Biophysical Characterization Of Structural And Energetic Differences Between H2a And H2a/H2b Heterodimer Variants From C. Elegans

The compaction of eukaryotic DNA occurs within the nucleus of the cell by the nucleoprotein complex chromatin. Histones, the primary protein components of chromatin, facilitate this packaging to fit full-length DNA into the nucleus. The histones arrange themselves into two H2A/H2B dimers and an H3/H4 tetramer to form a histone octamer. The octamer is surrounded by about 150 base pairs of DNA to form the fundamental repeating unit of chromatin called the nucleosome core particle (NCP). Compaction of DNA facilitated by the NCP is important for restricting access to DNA to regulate DNA-based chemistries including gene expression and replication. Structural changes in chromatin thus act as an essential regulatory mechanism. Differences in nucleosome structure and dynamics play fundamental roles in cell differentiation and in the progression of a variety of diseases. The properties of the nucleosome are often modulated by histone protein variants, specifically H2A variants whose amino acid sequences differ from that of H2A. The work compares how variants of H2A monomer (HTAS-1 and HTZ) alter the secondary structure and stability of the H2A/H2B heterodimer. These variants are known to have different transcription and replication behaviors. We hypothesize that differences in secondary structure and stability of the variants within the H2A/H2B dimer correlate with observed differences in transcription and replication. The secondary structure and thermodynamic stability of H2A variant monomers and the heterodimer composed of H2A variants and H2B were determined using chemical and thermal denaturation monitored with far-UV circular dichroism spectroscopy. This work provides insights into how the physical properties of the histone variants HTAS-1 and HTZ alter the H2A/H2B dimer within the nucleosome.