B,C Structure alignment of domain 3a with various SM proteins. Crystal structure of domain 3a from rat Munc18-1 K332E/K333E was aligned and superposed with rat Munc18-1 WT in complex with closed Syx1 (pdb entry: 3C98) (B), and with human Munc18-2 (pdb entry:4CCA), nSec1 from squid (pdb entry:1FVH), and rat Munc18-1 WT in complex with mouse Syx4 N-peptide (pdb entry: 3PUJ) (C). 