NASP maintains histone H3-H4 homeostasis through two distinct H3 binding modes

Area of Science:

• Molecular Biology
• Epigenetics
• Protein Structure and Function

Background:

• Histone chaperones are essential regulators of histone metabolism, impacting DNA replication, transcription, and DNA repair.
• Nuclear autoantigenic sperm protein (NASP) is a key histone chaperone, particularly for histone H3-H4 dimers, playing a vital role in preventing histone degradation and maintaining cellular histone pools.
• Understanding the precise mechanisms by which NASP interacts with histones is crucial for elucidating histone homeostasis.

Purpose of the Study:

• To identify and characterize the distinct histone binding modes of NASP.
• To elucidate the structural basis for NASP's function in maintaining histone H3-H4 supply.
• To reveal how NASP's interactions ensure histone homeostasis within the cell.

Main Methods:

• X-ray crystallography was employed to determine the structures of NASP complexes, including a sNASP dimer, a sNASP dimer with H3 α3 peptides, and a sNASP-H3-H4-ASF1b co-chaperone complex.
• Structural analysis identified two distinct histone binding modes involving the H3 α3 helix and the H3 αN region.
• Functional studies were conducted to validate the in vivo relevance of the identified binding modes.

Main Results:

• Two distinct histone binding modes for NASP were identified: one involving the H3 α3 helix and another involving the H3 αN region.
• Structural determination revealed the molecular interactions within NASP-histone complexes, including the sNASP dimer, sNASP-H3 peptide complex, and the sNASP-H3-H4-ASF1b co-chaperone complex.
• Functional assays confirmed that the H3 αN-interaction is the predominant binding mode in cellular contexts and that NASP shielding of this region is critical for maintaining the soluble pool of H3-H4 histones.

Conclusions:

• This study uncovers the molecular mechanisms underlying NASP's function as a major H3-H4 histone chaperone.
• The identified distinct binding modes, particularly the H3 αN-interaction, are essential for NASP's role in guarding histone homeostasis.
• The findings provide a structural and functional basis for NASP's critical role in maintaining the cellular supply of H3-H4 histones.