Histone octamer

File:Histone Octamer x-ray structure.jpg

Histone Octamer

The histone octamer is a core component of the nucleosome, which is the fundamental unit of chromatin structure in eukaryotic cells. The nucleosome plays a critical role in the organization and regulation of DNA, facilitating the compaction of DNA into the nucleus and influencing gene expression.

Structure

The histone octamer is composed of eight histone proteins: two each of H2A, H2B, H3, and H4. These histones are highly conserved proteins that form a protein core around which approximately 147 base pairs of DNA are wrapped in 1.65 left-handed superhelical turns.

Histone Fold Domain

Each histone protein contains a characteristic structural motif known as the "histone fold," which consists of three alpha helices connected by two loops. This motif facilitates the dimerization of histones, allowing them to form H2A-H2B and H3-H4 dimers. Two H3-H4 dimers associate to form a tetramer, and two H2A-H2B dimers join this tetramer to complete the octamer.

Assembly

The assembly of the histone octamer is a highly regulated process that occurs in the cell nucleus. Initially, the H3-H4 tetramer binds to DNA, followed by the addition of two H2A-H2B dimers to form the complete nucleosome core particle. This assembly is facilitated by histone chaperones, which prevent non-specific interactions and ensure proper nucleosome formation.

Function

The primary function of the histone octamer is to package DNA into a compact, organized structure, allowing it to fit within the confines of the cell nucleus. This packaging also plays a crucial role in regulating access to the DNA, thereby influencing processes such as DNA replication, DNA repair, and transcription.

Chromatin Remodeling

The positioning and composition of histone octamers can be altered by chromatin remodeling complexes, which use energy derived from ATP hydrolysis to reposition, eject, or restructure nucleosomes. This dynamic remodeling is essential for allowing access to DNA during transcription and other DNA-dependent processes.

Histone Modifications

Histone proteins within the octamer can undergo various post-translational modifications, such as acetylation, methylation, phosphorylation, and ubiquitination. These modifications occur primarily on the histone tails, which protrude from the nucleosome core, and play a key role in regulating chromatin structure and function. For example, acetylation of lysine residues is generally associated with transcriptional activation, while methylation can be associated with either activation or repression, depending on the specific residues modified.

Also see

• Nucleosome
• Chromatin
• Histone modification
• Epigenetics
• Chromatin remodeling