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

Post-translational modification of nuclear proteins with poly(ADP-ribo se) modulates chromatin structure, and may be required for DNA process ing events such as replication, repair and transcription. The polymer- catabolizing enzyme, poly(ADP-ribose) glycohydrolase, is crucial for t he regulation of polymer metabolism and the reversibility of the prote in modification. Previous reports have shown that glycohydrolase diges ts poly(ADP-ribose) via an exoglycosidic mechanism progressing from th e 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 gl ycohydrolase digestion of protein-bound poly(ADP-ribose) led to the pr oduction of protein-free oligomers of ADP-ribose. Second, partial glyc ohydrolase 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, endog lycosidic activity produced linear polymers from branched polymers alt hough branch points themselves were not a preferential target of cleav age. From these data, we propose a mechanism whereby poly(ADP-ribose) glycohydrolase degrades polymers in three distinct phases; (a) endogly cosidic cleavage, (b) endoglycosidic cleavage plus exoglycosidic, proc essive degradation, (c) exoglycosidic, distributive degradation.