Purification, characterization, and biosynthesis of margatoxin, a component of Centruroides margaritatus venom that selectively inhibits voltage-dependent potassium channels.

A novel peptidyl inhibitor of K+ channels has been purified to homogeneity from venom of the new world scorpion Centruroides margaritatus. The primary structure of this 39-amino-acid peptide, which we term margatoxin (MgTX), was determined by amino acid compositional analysis and peptide sequencing. Margatoxin potently inhibits binding of radiolabeled charybdotoxin (ChTX) to voltage-activated channels in brain synaptic plasma membranes. Like ChTX, MgTX blocks the n-type current of human T-lymphocytes (Kv1.3 channel), but compared to ChTX, is 20-fold more potent (half-block at approximately 50 pM), has a slower dissociation rate, and has no effect on calcium-activated channels. To demonstrate that these characteristics are due solely to the purified toxin, recombinant MgTX was expressed in Escherichia coli as part of a fusion protein. After cleavage and folding, purified recombinant MgTX displayed the same properties as native peptide. Replacement of the COOH-terminal histidine residue of MgTX with asparagine resulted in a peptide with a 10-fold reduction in potency. This was due to a faster apparent dissociation rate, suggesting that the COOH-terminal amino acid may play an important role in the binding of MgTX to the Kv1.3 channel. MgTX displays significant sequence homology with previously identified K+ channel inhibitors (e.g. ChTX, iberiotoxin, noxiustoxin, and kaliotoxin). However, given its potency and unique selectivity, MgTX represents an especially useful tool with which to study the physiologic role of Kv1.3 channels.