Structure of alpha-latrotoxin oligomers reveals that divalent cation-dependent tetramers form membrane pores

We report here the first three-dimensional structure of alpha-latrotoxin, a black widow spider neurotoxin, which forms membrane pores and stimulates s ecretion in the presence of divalent cations, We discovered that alpha-latr otoxin exists in two oligomeric forms: it is dimeric in EDTA but forms tetr amers in the presence of Ca2+ or Mg2+. The dimer and tetramer structures we re determined independently at 18 Angstrom and 14 Angstrom resolution, resp ectively, using cryo-electron microscopy and angular reconstitution. The al pha-latrotoxin monomer consists of three domains. The N- and C-terminal dom ains have been identified using antibodies and atomic fitting. The C4-symme tric tetramers represent the active form of alpha-latrotoxin; they have an axial channel and can insert into lipid bilayers with their hydrophobic bas e, providing the first model of alpha-latrotoxin pore formation.