Endoglycosidic cleavage of branched polymers by poly(ADP‐ribose) glycohydrolase

Abstract

Post‐translational modification of nuclear proteins with poly(ADP‐ribose) modulates chromatin structure and may be required for DNA processing events such as replication, repair and transcription. The polymer‐catabolizing enzyme, poly(ADP‐ribose) glycohydrolase, is crucial for the regulation of polymer metabolism and the reversibility of the protein modification. Previous reports have shown that glycohydrolase digests poly(ADP‐ribose) via an exoglycosidic mechanism progressing from the protein‐distal end of the polymer. Using two independent approaches, we investigated the possibility that poly(ADP‐ribose) glycohydrolase also engages in endoglycosidic cleavage of polymers. First, partial glycohydrolase digestion of protein‐bound poly(ADP‐ribose) led to the production of protein‐free oligomers of ADP‐ribose. Second, partial glycohydrolase digestion of a fixed number of protein‐free poly(ADP‐ribose) polymers resulted in a transient increase in the absolute number of polymers while polymer size continuously decreased. Furthermore, endoglycosidic activity produced linear polymers from branched polymers although branch points themselves were not a preferential target of cleavage. From these data, we propose a mechanism whereby poly(ADP‐ribose) glycohydrolase degrades polymers in three distinct phases; (a) endoglycosidic cleavage, (b) endoglycosidic cleavage plus exoglycosidic, processive degradation, (c) exoglycosidic, distributive degradation. Copyright © 1994, Wiley Blackwell. All rights reserved