IDENTIFICATION OF A NOVEL TYPE OF ALTERNATIVELY SPLICED EXON FROM THEACETYLCHOLINESTERASE GENE OF BUNGARUS FASCIATUS - MOLECULAR-FORMS OF ACETYLCHOLINESTERASE IN THE SNAKE LIVER AND MUSCLE

The venom of the snake Bungarus fasciatus contains a hydrophilic, mono meric species of acetylcholinesterase (AChE), characterized by a C-ter minal region that does not resemble the alternative T-or H-peptides. H ere, we show that the snake contains a single gene for AChE, possessin g a novel alternative exon (S) that encodes the C-terminal region of t he venom enzyme, located down-stream of the T exon. Alternative splici ng generates S mRNA in the venom gland and S and T mRNAs in muscle and liver. We found no evidence for the presence of an H exon between the last common ''catalytic'' exon and the T exon, where H exons are loca ted in Torpedo and in mammals. Moreover, COS cells that were transfect ed with AChE expression vectors containing the T exon with or without the preceding genomic region produced exclusively AChE, subunits, In t he snake tissues, we could not detect any glycophosphatidylinositol-an chored AChE form that would have derived from H subunits. In the liver , the cholinesterase activity comprises both AChE and butyrylcholinest erase components; butyrylcholinesterase corresponds essentially to non amphiphilic tetramers and AChE to nonamphiphilic monomers (G(1)(na)). In muscle, AChE is largely predominant: it consists of globular forms (G(1)(a) and G(4)(a)) and trace amounts of asymmetric forms (A(8) and A(12)), which derive from AChE(T) subunits. Thus, the Bungarus AChE ge ne possesses alternatively spliced T and S exons but no II exon; the a bsence of an H exon may be a common feature of AChE genes in reptiles and birds.