Abstract
MCM2 is a subunit of the replicative helicase machinery shown to interact with histones H3 and H4 during the replication process through its N-terminal domain. During replication, this interaction has been proposed to assist disassembly and assembly of nucleosomes on DNA. However, how this interaction participates in crosstalk with histone chaperones at the replication fork remains to be elucidated. Here, we solved the crystal structure of the ternary complex between the histone-binding domain of Mcm2 and the histones H3-H4 at 2.9 Å resolution. Histones H3 and H4 assemble as a tetramer in the crystal structure, but MCM2 interacts only with a single molecule of H3-H4. The latter interaction exploits binding surfaces that contact either DNA or H2B when H3-H4 dimers are incorporated in the nucleosome core particle. Upon binding of the ternary complex with the histone chaperone ASF1, the histone tetramer dissociates and both MCM2 and ASF1 interact simultaneously with the histones forming a 1:1:1:1 heteromeric complex. Thermodynamic analysis of the quaternary complex together with structural modeling support that ASF1 and MCM2 could form a chaperoning module for histones H3 and H4 protecting them from promiscuous interactions. This suggests an additional function for MCM2 outside its helicase function as a proper histone chaperone connected to the replication pathway.
Highlights
The nucleosome is the universal repeating unit of chromatin
In the presence of histones H3-H4, exactly the same set of resonances is affected between Mini Chromosome Maintenance 2 (MCM2) (1–160) and MCM2 (63–153), showing that the histone binding region can be restricted to the conserved domain
Each MCM2 molecule interacts with one H3-H4 dimer, and the tetramer is obtained by crystallographic symmetry operations
Summary
Its core particle contains 147 base pair of DNA wrapped around an octamer of two copies of the four core histones H2A, H2B, H3 and H4 (1) This particle exists in various forms using distinct histone variants for H3 (in humans, the S phase subtypes H3.1 and H3.2, the H3.3 variant deposited at all phases of the cell cycle and the centromeric CENP-A (2–4)), and for H2A (4)) as well as the large repertoire of posttranscriptional modifications (PTMs), called the ‘epigenetic code’ (5) Together these combinations can establish a chromatin landscape regulating gene expression programs. Mechanisms that regulate chromatin states during cell cycle are essential for maintaining the cellular identity upon cell division Consistent with this central function, deregulation of histone marks promotes tumoral progression (6). This raises the question of how parental histones and their marks (PTM and variants) are handled at the fork and whether they re-associate at the same po-
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