Bleomycin hydrolase Gal6
Saccharomyces cerevisiae (Baker's yeast)
Bleomycin is a glycopeptide, a short polymer of amino acids with one or more carbohydrates covalently linked to it. It has the ability to bind to and cleave double-stranded DNA. Hence it is frequently used as an anti-cancer drug to kill tumor cells by destroying their DNA. Bleomycin hydrolase (BH) is a cysteine protease (an enzyme, with cysteine in its active site, which degrades proteins by hydrolyzing their peptide bonds) that inactivates bleomycin by hydrolyzing it. High levels of BH in tumor cells renders them resistant to bleomycin, whereas its low levels in normal cells makes bleomycin toxic to normal tissue, especially skin and lungs. BH is expressed in all tissues in prokaryotes and eukaryotes and its normal cellular function is unknown. Gal6 is the homolog (derived from the same ancestral gene) of BH in yeast. It binds DNA and acts as a repressor in the Gal4 regulatory system - a system that regulates the expression or repression of galactose catabolic (GAL) genes in yeast. Thus BH's cellular functions, so far identified, involve both proteolysis (degrading proteins) and DNA-binding.
Shown above is the crystal structure of a single subunit of Gal6. Gal6 is a hexamer with a prominent central channel and an organization similar to the 20S proteosome. It has three subunits on top and three below. Each monomer has a catalytic (protease) domain, an oligomerization or hook domain and a helical domain. Each of the top three subunits forms dimeric interactions with the subunit below it by wrapping its hook domain around the helical domain of the bottom subunit. Trimer interactions occur between the top three subunits and the bottom three subunits respectively. A part of the helical domain of each subunit extends from the equator of the hexamer. The six papain-like active sites are situated in the central channel, which is lined with 60 lysine residues possibly involved in DNA binding. The carboxyl terminal arm of Gal6 extends into the active site cleft possibly serving a regulatory role. The DNA binding and proteolysis activities do not reside in distinct domains but are structurally combined in the protein.
Protein Data Bank (PDB)
author: Ashwini Patil