HMGN1 and 2 remodel core and linker histone tail domains within chromatin

The structure of the nucleosome, the basic building block of the chromatin fiber, plays a key role in epigenetic regulatory processes that affect DNA-dependent processes in the context of chromatin. Members of the HMGN family of proteins bind specifically to nucleosomes and affect chromatin structure and function, including transcription and DNA repair. To better understand the mechanisms by which HMGN 1 and 2 alter chromatin, we analyzed their effect on the organization of histone tails and linker histone H1 in nucleosomes. We find that HMGNs counteract linker histone (H1)-dependent stabilization of higher order ‘tertiary’ chromatin structures but do not alter the intrinsic ability of nucleosome arrays to undergo salt-induced compaction and self-association. Surprisingly, HMGNs do not displace H1s from nucleosomes; rather these proteins bind nucleosomes simultaneously with H1s without disturbing specific contacts between the H1 globular domain and nucleosomal DNA. However, HMGNs do alter the nucleosome-dependent condensation of the linker histone C-terminal domain, which is critical for stabilizing higher-order chromatin structures. Moreover, HMGNs affect the interactions of the core histone tail domains with nucleosomal DNA, redirecting the tails to more interior positions within the nucleosome. Our studies provide new insights into the molecular mechanisms whereby HMGNs affect chromatin structure.