DNA bending versus DNA end joining activity of HMGB1 protein is modulated in vitro by acetylation.

The ability of HMGB1 protein to recognize bent DNA and to induce bending in linear duplex DNA defines HMGB1 as an architectural factor. It has already been demonstrated that the binding affinity of the protein for various bent DNA structures is enhanced upon in vivo acetylation at Lys2. Here we investigate how this modification of HMGB1 affects its ability to bend DNA. We report that the modified protein cannot bend short DNA fragments but, instead, stimulates joining of the same fragments via their ends. The same properties are exhibited in vivo by acetylated HMGB1 lacking its acidic tail. Further, in vitro acetylation of the truncated protein at Lys81 (possible upon tail removal only) restores the protein's bending ability, while the level of stimulation of DNA end joining is strongly reduced. We conclude, therefore, that the ability of HMGB1 to bend DNA or to stimulate end joining is modulated in vitro by acetylation. In an attempt to explain the properties of in vivo-acetylated HMGB1, its complexes with DNA have been analyzed by both protein-DNA cross-linking and atomic force microscopy. Unlike the parental protein, bound mainly within the internal sequences, acetylated HMGB1 binds preferentially to DNA ends. We propose that the loading of acetylated protein on DNA ends accounts for both the failure to bend DNA and the stimulation of DNA end joining.

The inhibitory effect of HMGB-1 protein on the repair of cisplatin-damaged DNA is accomplished through the acidic domain.

The well established inhibitory effect of HMGB-1 on repair of cisplatin-damaged DNA has been studied with two modified forms of the protein, shown to bind platinated DNA with higher affinity than the original protein: in vivo acetylated HMGB-1 and HMGB-1 lacking its C-terminal domain. The native and the modified proteins were assayed for their effects on adduct removal by using cell-free extract capable of repairing cisplatinated DNA in vitro. The inhibition observed with the native HMGB-1 was reduced in the presence of acetylated HMGB-1 and completely abolished when the assay was carried out with the truncated protein. When the repair assay was performed in the presence of a synthetic polypeptide identical to the C-terminal tail, either alone or together with the truncated protein, the inhibitory effect was partially recovered in a concentration-dependent manner. These findings strongly suggest that the HMGB-1-induced inhibition of cisplatin-DNA adduct repair is accomplished through the acidic domain. The results obtained are discussed in terms of the repair events that may occur in the presence of HMGB-1 protein.