Mutually exclusive exon use and reactive center diversity in insect serpins.

We have isolated 38 cDNA clones for serpins from a lepidopteran insect, Manduca sexta, and have found that they are identical in sequence, except for a region encoding the carboxyl-terminal 40-45 residues, which includes the reactive center. Among these clones, there are 11 variants of the reactive center region, each encoded by a different version of the ninth exon in the serpin gene. Thus, evolution of this insect serpin gene has resulted from duplication and sequence divergence of only the exon encoding the reactive site. Alternative pre-mRNA splicing then generates inhibitor diversity and the potential to regulate a variety of proteinases, using the same protein framework joined to different reactive site region cassettes.