Amphibian DNases I are characterized by a C-terminal end with a unique, cysteine-rich stretch and by the insertion of a serine residue into the Ca2+-binding site

Abstract

We purified four amphibian deoxyribonucleases I from the pancreases of one toad, two frog and one newt species, by using three different column chromatography methods in sequence. Each of the purified enzymes had a molecular mass of approx. 40kDa and an optimal pH for activity of approx. 8.0. These values were significantly greater than those for other vertebrate DNases I. The full-length cDNA encoding each amphibian DNase I was constructed from the total RNA of the pancreas by using rapid amplification of cDNA ends. Nucleotide sequence analyses revealed two structural characteristics unique to amphibian DNases I: a stretch of approx. 70 amino acids with a high cysteine content (approx. 15%) in the C-terminal region, and the insertion of a serine residue at position 205 (in a domain containing an essential Ca2+-binding site). Expression analysis of a series of mutant constructs indicated that both of these structures are essential in generating the active form of the enzyme. ‘DNase I signature sequences’, which are well conserved in other vertebrate DNases I, could not be found in any of the amphibian DNases I tested, whereas a ‘somatomedin B motif’ was identified in the Cys-rich stretches of all four. Although DNase I has so far been considered to be a secretory glycoprotein, amphibian DNase I seems to be non-glycosylated. These structural findings indicate strongly that amphibian DNases I are situated in a unique position on the phylogenetic tree of the DNase I family.