Noncovalent interactions of poly(adenosine diphosphate ribose) with histones

Covalent linkage of ADP-ribose polymers to proteins is generally considered essential for the posttranslational modification of protein function by poly(ADP-ribosyl)ation. Here we demonstrate an alternative way by which ADP-ribose polymers may modify protein function. Using a highly stringent binding assay in combination with DNA sequencing gels, we found that ADP-ribose polymers bind noncovalently to a specific group of chromatin proteins, i.e., histones H1, H2A, H2B, H3, and H4 and protamine. This binding resisted strong acids, chaotropes, detergents, and high salt concentrations but was readily reversible by DNA. When the interactions of variously sized linear and branched polymer molecules with individual histone species were tested, the hierarchies of binding were branched polymers greater than long, linear polymers greater than short, linear polymers and H1 greater than H2A greater than H2B = H3 greater than H4. For histone H1, the target of polymer binding was the carboxy-terminal domain, which is also the domain most effective in inducing higher order structure of chromatin. Thus, noncovalent interactions may be involved in the modification of histone functions in chromatin.