D The flexibility of SM proteins in domain 3a and domain 1. Structures of rat Munc18-1 WT in complex with closed Syx1, human Munc18-2, nSec1 from squid, and rat Munc18-1 WT in complex with mouse Syx4 N-peptide were aligned and superposed with rat Munc18-1 K332E/K333E. Enlarged views of domain 3a and domain 1 are shown in dashed boxes. 