Genomic organization and chromosome localization of the human cathepsin K gene (CTSK)

Human cathepsin K is a recently described cysteine protease with high sequence homology to cathepsins S and L, members of the papain superfamily of cysteine proteases. Cathepsin K is abundantly and selectively expressed in osteoclasts and may perform a specialized role in osteoclast-mediated bone resorption. In the present study, the genomic organization and chromosomal localization of human cathepsin K (HGMW-approved symbol CTSK) were determined. Intron-exon boundaries were identified by PCR on human genomic DNA, and subsequently a P1 genomic clone containing the full-length gene was isolated. Cathepsin K spans approximately 12.1 kb of genomic DNA and is composed of eight exons and seven introns. The genomic organization of cathepsin K is similar to that of cathepsins S and L. The gene was mapped to chromosome 1q21 by fluorescence in situ hybridization. Primer walking on the P1 genomic clone identified 1108 bp of 5' flanking sequence and 459 bp of 3' flanking sequence. Ribonuclease protection assay and 5' RACE indicated a single transcriptional start site 49 bp upstream of the initiator Met codon. Analysis of the 5' flanking region indicates that this gene lacks canonical TATA and CAAT boxes and contains multiple potential transcription regulatory sites. The characterization of the cathepsin K gene and its promoter may provide valuable insights not only into its osteoclast-selective expression, but also into the molecular mechanisms responsible for osteoclast activation.