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#H3

2 public questions tagged with this topic.

Nucleosome Packing

CENP-A is a variant of histone:

Centromeric chromatin contains distinctive histone variant CENP-A that epigenetically specifies centromere location independent of underlying DNA sequence in most eukaryotes. CENP-A resembles canonical histone H3 in histone fold domain sharing about sixty percent similarity, but possesses divergent N-terminus and loop1 centromere targeting domain CATD directing specific deposition via HJURP chaperone during G1. It replaces H3 in subset of nucleosomes at active centromere creating specialized octamer recruiting inner kinetochore. H1 is linker histone, H2A variants include H2A.Z, H4 lacks centromeric-specific variant analogous to CENP-A functionally.

Ref: Palmer et al., 1987 PNAS; Alberts et al., Molecular Biology of the Cell, Chapter 4: CENP-A is Variant of H3

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Nucleosome Packing

Which histones form tetramer in nucleosome core?

Histone octamer assembly follows ordered hierarchical pathway in vitro and via chaperones in vivo. H3 and H4 first form heterodimer via histone fold helices interaction, two dimers associate via H3-H3 four-helix bundle to form stable H3-H4 tetramer (H3-H4)2. This tetramer binds DNA first during replication-coupled assembly, serving as kernel onto which two H2A-H2B dimers deposit to complete octamer. H2A-H2B alone cannot tetramerize; H1 associates externally after nucleosome formation. Conservation of tetramer during replication ensures epigenetic inheritance of H3-H4 modifications to daughter strands, maintaining heterochromatin memory.

Ref: Tagami H. et al., Cell 2004; Lodish et al., Molecular Cell Biology, Chapter 8: H3-H4 Tetramer Formation

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