Heterogeneity in the cysteine protease inhibitor clitocypin gene family

Clitocypin from the basidiomycete Clitocybe nebularis is the first fungal protein cysteine protease inhibitor to be characterised in detail, yet no information on its molecular genetics is available. Owing to its unique characteristics, it was assigned as the only member of a new family of cysteine protease inhibitors in the MEROPS inhibitor classification. Here we describe the full-length sequence of the clitocypin gene. A BLAST search confirmed its lack of significant sequence similarity to any other gene. The gene is composed of four exons and three short introns and belongs to a small family of closely related genes with more than 90% identity. Sequence variability is evenly distributed in introns and exons and deduced amino acid substitutions are distributed throughout the protein sequence. Basidiocarps collected at two distant locations were examined and the level of heterogeneity found in one basidiocarp is similar to that between the two. Sequencing of the ribosomal DNA spacers from the two basidiocarps confirmed that the heterogeneity observed in the clitocypin gene is not due to evolutionary divergence of the two specimens caused by geographic separation. Clitocypin is expressed in different parts of the basidiocarp and in cultured mycelia in a manner suggesting regulation by developmental and/or environmental factors.