Fig. 5.

Comparison of hexamer rings of open and closed Munc13C found in the 2D crystal. (A) Depiction of the innermost ring of six Munc13C molecules from the trimeric hexamer of open Munc13C molecules found in the crystal (Fig. 2B). Positions of medial and outer subunits are indicated by empty silhouette outlines. (B) Close-up view of the six inner Munc13C molecules in A. (C) C2B domains (pale blue) from the six open Munc13C molecules in A and B aligned with C2B domains (dark blue) from the hexagonal cage of closed Munc13C molecules (Fig. 3). The two C2B arrangements are related by rigid-body translations of ∼1.2 nm corresponding to outward movement of the C2B domains from the center. (D) View of the hexameric cage of closed Munc13C molecules, oriented so its C2B domains are aligned with C2B domains of the six open Munc13C molecules in B. Comparison of open and closed Munc13C arrangements from B and D, respectively, reveals an economic motion in which the C2B “feet” splay outwards, while jackknife closure by the rest of the molecule brings C2C domains into contact with their counterclockwise, lateral neighbors. See Movie S2.