NASP

NASP is an essential, evolutionarily conserved TPR-domain histone chaperone that maintains soluble pools of histones H1, H3, and H3-H4 dimers, thereby coupling histone supply to DNA replication-dependent and -independent chromatin assembly. In the cytoplasm, NASP prevents H3 aggregation and degradation, while nuclear sNASP receives monomeric H3 from importin-5 in a RanGTP-dependent handoff and cooperates with ASF1, Hsc70/Hsp90, and the INO80 complex to deliver histones for chromatin assembly and to direct excess histones for degradation (PMID:22195965, PMID:36066346, PMID:40804522). NASP is essential for S-phase progression and embryonic viability, as its loss causes replication defects, globally increased chromatin accessibility, and embryonic lethality in mice and Drosophila (PMID:16728391, PMID:36930688). In spermatogenic cells, the testicular isoform tNASP modulates meiotic progression by forming a complex with HSPA2 and CDC2 that inhibits CDC2/cyclin B1 kinase activity (PMID:19553603).

1. 1992 Medium

   Identification of human tNASP as a testis-expressed homolog of Xenopus N1/N2 with conserved histone-binding domains and nuclear localization established NASP as a candidate histone chaperone in mammalian spermatogenesis.

   Evidence Sequence alignment, immunolocalization in testis, Northern blot

   PMID:1426632

   Open questions at the time

   • No direct demonstration of histone binding activity
   • Somatic expression and function unknown
2. 2000 High

   Demonstrating that somatic sNASP physically associates with linker histone H1 and that its expression is S-phase regulated established NASP as a cell-cycle-coupled histone chaperone beyond the testis.

   Evidence Affinity chromatography, synchronized HeLa and 3T3 cell cultures

   PMID:10893414

   Open questions at the time

   • Whether NASP binds core histones H3/H4 was unknown
   • No functional consequence of sNASP depletion tested
3. 2004 High

   Showing that tNASP binds HSP90, stimulates its ATPase activity, and independently transports linker histones into the nucleus in an energy- and NLS-dependent manner revealed NASP as an active nuclear import factor for histones that integrates chaperone-assisted folding.

   Evidence Cross-linking mass spectrometry, co-IP, in vitro ATPase assay, permeabilized HeLa cell import assay

   PMID:15533935

   Open questions at the time

   • Whether sNASP uses the same import mechanism was untested
   • In vivo relevance of HSP90 stimulation not shown
4. 2006 High

   NASP knockout mice die at the blastocyst stage and siRNA depletion blocks S-phase progression, establishing NASP as essential for DNA replication and viability.

   Evidence NASP-null mouse model, siRNA knockdown with cell cycle analysis in HeLa and U2OS cells

   PMID:16728391

   Open questions at the time

   • Molecular mechanism of replication failure not defined
   • Whether histone supply is the sole critical function was unclear
5. 2007 High

   Discovery that fission yeast Sim3 (NASP homolog) is required for CENP-A deposition at centromeres extended the chaperone function to centromeric histone variants and replication-independent chromatin assembly.

   Evidence Genetic screen, ChIP, GFP-CENP-A localization in S. pombe

   PMID:18158900

   Open questions at the time

   • Whether human NASP similarly chaperones CENP-A was not tested
   • Mechanism of selectivity for CENP-A vs. canonical H3 unknown
6. 2008 High

   Demonstrating that NASP forms high-affinity complexes with H3 and H4 in addition to H1 and functions in H1-independent chromatin assembly expanded its chaperone specificity beyond linker histones.

   Evidence Native gel electrophoresis, surface plasmon resonance, in vitro chromatin assembly assay

   PMID:18782834

   Open questions at the time

   • Structural basis of H3/H4 recognition not determined
   • Relative in vivo contributions of H1 vs. H3-H4 chaperoning unclear
7. 2009 Medium

   Identification of the tNASP–HSPA2–CDC2 complex in spermatocytes, where linker histone binding stimulates HSPA2 ATPase activity and sequesters CDC2 from cyclin B1, revealed a meiosis-specific regulatory role for NASP in cell cycle control.

   Evidence Co-IP, in vitro ATPase assay, kinase activity assay in primary spermatocytes

   PMID:19553603

   Open questions at the time

   • Single-lab finding without independent replication
   • Whether this mechanism applies to mitotic cells untested
   • In vivo meiotic progression phenotype of tNASP-specific loss not shown
8. 2011 High

   Showing that NASP maintains a soluble H3-H4 reservoir and, together with Hsc70/Hsp90, triages excess histones for degradation established NASP as the central buffering node in histone proteostasis.

   Evidence Histone overexpression, replication stress, Asf1 depletion, pulse-chase, quantitative histone measurement in human cells

   PMID:22195965

   Open questions at the time

   • Degradation pathway (chaperone-mediated autophagy vs. proteasome) not fully delineated
   • How NASP senses histone excess unclear
9. 2022 High

   Structural determination of the AtNASP–H3 α3 peptide complex and demonstration that the binding mode is conserved in human NASP, combined with evidence that NASP and ASF1 form a co-chaperone complex promoting tetrasome assembly, provided an atomic-level view of NASP's histone recognition mechanism.

   Evidence X-ray crystallography of AtNASP–H3 complex, in vitro tetrasome assembly, co-IP for ASF1 interaction

   PMID:35587028

   Open questions at the time

   • Full-length human NASP structure not solved
   • How H3-H4 dimer vs. monomer specificity is achieved structurally unknown
10. 2022 High

    Demonstrating that importin-5 delivers cytoplasmic monomeric H3 to nuclear sNASP via RanGTP-dependent competition, and that sNASP specifically binds monomeric H3 while both isoforms bind H3-H4 dimers, resolved how newly synthesized H3 enters the nucleus and defined isoform-specific substrate preferences.

    Evidence Pulse-chase, in vitro binding/competition assays, cytoplasm-tethering experiments in human cells

    PMID:36066346

    Open questions at the time

    • Whether additional importins contribute in vivo not excluded
    • Fate of H3 after sNASP handoff to downstream chaperones not fully mapped
11. 2023 High

    Genetic ablation of Drosophila NASP caused degradation of the soluble H3-H4 pool and early embryonic lethality, providing in vivo confirmation across species that NASP is the critical H3-H4 chaperone for maintaining histone stores during rapid cell divisions.

    Evidence NASP-null Drosophila mutant, maternal-effect genetic analysis, quantitative histone fractionation

    PMID:36930688

    Open questions at the time

    • Whether replication-independent chromatin assembly is also affected not addressed
    • Potential redundancy with other chaperones in later development not tested
12. 2025 High

    Discovery that NASP stabilizes histones evicted by PARP-mediated chromatin remodeling and cooperates with PARP1 and INO80 for histone turnover at replication forks, with NASP loss sensitizing tumors to PARP inhibitors, linked NASP's histone chaperoning to the DNA damage response and therapeutic vulnerability.

    Evidence Functional genetic screens, NASP-KO cell lines, PARPi sensitivity assays in vitro and in vivo, DNA fiber assay

    PMID:40804522

    Open questions at the time

    • Whether NASP directly contacts PARP1 or INO80 is not established
    • Applicability across tumor types beyond those tested unclear

• Key unresolved questions include the full-length structure of human NASP, the precise mechanism by which NASP distinguishes monomeric H3 from H3-H4 dimers to route them through different chaperoning pathways, and whether the tNASP-mediated meiotic cell cycle regulation observed in spermatocytes is functionally conserved in other contexts.
• Full-length human NASP structure not available
• Mechanism of substrate discrimination between H3 monomer and H3-H4 dimer unknown
• In vivo meiotic phenotype of tNASP-specific knockout not reported