Poly(ADP-ribose) Synthetase, a Main Acceptor of Poly(ADP-ribose) in Isolated Nuclei

Poly(ADP-ribose) synthetase was identified as the main acceptor of this polymer produced in isolated nuclei of rat liver. When the nuclei were incubated with [32P]NAD at a limited concentration (2.4 microM) and for a brief period (10 s), a protein with Mr = 110,000 was predominantly poly(ADP-ribosyl)ated, as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The modification of this protein increased upon longer incubations or at higher NAD concentrations, and induced a marked increase in the apparent molecular weight. A comparison with poly(ADP-ribose) synthetase (Mr = 110,000) of rat liver under various conditions suggested that the increase in the molecular weight of the acceptor resembled that of the synthetase undergoing multiple auto-poly(ADP-ribosyl)ation. This interpretation was further supported by the following observations: 1) [32P]poly(ADP-ribose) attached to the acceptor co-eluted with the synthetase activity from a hydroxyapatite column; 2) the [32P]poly(ADP-ribose).acceptor complex isolated on the column was converted to a very large complex by further incubation with NAD; and 3) a group of large poly(ADP-ribose).acceptor complexes were reduced to a single molecular species with Mr = 110,000 by extensive digestion with poly(ADP-ribose) glycohydrolase. These findings altogether suggested that poly(ADP-ribose) synthesized in isolated nuclei was principally bound to the synthetase itself.