The functional role of positively charged amino acid side chains in alpha-bungarotoxin revealed by site-directed mutagenesis of a His-tagged recombinant alpha-bungarotoxin.

Abstract

A polyhistidine tag was added to the N-terminus of alpha-bungarotoxin (Bgtx) recombinantly expressed in E. coli. The His-tagged Bgtx was identical to native, venom-derived Bgtx in its apparent affinity for the nicotinic acetylcholine receptor (nAChR) in Torpedo electric organ membranes. Furthermore, in a physiological assay involving mouse muscle nAChR expressed in Xenopus oocytes, the His-tagged Bgtx was as effective as authentic Bgtx at blocking acetylcholine-evoked currents. Ala-substitution mutagenesis of His-tagged Bgtx was used to evaluate the functional contribution of Arg36, a residue that is invariant among all alpha-neurotoxins. Replacement with Ala resulted in a 90-fold decrease in the apparent affinity for the Torpedo nAChR and a corresponding 150-fold increase in the IC50 for block of heterologously expressed mouse muscle nAChR, demonstrating the critical importance of this positive charge for the binding and functional activity of a long alpha-neurotoxin. The observed decrease in affinity corresponds to a DeltaDeltaG of 2.7 kcal/mol and indicates that Arg36 makes a major contribution to complex formation. This finding is consistent with the proposal that Arg36 mimics the positive charge found on acetylcholine and directs the toxin to interact with receptor sites normally involved in acetylcholine recognition. In comparison, Ala-substitution of the highly conserved Lys26 resulted in only a 9-fold decrease in apparent affinity. Truncation of the His-tagged Bgtx following residue 67 produces a toxin lacking the seven C-terminal residues including the two positively charged residues Lys70 and Arg72. Truncation leads to a 7-fold decrease in apparent binding affinity.