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Research Report: Saccharomyces cerevisiae NAP1 (UniProt P25293; gene NAP1, systematic YKR048C) — Functional Annotation

Executive summary

NAP1 encodes the conserved nucleosome assembly protein 1 (Nap1), a canonical histone chaperone with strong functional specificity for H2A–H2B dimers. In budding yeast, Nap1 is primarily cytosolic for handling newly synthesized H2A–H2B, but also acts in the nucleus where it supports targeted histone release/import, nucleosome assembly and remodeling during transcription and other chromatin transactions. Recent mechanistic advances (2023–2024) resolved how Nap1 removes H2A–H2B from partially unwrapped nucleosomes and how Nap1 cooperates with the importin Kap114 and RanGTP to transfer H2A–H2B for nucleosome assembly. (ohkuni2003genomewideexpressionanalysis pages 1-2, fung2024nap1andkap114 pages 1-2, nagae2023histonechaperonenap1 pages 1-2)

1) Key concepts and definitions (current understanding)

1.1 What Nap1 is

Nap1 is defined as a nucleosome assembly protein and core histone chaperone that binds core histones—especially H2A and H2B—to promote correct nucleosome organization and to prevent inappropriate histone–DNA interactions. This view is consistent across classic yeast functional genetics and modern mechanistic structural work. (ohkuni2003genomewideexpressionanalysis pages 1-2, fung2024nap1andkap114 pages 1-2)

1.2 What a histone chaperone does (as applied to Nap1)

In the context of Nap1, “histone chaperone” denotes a protein that:
- binds H2A–H2B and shields their basic, DNA-binding surfaces to prevent non-specific aggregation, and
- facilitates productive nucleosome incorporation of H2A–H2B rather than random histone–DNA association. (fung2024nap1andkap114 pages 1-2)

1.3 Primary molecular function (substrate specificity)

The primary substrate of Nap1 is the H2A–H2B dimer. Mechanistically:
- Nap1 forms a stable homodimer (often referred to as Nap1_2 or “Nap12” in the Kap114 work) that binds H2A–H2B and participates in nucleosome assembly pathways. (fung2024nap1andkap114 pages 1-2, nagae2023histonechaperonenap1 pages 1-2)
- Nap1 can also act in H2A–H2B eviction/dismantling, particularly when nucleosomal DNA is partially unwrapped (e.g., during collisions with translocases), supporting a role in nucleosome disassembly/reassembly cycles linked to transcription and chromatin repair. (nagae2023histonechaperonenap1 pages 1-2, nagae2023histonechaperonenap1 pages 2-2)

1.4 Biological processes and pathways

Evidence from yeast genetics and mechanistic biochemistry places Nap1 in multiple chromatin-centered processes:
- Nucleosome maintenance/spacing and transcriptional regulation in vivo, with deletion leading to widespread transcriptional changes and cluster-like behavior of affected genes. (ohkuni2003genomewideexpressionanalysis pages 1-2)
- Histone trafficking/import and targeted nuclear release, via cooperation with Kap114 and the RanGTP system to deliver H2A–H2B for nucleosome assembly. (fung2024nap1andkap114 pages 1-2, fung2024nap1andkap114 pages 2-4)
- Links to cell-cycle/mitotic regulation, via reported interactions with Clb2 (B-type cyclin) and Gin4 (septum-related kinase), suggesting coupling between chromatin management and mitotic morphogenesis programs. (ohkuni2003genomewideexpressionanalysis pages 1-2, ohkuni2003genomewideexpressionanalysis pages 4-5)

2) Recent developments and latest research (prioritized 2023–2024)

2.1 2024: Nap1–Kap114–RanGTP quaternary complex and targeted histone release

A 2024 Journal of Cell Biology study provided biochemical and cryo-EM evidence that yeast Nap1 and Kap114 co-chaperone H2A–H2B and facilitate targeted histone release in the nucleus, addressing a central question of how import factors and histone chaperones coordinate to deliver H2A–H2B to assembling nucleosomes. (Publication date: Nov 2024; URL: https://doi.org/10.1083/jcb.202408193) (fung2024nap1andkap114 pages 1-2, fung2024nap1andkap114 pages 2-4)

Key mechanistic points reported include:
- Nap1 is described as the principal cytosolic H2A–H2B chaperone that is mostly cytoplasmic but also functions in the nucleus (implying nucleocytoplasmic shuttling). (fung2024nap1andkap114 pages 1-2)
- Kap114, H2A–H2B, and Nap1_2 form equimolar complexes, including a quaternary Nap1_2•H2A–H2B•Kap114•RanGTP assembly. (fung2024nap1andkap114 pages 1-2, fung2024nap1andkap114 pages 2-4)
- The study provides residue-level insight: Nap1_2 β-hairpin residues (E288/R290/Q292) are critical for Kap114 binding and nuclear localization phenotypes. (fung2024nap1andkap114 pages 2-4)

2.2 2023: Mechanism of Nap1-mediated H2A–H2B dismantling from partially unwrapped nucleosomes

A 2023 Nucleic Acids Research paper used in vitro transcription assays and molecular simulations to show that partial nucleosome unwrapping by a translocase dramatically facilitates Nap1-mediated H2A–H2B dimer dismantling. (Publication date: May 2023; URL: https://doi.org/10.1093/nar/gkad396) (nagae2023histonechaperonenap1 pages 1-2)

Mechanistic conclusions include:
- Nap1 acidic C-terminal flexible tails can engage an H2A–H2B interface that is normally buried in the nucleosome and not accessible to Nap1’s globular domains, consistent with a “penetrating fuzzy binding” mode of chaperone–histone interaction. (nagae2023histonechaperonenap1 pages 1-2)
- Nap1 can remove H2A–H2B from fully wrapped nucleosomes only slowly at low temperature, but removal is accelerated when DNA is partially unwrapped (e.g., by polymerase-like collisions). (nagae2023histonechaperonenap1 pages 2-2)

2.3 2024 (non-yeast, NAP family insight): Modulation of acidic disordered regions and chaperone efficiency

Although not specific to S. cerevisiae Nap1, a 2024 iScience study analyzed Nap1/NAP1-like acidic disordered regions and showed that post-translational modification (glutamylation) can increase DNA mimicry and histone chaperone efficiency, reinforcing the functional importance of the acidic disordered regions implicated in yeast Nap1’s mechanism. (Publication date: Apr 2024; URL: https://doi.org/10.1016/j.isci.2024.109458) (lorton2024glutamylationofnpm2 pages 15-16)

3) Cellular localization and where Nap1 acts

3.1 Cytosolic functions

Nap1 is presented as the primary cytosolic H2A–H2B chaperone that chaperones newly synthesized and folded H2A–H2B and shields them from inappropriate interactions. (fung2024nap1andkap114 pages 1-2)

3.2 Nuclear/chromatin functions

Nap1 also functions in the nucleus in pathways including nucleosome assembly/remodeling and transcription-associated nucleosome processing, with cooperative action with Kap114/RanGTP enabling targeted histone release and transfer toward assembling nucleosomes. (fung2024nap1andkap114 pages 1-2)

4) Expert opinions and authoritative analysis (from primary sources)

4.1 Chromatin organization and gene expression in vivo (classic yeast genetics)

Ohkuni et al. interpret nap1Δ expression patterns as consistent with Nap1 functioning to maintain ordered nucleosome arrangement, potentially influencing cluster-wide transcriptional states (“tight” vs “loose” chromatin regions corresponding to cluster repression vs expression). (Publication date: Jun 2003; URL: https://doi.org/10.1016/S0006-291X(03)00907-0) (ohkuni2003genomewideexpressionanalysis pages 4-5, ohkuni2003genomewideexpressionanalysis pages 1-2)

4.2 Chaperoning as a regulated handoff pathway (modern mechanistic view)

Fung et al. argue that Nap1 and Kap114 cooperatively create a sheltered pathway for H2A–H2B that enables transfer from importin-bound histone cargo to chaperone-bound cargo and onward to nucleosome assembly substrates, providing a mechanistic framework for targeted nuclear release. (fung2024nap1andkap114 pages 1-2, fung2024nap1andkap114 media e5e2dd1a)

5) Relevant statistics and quantitative data

5.1 Genome-wide transcriptional effects of nap1Δ in yeast

In three independent Affymetrix yeast GeneChip experiments, ~8.4–12.0% of ORFs showed ≥2-fold expression changes in nap1Δ, with experiment-level breakdowns (down/up/total) reported as: 3.2% down & 8.7% up (12.0% total); 1.4% down & 8.8% up (10.2% total); 6.3% down & 2.1% up (8.4% total). (ohkuni2003genomewideexpressionanalysis pages 1-2)

Among genes changing >2-fold, ~35.4% were located in genomic “clusters” in nap1Δ (versus ~12.5–12.7% in two comparator deletions), suggesting a distinctive spatial organization component to Nap1-dependent transcriptional effects. (ohkuni2003genomewideexpressionanalysis pages 2-4)

5.2 Stoichiometry and structural resolution for Nap1/Kap114/H2A–H2B/RanGTP assemblies (2024)

Fung et al. report:
- 1:1 Kap114:Nap1_2 complex formation and 1:1:1 Kap114:Nap1_2:H2A–H2B complexes.
- A quaternary assembly consistent with 1:1:1:1 Kap114:Nap1_2:H2A–H2B:RanGTP.
- Cryo-EM structure solved at 2.9 Å (local refinement 4.0 Å).
- Approximate interface areas: H2A–H2B with Kap114 ~1,600 Ų; Kap114–Nap1_2 ~120 Ų.
- Key Nap1 β-hairpin residues E288/R290/Q292 (alanine substitutions) abolish pull-down interaction, supporting a direct functional interface. (fung2024nap1andkap114 pages 2-4)

5.3 Biochemical/structural parameters for Nap1-mediated nucleosome processing (2023)

Nagae et al. describe Nap1 as a ~48 kDa monomer that forms a stable homodimer and binds a single H2A–H2B dimer with nanomolar affinity. They model Nap1 using a globular core (residues 74–365) with disordered regions (residues 1–73 and 366–417), consistent with mechanistic emphasis on flexible tails. (nagae2023histonechaperonenap1 pages 2-2, nagae2023histonechaperonenap1 pages 2-3)

6) Current applications and real-world implementations

6.1 In vitro chromatin biochemistry and nucleosome reconstitution

Yeast Nap1 is used as a reagent in mechanistic assays that reconstitute nucleosomes (e.g., Widom 601 substrates) and evaluate transcription-coupled histone exchange or dimer removal. Nagae et al. implemented in vitro transcription assays with T7 RNAP and Nap1 to quantify H2A–H2B dimer dismantling, using EMSA, MNase digestion, and pull-down workflows under defined reaction conditions (e.g., 6 µM Nap1 dimers, 0.2 µM nucleosomes, 0.8 µM T7 RNAP, 200 mM NaCl). (nagae2023histonechaperonenap1 pages 2-3)

6.2 Structural biology of histone import/co-chaperoning

The Nap1/Kap114 study demonstrates a common real-world implementation: biochemical reconstitution of multi-protein assemblies coupled to cryo-EM to reveal how histone cargo is shielded and transferred for nucleosome assembly, along with nucleosome assembly and DNA-competition assays to validate function. (fung2024nap1andkap114 pages 1-2, fung2024nap1andkap114 media 07f742f1)

6.3 Single-molecule biophysics using NAP-family folds (contextual application)

A 2023 single-molecule optical tweezers/confocal fluorescence study used a NAP1-fold-containing histone chaperone (SET/TAF-1β) to track chaperone dynamics and nucleosome unwrapping/eviction in real time, illustrating how NAP-family folds are deployed in quantitative chromatin biophysics pipelines (a methodological direction directly relevant to yeast Nap1 studies even if the protein differs). (buzon2023thehistonechaperones pages 1-2)

Visual evidence (selected)

The cryo-EM architecture of the quaternary Nap1_2•H2A–H2B•Kap114•RanGTP complex and a mechanistic model for Nap1–Kap114 co-chaperoning and targeted histone release/transfer are shown in figures from Fung et al. (2024). (fung2024nap1andkap114 media e5e2dd1a, fung2024nap1andkap114 media 07f742f1)

Evidence map (compact table)

Table: This table summarizes experimentally supported functions, mechanisms, localization, and quantitative findings for Saccharomyces cerevisiae NAP1 (UniProt P25293; YKR048C). It is useful as a compact evidence map for functional annotation focused on the yeast protein rather than similarly named proteins in other organisms.

Conclusions and functional annotation statement

Functional annotation (most supported): Yeast NAP1 (YKR048C; UniProt P25293) encodes a dimeric H2A–H2B histone chaperone central to histone handling and nucleosome dynamics. Nap1’s primary molecular function is to bind/shield H2A–H2B and mediate their correct delivery into nucleosomes, including a Kap114/RanGTP-coordinated handoff pathway for targeted nuclear release and nucleosome assembly. Nap1 also participates in transcription-coupled chromatin dynamics by enabling H2A–H2B eviction from partially unwrapped nucleosomes via acidic disordered tails. In vivo, Nap1 contributes to chromatin organization that can influence clustered transcriptional outputs, and it has reported functional interactions with cell-cycle regulators (Clb2, Gin4). (ohkuni2003genomewideexpressionanalysis pages 4-5, fung2024nap1andkap114 pages 1-2, nagae2023histonechaperonenap1 pages 1-2)

Key references (URLs, publication dates)

• Ohkuni K. et al. Biochemical and Biophysical Research Communications (Jun 2003). “Genome-wide expression analysis of NAP1 in Saccharomyces cerevisiae.” https://doi.org/10.1016/S0006-291X(03)00907-0 (ohkuni2003genomewideexpressionanalysis pages 1-2)
• Nagae F. et al. Nucleic Acids Research (May 2023). “Histone chaperone Nap1 dismantles an H2A/H2B dimer from a partially unwrapped nucleosome.” https://doi.org/10.1093/nar/gkad396 (nagae2023histonechaperonenap1 pages 1-2)
• Fung H.Y.J. et al. Journal of Cell Biology (Nov 2024). “Nap1 and Kap114 co-chaperone H2A-H2B and facilitate targeted histone release in the nucleus.” https://doi.org/10.1083/jcb.202408193 (fung2024nap1andkap114 pages 1-2)
• Lorton B.M. et al. iScience (Apr 2024). “Glutamylation of Npm2 and Nap1 acidic disordered regions increases DNA mimicry and histone chaperone efficiency.” https://doi.org/10.1016/j.isci.2024.109458 (lorton2024glutamylationofnpm2 pages 15-16)
• Buzón P. et al. Advanced Science (Aug 2023). “The Histone Chaperones SET/TAF‐1β and NPM1….” https://doi.org/10.1002/advs.202301859 (buzon2023thehistonechaperones pages 1-2)

References

1. (ohkuni2003genomewideexpressionanalysis pages 1-2): Kentaro Ohkuni, Katsuhiko Shirahige, and Akihiko Kikuchi. Genome-wide expression analysis of nap1 in saccharomyces cerevisiae. Biochemical and biophysical research communications, 306 1:5-9, Jun 2003. URL: https://doi.org/10.1016/s0006-291x(03)00907-0, doi:10.1016/s0006-291x(03)00907-0. This article has 94 citations and is from a peer-reviewed journal.

2. (fung2024nap1andkap114 pages 1-2): Ho Yee Joyce Fung, Jenny Jiou, Ashley B. Niesman, Natalia E. Bernardes, and Yuh Min Chook. Nap1 and kap114 co-chaperone h2a-h2b and facilitate targeted histone release in the nucleus. The Journal of Cell Biology, Nov 2024. URL: https://doi.org/10.1083/jcb.202408193, doi:10.1083/jcb.202408193. This article has 7 citations.

3. (nagae2023histonechaperonenap1 pages 1-2): Fritz Nagae, Shoji Takada, and Tsuyoshi Terakawa. Histone chaperone nap1 dismantles an h2a/h2b dimer from a partially unwrapped nucleosome. Nucleic Acids Research, 51:5351-5363, May 2023. URL: https://doi.org/10.1093/nar/gkad396, doi:10.1093/nar/gkad396. This article has 19 citations and is from a highest quality peer-reviewed journal.

4. (nagae2023histonechaperonenap1 pages 2-2): Fritz Nagae, Shoji Takada, and Tsuyoshi Terakawa. Histone chaperone nap1 dismantles an h2a/h2b dimer from a partially unwrapped nucleosome. Nucleic Acids Research, 51:5351-5363, May 2023. URL: https://doi.org/10.1093/nar/gkad396, doi:10.1093/nar/gkad396. This article has 19 citations and is from a highest quality peer-reviewed journal.

5. (fung2024nap1andkap114 pages 2-4): Ho Yee Joyce Fung, Jenny Jiou, Ashley B. Niesman, Natalia E. Bernardes, and Yuh Min Chook. Nap1 and kap114 co-chaperone h2a-h2b and facilitate targeted histone release in the nucleus. The Journal of Cell Biology, Nov 2024. URL: https://doi.org/10.1083/jcb.202408193, doi:10.1083/jcb.202408193. This article has 7 citations.

6. (ohkuni2003genomewideexpressionanalysis pages 4-5): Kentaro Ohkuni, Katsuhiko Shirahige, and Akihiko Kikuchi. Genome-wide expression analysis of nap1 in saccharomyces cerevisiae. Biochemical and biophysical research communications, 306 1:5-9, Jun 2003. URL: https://doi.org/10.1016/s0006-291x(03)00907-0, doi:10.1016/s0006-291x(03)00907-0. This article has 94 citations and is from a peer-reviewed journal.

7. (lorton2024glutamylationofnpm2 pages 15-16): Benjamin M. Lorton, Christopher Warren, Humaira Ilyas, Prithviraj Nandigrami, Subray Hegde, Sean Cahill, Stephanie M. Lehman, Jeffrey Shabanowitz, Donald F. Hunt, Andras Fiser, David Cowburn, and David Shechter. Glutamylation of npm2 and nap1 acidic disordered regions increases dna mimicry and histone chaperone efficiency. iScience, 27:109458, Apr 2024. URL: https://doi.org/10.1016/j.isci.2024.109458, doi:10.1016/j.isci.2024.109458. This article has 10 citations and is from a peer-reviewed journal.

8. (fung2024nap1andkap114 media e5e2dd1a): Ho Yee Joyce Fung, Jenny Jiou, Ashley B. Niesman, Natalia E. Bernardes, and Yuh Min Chook. Nap1 and kap114 co-chaperone h2a-h2b and facilitate targeted histone release in the nucleus. The Journal of Cell Biology, Nov 2024. URL: https://doi.org/10.1083/jcb.202408193, doi:10.1083/jcb.202408193. This article has 7 citations.

9. (ohkuni2003genomewideexpressionanalysis pages 2-4): Kentaro Ohkuni, Katsuhiko Shirahige, and Akihiko Kikuchi. Genome-wide expression analysis of nap1 in saccharomyces cerevisiae. Biochemical and biophysical research communications, 306 1:5-9, Jun 2003. URL: https://doi.org/10.1016/s0006-291x(03)00907-0, doi:10.1016/s0006-291x(03)00907-0. This article has 94 citations and is from a peer-reviewed journal.

10. (nagae2023histonechaperonenap1 pages 2-3): Fritz Nagae, Shoji Takada, and Tsuyoshi Terakawa. Histone chaperone nap1 dismantles an h2a/h2b dimer from a partially unwrapped nucleosome. Nucleic Acids Research, 51:5351-5363, May 2023. URL: https://doi.org/10.1093/nar/gkad396, doi:10.1093/nar/gkad396. This article has 19 citations and is from a highest quality peer-reviewed journal.

11. (fung2024nap1andkap114 media 07f742f1): Ho Yee Joyce Fung, Jenny Jiou, Ashley B. Niesman, Natalia E. Bernardes, and Yuh Min Chook. Nap1 and kap114 co-chaperone h2a-h2b and facilitate targeted histone release in the nucleus. The Journal of Cell Biology, Nov 2024. URL: https://doi.org/10.1083/jcb.202408193, doi:10.1083/jcb.202408193. This article has 7 citations.

12. (buzon2023thehistonechaperones pages 1-2): Pedro Buzón, Alejandro Velázquez‐Cruz, Laura Corrales‐Guerrero, Antonio Díaz‐Quintana, Irene Díaz‐Moreno, and Wouter H. Roos. The histone chaperones set/taf‐1β and npm1 exhibit conserved functionality in nucleosome remodeling and histone eviction in a cytochrome c‐dependent manner. Advanced Science, Aug 2023. URL: https://doi.org/10.1002/advs.202301859, doi:10.1002/advs.202301859. This article has 7 citations and is from a peer-reviewed journal.

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Citations

1. ohkuni2003genomewideexpressionanalysis pages 1-2
2. ohkuni2003genomewideexpressionanalysis pages 2-4
3. buzon2023thehistonechaperones pages 1-2
4. ohkuni2003genomewideexpressionanalysis pages 4-5
5. https://doi.org/10.1083/jcb.202408193
6. https://doi.org/10.1093/nar/gkad396
7. https://doi.org/10.1016/j.isci.2024.109458
8. https://doi.org/10.1016/S0006-291X(03
9. https://doi.org/10.1093/nar/gkad396;
10. https://doi.org/10.1002/advs.202301859
11. https://doi.org/10.1016/s0006-291x(03
12. https://doi.org/10.1083/jcb.202408193,
13. https://doi.org/10.1093/nar/gkad396,
14. https://doi.org/10.1016/j.isci.2024.109458,
15. https://doi.org/10.1002/advs.202301859,