G Cartoon illustration of the templating mechanism of domain 3a. The extended domain 3a (H11 and H12) binds to Syx1-H3 and Syb2 simultaneously. Color codes are as follows: Munc18-1, light gray; Munc18-1 H11 and H12, cyan; Syx1-H3, blue; Syb2, red. Key binding sites (L348, K332, and K333) on the H12 are highlighted with orange spheres. 