Maurotoxin and the Kv1.1 channel: voltage-dependent binding upon enantiomerization of the scorpion toxin disulfide bridge Cys(31)-Cys(34)

Maurotoxin (MTX) is a 34-amino acid polypeptide cross-linked by four disulf ide bridges that has been isolated from the venom of the scorpion Scorpio m aurus palmatus and characterized. Maurotoxin competed with radiolabeled apa min and kaliotoxin for binding to rat brain synaptosomes and blocked K+ cur rents from Kv1 channel subtypes expressed in Xenopus oocytes. Structural ch aracterization of the synthetic toxin identified half-cystine pairings at C ys(3)-Cys(24), Cys(9)-Cys(29) Cys(13)-Cys(19) and Cys(31)-Cys(34.) This dis ulfide bridge pattern is unique among known scorpion toxins, particularly t he existence of a C-terminal '14-membered disulfide ring' (i.e. cyclic doma in 31-34), We therefore studied structure-activity relationships by investi gating the structure and pharmacological properties of synthetic MTX peptid es either modified at the C-terminus {i.e. MTX(1-29), [Abus(31),(34)]-MTX a nd [Cys(31),(34), Tyr(32)](D)-MTX} or mimicking the cyclic C-terminal domai n [i.e. MTX(31-34)]. Unexpectedly, the absence of a disulfide bridge Cys(31 )-Cys(34) i, [Abu (31,34)]-MTX and MTX(1-29) resulted in MTX-unrelated half -cystine pairings of the three remaining disulfide bridges for the two anal ogs, which is likely to be responsible for their inactivity against Ky1 cha nnel subtypes. Cyclic MTX(31-34) was also biologically inactive. [CYS31,34, Tyr(32)](D)-MTX, which had a 'native', MTX-related, disulfide bridge organ ization, but a D-residue-induced reorientation of the C-terminal disulfide bridge, was potent at blocking the Kv1.1 channel. This peptide-induced Kv1. 1 blockage was voltage-dependent (a property not observed for MTX), maximal in the low depolarization range and associated with on-rate changes in lig and binding. Thus, the cyclic C-terminal domain of MTX seems to be crucial for recognition of Kv1.3, and to a lesser extent, Kv1.2 channels and it may contribute to the stabilization and strength of the interaction between th e toxin and the Kv1.1 channel.