The Structure of the Bovine Cathepsin B Gene

Cathepsin B is a cysteine proteinase which plays an important role in lysosomal proteolysis. We report here the characterization of a 15‐kbp genomic clone of the bovine cathepsin B gene. Bovine preprocathep‐sin B is encoded by nine exons from translational initiation to stop codon. The last exon is sandwiched between Alu‐like short interspersed nuclear elements. Alternate use of polyadenylation sites generates three transcripts encoding bovine cathepsin B. In all tissues tested, 3′ end cleavage was found to occur in equal proportion at the first and second polyadenylation site, producing transcripts of 2.6‐kb. Polyadenylation at the third polyadenylation site generates a 3.2‐kb mRNA, only expressed at low levels and in a developmental and tissue‐specific manner. Due to micro‐heterogeneities between genomic and cDNA clones, sequence polymorphism was investigated in the trailer region. DNA sequencing of PCR‐amplified genomic fragments revealed that, in contrast to the protein‐encoding region, genetic variability exists in the 3′ untranslated region. Polymorphism in the trailer was confirmed in cathepsin B mRNA by ribo‐nuclease‐protection assays. We finally emphasize that while exon‐exon boundaries in mature cathepsin B are well conserved from nematodes to mammals, exons also tend to correspond to discrete units of cathepsin B structure.