ENDOGLYCOSIDIC CLEAVAGE OF BRANCHED POLYMERS BY POLY(ADP-RIBOSE) GLYCOHYDROLASE

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.