Robustoxin (δ-ACTX-Ar1), sometimes misspelled robustotoxin, and alternatively known as delta atracotoxin, is a low-molecular-weight neurotoxic polypeptide found in the venom of the Sydney funnel-web spider (Atrax robustus).
Delta atracotoxin produces potentially fatal neurotoxic symptoms in primates by slowing the inactivation of sodium ion channels in autonomic and motor neurons. In the spiders' intended insect prey, the toxin exerts this same activity upon potassium and calcium ion channels.
The structure of atracotoxin comprises a core beta region with a cystine knot motif, a feature seen in other neurotoxic polypeptides.