Two novel proteins, BM8 and BM14, were isolated from Bungarus multicinctus (Taiwan banded krait) venom using the combination of chromatography on a SPSephadex C-25 column and a reversephase HPLC column. Both proteins contained 82 amino acid residues including 10 cysteine residues, but there were two amino acid substitutions at positions 37 and 38 (Glu37Ala38 in BM8; Lys37Lys38 in BM14). CD spectra and acrylamide quenching studies revealed that the gross conformation of BM8 and BM14 differed. In contrast to BM8, BM14 inhibited the binding of [3H]quinuclidinyl benzilate to the M2 muscarinic acetylcholine (mAchR) receptor subtype. Trinitrophenylation of Lys residues abolished the mAchRbinding activity of BM14, indicating that the Lys substitutions at positions 37 and 38 played a crucial role in the activity of BM14. The genomic DNA encoding the precursor of BM14 was amplified by PCR. The gene shared virtually identical structural organization with αneurotoxin and cardiotoxin genes. The intron sequences of these genes shared a sequence identity up to 84%, but the proteincoding regions were highly variable. These results suggest that BM8, BM14, neurotoxins and cardiotoxins may have originated from a common ancestor, and the evolution of snake venom proteins shows a tendency to diversify their functions.
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