaks. Magnification of the decorated LNTs (bottom) shows regions of both densely (right) and sparsely (left) packed protein on the LNT surface (red arrows), with scattered elongated densities (cyan arrows) extending out from the lipid surface. The protein density on the membrane surface (red arrows) is best approximated by the Syt1 C2 domain, with the SNAREpins corresponding to the auxiliary density (cyan arrow) on the top. It appears that the SNAREpins are displaced from the primary-binding site on the Syt1 protein