Characterization of genes encoding rat tonin and a kallikrein-like serine protease

Tissue kallikreins are a group of serine proteases which may function as peptide hormone processing enzymes. Two rat kallikrein genomic clones (RSKG-5 and RSKG-50) were sequenced and characterized. The rat tonin gene and a kallikrein-like gene were found in clones RSKG-5 and RSKG-50, respectively. The tonin gene is 4146 base pairs in length, with both the variant CCAAA and TTTAAA boxes in the 5'-end region and an AATAAA polyadenylation signal at the 3' end of the gene. It has five exons which are separated by four introns. Sequence analysis of 3.7-kb 5' upstream and 7.5-kb 3' downstream of the tonin gene failed to reveal a second kallikrein gene. Sequence comparisons of the RSKG-5 exons with tonin cDNA revealed that only one base in the 3'-noncoding region was different from that in the previously reported rat tonin cDNA. Characteristic TC- and TG-repeated sequences were also found in the first and second introns of the tonin gene. The tonin gene encodes a preprotonin of 259 amino acids (aa). The active enzyme consists of 235 aa and is preceded by a deduced signal peptide of 17 aa and a profragment of 7 aa. Northern blot analysis indicates that RSKG-5 is expressed in a sex-dependent manner in rat submandibular gland, with a higher level expressed in males. The RSKG-50 gene was truncated at an EcoRI site in the second intron, excluding its 5' end. Compared to the coding sequence of pancreatic kallikrein, 12 nucleotides have been deleted in exon 3 of the RSKG-50 gene. The nucleotide sequences of the third, fourth, and fifth exons of the RSKG-50 gene encode a polypeptide of 188 aa residues. The translated peptide is 80% homologous to rat pancreatic kallikrein and 75% homologous to rat tonin in the corresponding regions. Key residues in the RSKG-50 gene product indicate a serine protease with kallikrein-like cleavage specificity at basic amino acids.