Molecular mass, stoichiometry, and assembly of 20 s particles

N-Ethylmaleimide-sensitive factor (NSF), soluble NSF attachment proteins (SNAPs), and SNAP receptor (neuronal SNARE) complexes form 20 S particles with a mass of 788 +/- 122 kDa as judged by scanning transmission electron microscopy. A single NSF hexamer and three alphaSNAP monomers reside within a 20 S particle as determined by quantitative amino acid analysis. In order to study the binding of alphaSNAP and NSF in solution, to define their binding domains, and to specify the role of oligomerization in their interaction, we fused domains of alphaSNAP and NSF to oligomerization modules derived from thrombospondin-1, a trimer, and cartilage oligomeric matrix protein, a pentamer, respectively. Binding studies with these fusion proteins reproduced the interaction of alphaSNAP and NSF N domains in the absence of the hexamerization domain of NSF (D2). Trimeric alphaSNAP (or its C-terminal half) is sufficient to recruit NSF even in the absence of SNARE complexes. Furthermore, pentameric NSF N domains are able to bind alphaSNAP in complex with SNAREs, whereas monomeric N domains do not. Our results demonstrate that the oligomerization of both NSF N domains and alphaSNAP provides a critical driving force for their interaction and the assembly of 20 S particles.