Do structural deviations between toxins adopting the same fold reflect functional differences?

Three-finger proteins form a structurally related family of compounds that exhibit a great variety of biological properties. To address the question o f the prediction of functional. areas on their surfaces, we tentatively con ferred the acetylcholinesterase inhibitory activity of fasciculins on a sho rt-chain curaremimetic toxin. For this purpose, we assimilated the three-di mensional structure of fasciculin 2 with the one of toxin a. This compariso n revealed that the tips of the first and second loops, together with the C terminus residue, deviated most. A first recombinant fasciculin/toxin a ch imera was designed by transferring loop 1 in its entirety together with the tip of loop 2 of fasciculin 2 into the toxin a scaffold. A second chimera (rChII) was obtained by adding the point Asn-61 --> Tyr substitution. Compa rison of functional and structural properties of both chimeras show that rC hII can accommodate the imposed modifications and displays nearly all the a cetylcholinesterase-blocking activities of fasciculins. The three-dimension al structure of rChII demonstrates that rChII adopts a typical three-finger ed fold with structural features of both parent toxins. Taken together, the se results emphasize the great structural flexibility and functional adapta bility of that fold and confirm that structural deviations between fascicul ins and short-chain neurotoxins do indeed reflect functional diversity.