MAUROTOXIN, A 4 DISULFIDE BRIDGES SCORPION TOXIN ACTING ON K+ CHANNELS

Maurotoxin, a toxin from the venom of the Tunisian chactoid scorpion S corpio maurus, has been purified to homogeneity by gel filtration/reve rsed-phase HPLC, and characterized. It is a basic and C-terminal amida ted 34-residue polypeptide cross-linked by four disulfide bridges. Fro m Edman sequencing results, only six different pairings between the fi rst six half-cystines were retained whereas a disulfide bridge was pre dicted between the two half-cystines in positions 31 and 34. Modelling based on the structure of charybdotoxin favored two different pairing s, one of which possessed two disulfides in common with the general mo tif of scorpion toxins. The solid-phase technique was used to obtain s ynthetic maurotoxin, sMTX. The half-cystine pairings of sMTX were dete rmined by enzymatic cleavage and were found to be Cys3-Cys24, Cys9-Cys 29, Cys13-Cys19, and Cys31-34, in agreement with experimental data obt ained with natural maurotoxin. Both natural and synthetic maurotoxins were lethal to mice following intracerebroventricular injection (LD50, 80 ng/mouse). They blocked the Kv1.1, Kv1.2, and Kv1.3 channels expre ssed in Xenopus oocytes with almost identical half-effects (IC50) in t he range of 40, 0.8 and 150 nM, respectively. They also competed with I-125-apamin (SKca channel blocker) and I-125-kaliotoxin (Kv channel b locker) for binding to rat brain synaptosomes with IC50 Of about 5 and 0.03 nM. As the natural and synthetic maurotoxins exhibit indistingui shable physicochemical and pharmacological properties, they are likely to adopt the same half-cystine pairing pattern which is unique among known scorpion toxins. However, this disulfide organization is differe nt from those reported for Pandinus imperator and Heterometrus spinnif er toxins 1 (Pi1 and HsTxl), two novel four-disulfide bridged K+ chann el-acting scorpion toxin sharing about 50-70% sequence identity with m aurotoxin. (C) 1998 Elsevier Science Ltd. All rights reserved.