Fig. 1.

Coarse-grained model of Syt C2AB domain and oligomeric Syt rings. (A) Synaptic vesicles carry ∼15 copies of Syt, with each comprising a TMD, a flexible juxtamembrane LD, and the C2A and C2B globular domains (schematic, not to scale). The polylysine patch (blue) on C2B mediates Ca²⁺-independent binding to anionic membranes. (B) Crystal structure of Syt (PDB ID: 2R83) (Left) and our coarse-grained representation (Right). Each C2 domain is represented as two overlapping beads, radius $R_{\text{bead}}$, giving length $a=5$ nm and width $b=3$ nm. The positively charged polylysine patch (blue) on C2B is treated as a point charge. (C) Coarse-grained model representation of a Syt ring of radius R, Top view. (D) Rendition of a Syt ring bound to a membrane, based on EM reconstruction of Syt oligomers assembled on phospholipid monolayer tubes (43) (cyan, C2B; gray, C2A; blue, polylysine patch; yellow, Ca²⁺ binding loops on C2B). (E) C2AB subunit in a ring interacting with planar membrane (Side view). The polylysine patch lies below the plane of the ring, at an angle $\theta =30°$ suggested by EM reconstruction, D. In the ring, the C2AB unit cannot rotate downward, so electrostatic attraction bends the charged membrane up toward the patch (Right).