B Example histograms of smFRET efficiency of Syx1 in complex with Munc18-1 WT in the absence and presence of the MUN domain, and within the SNARE complex. C−E Example histograms of smFRET efficiency of Syx1 bound to Munc18-1 QKAA (C), KKEE (D), and L348R (E) in the absence or presence of the MUN domain. The QKAA mutation strongly impaired the shift of smFRET efficiency in the presence of the MUN domain. 