The success and efficiency of fusion of transport vesicles to target membranes depend upon sets of proteins that are functionally and evolutionarily conserved. The solubleN-ethylmaleimide-sensitive fusion (NSF)-attachment protein receptor (SNARE) family and the Sec1/Munc18 family, also termed SM proteins, are central players in this process. SM proteins interact with syntaxins of SNARE family, which has been regarded intrinsic for function. By exploring the bi-molecular interactions of SM proteins with syntaxins and their functional implicationsin vivoin several eukaryotes, which has been enormously facilitated by the availability of the three-dimensional structures of members of both protein families, the long-standing assumption that SM proteins fulfill their regulatory role by binding to syntaxins has to be reconsidered.