Maurotoxin is a toxin isolated from the venom of the Tunisian chactoid scorpion Scorpio maurus. It is a 34-amino-acid peptide cross-linked by four disulfide bridges. Maurotoxin competes with radiolabeled apamin and kaliotoxin for binding to rat-brain synaptosomes. Due to its very low concentration in venom (0.6% of the proteins), maurotoxin was chemically synthesized by means of an optimized solid-phase technique. The synthetic maurotoxin was characterized. It was lethal to mice following intracerebroventricular injection (LD50, 80 ng/mouse). The synthetic maurotoxin competed with 125I-apamin and 125I-kaliotoxin for binding to rat-brain synaptosomes with half-maximal effects at concentrations of 5 nM and 0.2 nM, respectively. Synthetic maurotoxin was tested on K+ channels and was found to block the Kv1.1, Kv1.2, and Kv1.3 currents with half-maximal blockage (IC50) at 37, 0.8 and 150 nM, respectively. Thus, maurotoxin is a scorpion toxin with four disulfide bridges that acts on K+ channels. The half-cystine pairings of synthetic maurotoxin were identified by enzymatic cleavage. The pairings were Cys3-Cys24, Cys9-Cys29, Cys13-Cys19 and Cys31-Cys34. This disulfide organization is unique among known scorpion toxins. The physicochemical and pharmacological properties of synthetic maurotoxin were indistinguishable from those of natural maurotoxin, which suggests that natural maurotoxin adopts the same half-cystine pairing pattern. The conformation of synthetic maurotoxin was investigated by means of circular dichroism spectroscopy and molecular modeling. In spite of its unusual half-cystine pairings, the synthetic-maurotoxin conformation appears to be similar to that of other short scorpion toxins.