histonchaperoner

Histonchaperoner, also referred to as histone chaperone, denotes a class of nuclear proteins that bind histones and regulate the assembly and disassembly of nucleosomes in eukaryotic chromatin. They are not enzymes that modify histones themselves; rather they shield histones from non-specific interactions, prevent histone aggregation, and escort histones to DNA to facilitate proper chromatin assembly and remodeling.

An essential feature of histonchaperoners is their ability to present histones H3-H4 or H2A-H2B to DNA in a deposition-competent form. They operate across cellular processes including DNA replication, transcription, and DNA repair. By influencing the timing and location of histone deposition, they help maintain chromatin structure and contribute to epigenetic memory through the selective incorporation of histone variants and modifications.

Key families and examples include:

- CAF-1 (chromatin assembly factor 1): a replication-dependent chaperone that deposits H3-H4 onto DNA during S phase, coordinating nucleosome assembly with DNA synthesis.

- ASF1 (anti-silencing function 1): shuttles H3-H4 between replication forks and other chaperones, supplying histones for multiple chromatin contexts.

- HIRA: a replication-independent chaperone that deposits the variant H3.3 at active genes and other regions.

- FACT (facilitates chromatin transcription): reorganizes nucleosomes ahead of RNA polymerase II to assist transcription and chromatin reassembly after passage.

- DAXX-ATRX: specialized chaperone complex for depositing H3.3 at telomeres and heterochromatin.

- NAP1: a general chaperone for H2A-H2B-containing nucleosomes.

Histonchaperoners are essential for genome stability and proper development; their misregulation can affect transcription, replication fidelity, and epigenetic regulation.