Insecticidal toxins from black widow spider venom.

The biological effects of Latrodectus spider venom are similar in animals from different phyla, but these symptoms are caused by distinct phylum-specific neurotoxins (collectively called latrotoxins) with molecular masses ranging from 110 to 140 kDa. To date, the venom has been found to contain five insecticidal toxins, termed alpha, beta, gamma, delta and epsilon-latroinsectotoxins (LITs). There is also a vertebrate-specific neurotoxin, alpha-latrotoxin (alpha-LTX), and one toxin affecting crustaceans, alpha-latrocrustatoxin (alpha-LCT). These toxins stimulate massive release of neurotransmitters from nerve terminals and act (1) by binding to specific receptors, some of which mediate an exocytotic signal, and (2) by inserting themselves into the membrane and forming ion-permeable pores. Specific receptors for LITs have yet to be identified, but all three classes of vertebrate receptors known to bind alpha-LTX are also present in insects. All LTXs whose structures have been elucidated (alpha-LIT, delta-LIT, alpha-LTX and alpha-LCT) are highly homologous and have a similar domain architecture, which consists of a unique N-terminal sequence and a large domain composed of 13-22 ankyrin repeats. Three-dimensional (3D) structure analysis, so far done for alpha-LTX only, has revealed its dimeric nature and an ability to form symmetrical tetramers, a feature probably common to all LTXs. Only tetramers have been observed to insert into membranes and form pores. A preliminary 3D reconstruction of a delta-LIT monomer demonstrates the spatial similarity of this toxin to the monomer of alpha-LTX.

The multiple actions of black widow spider toxins and their selective use in neurosecretion studies.

The black widow spider venom contains several large protein toxins--latrotoxins--that are selectively targeted against different classes of animals: vertebrates, insects, and crustaceans. These toxins are synthesised as large precursors that undergo proteolytic processing and activation in the lumen of the venom gland. The mature latrotoxins demonstrate strong functional structure conservation and contain multiple ankyrin repeats, which mediate toxin oligomerisation. The three-dimensional structure has been determined for alpha-latrotoxin (alphaLTX), a representative venom component toxic to vertebrates. This reconstruction explains the mechanism of alphaLTX pore formation by showing that it forms tetrameric complexes, harbouring a central channel, and that it is able to insert into lipid membranes. All latrotoxins cause massive release of neurotransmitters from nerve terminals of respective animals after binding to specific neuronal receptors. A G protein-coupled receptor latrophilin and a single-transmembrane receptor neurexin have been identified as major high-affinity receptors for alphaLTX. Latrotoxins act by several Ca(2+)-dependent and -independent mechanisms based on pore formation and activation of receptors. Mutant recombinant alphaLTX that does not form pores has been used to dissect the multiple actions of this toxin. As a result, important insights have been gained into the receptor signalling and the role of intracellular Ca(2+) stores in the effect of alphaLTX.

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 secretion in the presence of divalent cations. We discovered that alpha-latrotoxin exists in two oligomeric forms: it is dimeric in EDTA but forms tetramers in the presence of Ca2+ or Mg2+. The dimer and tetramer structures were determined independently at 18 A and 14 A resolution, respectively, using cryo-electron microscopy and angular reconstitution. The alpha-latrotoxin monomer consists of three domains. The N- and C-terminal domains have been identified using antibodies and atomic fitting. The C4-symmetric tetramers represent the active form of alpha-latrotoxin; they have an axial channel and can insert into lipid bilayers with their hydrophobic base, providing the first model of alpha-latrotoxin pore formation.