Emergence of the active site of spleen exonuclease upon association of the two basic monomers of the tetrameric enzyme

Abstract

The 5′→3′ exonuclease from beef spleen is a 160-kDa tetramer consisting of four subunits of two types. Partial reduction of the tetramer led to one stable intermediate state of the enzyme with M r=80 kDa. The aim of this paper was to attribute the exonucleolytic activity to one of the two monomers, to the dimer or to the tetramer. The different forms of the exonuclease were separated by SDS-polyacrylamide gel electrophoresis, transferred on an Immobilon-P membrane and subsequently renaturated. Antibodies monospecific against each of the two monomers as well as against the dimer were isolated and their inhibitory effect on the holoenzyme determined. It was found that after renaturation the two monomers did not possess any exonuclease activity while the 80-kDa dimer showed a lower recovery of the specific activity of the enzyme (20.8±0.23 nkat/nmol, ( n=5)) in comparison with the 160-kDa tetramer (64.8±0.75 nkat/nmol ( n=5)). It was demonstrated that the antibodies monospecific against the dimer caused 53% maximum inhibition of the 160-kDa exonuclease. The antibodies monospecific against 25- and 55-kDa monomers did not inhibit the activity of the holoenzyme. No single-strand endonuclease activity of the spleen exonuclease was observed when using supercoiled Bluescript KS+ plasmid DNA as a substrate. This data suggest the emergence of an 80 kDa active form of beef spleen exonuclease upon association of two monomers of the tetrameric enzyme.