Phospho-KAT5 (S90) Antibody

What is KAT5/Tip60 and what are its primary functions?

KAT5 (Lysine Acetyltransferase 5), also known as Tip60 (Tat-interactive protein 60 kDa), is the catalytic subunit of the NuA4 histone acetyltransferase complex. This complex primarily acetylates nucleosomal histones H2A and H4, altering nucleosome-DNA interactions and promoting interaction with proteins that regulate transcription . KAT5 is involved in multiple cellular processes including:

• Transcriptional activation of select genes

• DNA repair, particularly double-strand breaks (DSBs)

• Apoptosis and cell cycle regulation

• Signal transduction

• Chromatin remodeling

KAT5 plays crucial roles in diverse cellular pathways including DNA double-strand break repair by promoting homologous recombination through inhibition of TP53BP1 binding to chromatin and catalyzing acetylation of histone H2A at Lys-15 (H2AK15ac) . It's also essential for activating transcriptional programs associated with growth regulation, senescence, and tumor suppression.

What is the significance of KAT5/Tip60 phosphorylation at S90?

Phosphorylation of KAT5 at serine 90 (S90) represents a critical post-translational modification that regulates its function and interactions. Research has demonstrated that CDK9 phosphorylates TIP60 at S90, and this modification specifically regulates TIP60's affinity for histone H3 and RNA Polymerase II . The phosphorylation event promotes recruitment of TIP60 to chromatin, which is essential for its function in transcriptional regulation and DNA repair processes .

S90 phosphorylation appears to be a key regulatory mechanism that positions KAT5 at specific genomic loci where it can acetylate target proteins. In experimental studies, phosphorylation-deficient S90A mutants show reduced association with chromatin and subsequently diminished functional activity in multiple cellular processes .

How does phospho-KAT5 (S90) differ from other post-translational modifications of KAT5?

KAT5 undergoes multiple post-translational modifications that regulate different aspects of its function:

While S90 phosphorylation specifically enhances chromatin recruitment through histone H3 interaction, S86 phosphorylation may have distinct but complementary functions. Research indicates that under starvation conditions, S86 phosphorylation by GSK3 activates acetyltransferase activity toward autophagy regulators like ULK1 .

Each modification appears to fine-tune KAT5's activity in specific cellular contexts or in response to different stimuli, creating a complex regulatory network that controls this important acetyltransferase.

What kinases phosphorylate KAT5 at S90?

CDK9 (Cyclin-Dependent Kinase 9) has been identified as the primary kinase responsible for phosphorylating KAT5 at S90 . This has been demonstrated through multiple experimental approaches:

• Treatment with CDK9-inhibitory molecules like SNS-032 resulted in decreased phospho-S90 signal and band shift of TIP60

• siRNA-mediated knockdown of CDK9 correlated with reduced phospho-S90 signal intensity

• Treatment with phosphatase inhibitor Calyculin A increased pS90TIP60 signals, but this effect was diminished when combined with CDK9 inhibitors SNS-032 or DRB

• In vitro kinase assays demonstrated direct phosphorylation of TIP60 by active CDK9/CycT1

Interestingly, research has observed that after CDK9 inhibition, the phospho-specific band may reappear at later time points (e.g., 3 hours), suggesting that another kinase insensitive to SNS-032 might compensate for CDK9 under certain conditions . This indicates potential redundancy in the regulatory mechanisms controlling this critical post-translational modification.

How does CDK9-mediated phosphorylation of KAT5 at S90 regulate its chromatin interactions?

CDK9-mediated phosphorylation of KAT5 at S90 serves as a molecular switch that promotes recruitment of TIP60 to chromatin through enhanced interaction with histone H3 . This mechanism involves several steps:

• CDK9 phosphorylates TIP60 at serine 90

• This phosphorylation creates or enhances a binding interface between TIP60 and histone H3

• Enhanced histone H3 binding facilitates TIP60's chromatin recruitment

• Chromatin-associated TIP60 can then acetylate nucleosomal histones H2A and H4

• These acetylation events alter chromatin structure and promote transcriptional activation

Research using TIP60 S90A phosphorylation-deficient mutants demonstrated reduced association with chromatin compared to wild-type TIP60 . This diminished chromatin association likely explains the reduced functional activity of TIP60 S90A in various cellular processes, including cell proliferation.

The regulation of TIP60's chromatin binding through S90 phosphorylation represents a critical control point for coordinating TIP60's diverse nuclear functions, from transcriptional regulation to DNA repair processes.

What functional differences exist between KAT5 S90 and S86 phosphorylation?

Phosphorylation at S90 and S86 appear to regulate distinct but potentially overlapping aspects of KAT5 function:

Experimental studies using xCELLigence real-time cell analysis showed that expression of TIP60 S90A substantially slowed cell proliferation compared to wild-type TIP60, but TIP60 S86A had an even more pronounced inhibitory effect on proliferation . This suggests differential regulation of cell growth by these two phosphorylation sites.

Interestingly, cells expressing either phosphorylation-deficient mutant eventually lost their initial growth retardation, suggesting the existence of compensation mechanisms that can overcome the absence of these regulatory modifications .

How does S90 phosphorylation affect KAT5's role in DNA repair pathways?

While direct experimental evidence linking S90 phosphorylation specifically to DNA repair functions is limited in the search results, we can infer its importance based on what we know about KAT5's roles in DNA repair and chromatin recruitment:

KAT5 contributes to DNA double-strand break (DSB) repair through several mechanisms:

• It catalyzes acetylation of histone H2A at Lys-15 (H2AK15ac), blocking the ubiquitination mark required for TP53BP1 localization at DNA breaks, thereby promoting homologous recombination

• It mediates acetylation of histone H2AX at Lys-5 (H2AXK5ac), promoting NBN/NBS1 assembly at DNA damage sites

• It can catalyze lactylation of NBN/NBS1 in response to DNA damage, promoting DSB repair via homologous recombination

Since S90 phosphorylation regulates KAT5's chromatin recruitment through enhanced histone H3 binding , this modification likely positions KAT5 at sites of DNA damage where it can acetylate these targets. Impaired S90 phosphorylation would potentially compromise KAT5's ability to localize to DNA damage sites, thereby reducing its effectiveness in promoting DNA repair.

What experimental approaches can study the dynamics of KAT5 S90 phosphorylation?

Several sophisticated approaches have been employed to study KAT5 S90 phosphorylation dynamics:

• Pharmacological manipulation and western blotting:

  • CDK9 inhibition using SNS-032 or DRB followed by western blotting with phospho-specific antibodies

  • Phosphatase inhibition with Calyculin A to increase phosphorylation levels

  • Combined treatments to demonstrate specificity of CDK9 as the kinase

• Genetic approaches:

  • siRNA-mediated knockdown of CDK9, followed by western blotting for phospho-S90

  • Generation and characterization of phospho-site mutants (S90A) in cellular assays

• Biochemical assays:

  • In vitro kinase assays with purified components:

    • FLAG-tagged TIP60 immunoprecipitation and purification

    • Dephosphorylation with alkaline shrimp phosphatase

    • Incubation with active CDK9/CycT1

    • Detection with phospho-specific antibodies

• Chromatin association studies:

  • Chromatin immunoprecipitation to assess binding to specific genomic loci

  • Nuclear fractionation to separate chromatin-bound from soluble nuclear proteins

These approaches could be combined with various cellular stressors (e.g., DNA damage, replication stress, metabolic stress) to study how S90 phosphorylation dynamics respond to different cellular conditions.

What compensation mechanisms exist when S90 phosphorylation is blocked?

Research has observed that cells expressing phosphorylation-deficient TIP60 mutants (S90A or S86A) initially exhibit growth retardation compared to wild-type TIP60, but at later time points, they lose this relative growth disadvantage . This suggests the existence of compensation mechanisms, although the specific nature of these mechanisms remains to be fully elucidated.

Potential compensation mechanisms might include:

• Activation of alternative signaling pathways that promote cell proliferation independently of TIP60 phosphorylation

• Upregulation of other acetyltransferases that can functionally substitute for phospho-TIP60, such as other MYST family members or p300/CBP

• Post-translational modifications at alternative sites on TIP60 that could restore its function in the absence of S90 phosphorylation

• Adaptation of cellular machinery to utilize unphosphorylated TIP60 more efficiently

• Changes in TIP60 protein levels through altered transcription, translation, or protein stability to compensate for reduced activity

The observation that another kinase might phosphorylate TIP60 at S90 when CDK9 is inhibited suggests redundancy in the regulatory mechanisms controlling this modification, which could contribute to the observed compensation.

What techniques are recommended for detecting KAT5 S90 phosphorylation?

Multiple techniques can be employed to detect KAT5 S90 phosphorylation, each with specific applications and considerations:

Several validated phospho-specific antibodies are commercially available:

• Rabbit polyclonal antibodies from multiple vendors (e.g., Abcam ab111588 , Boster Bio A01393S90 , St. John's Labs STJ91271 )

• These antibodies specifically detect KAT5 only when phosphorylated at S90

For optimal results, researchers should include appropriate controls to verify specificity and optimize conditions for their specific experimental system.

How can phospho-KAT5 (S90) antibodies be validated for specificity?

Rigorous validation of phospho-specific antibodies is essential for reliable results. Several approaches can be combined:

• Genetic approaches:

  • Use phosphorylation-deficient mutants (S90A) as negative controls

  • Compare wild-type cells with KAT5 knockout/knockdown cells

• Enzymatic treatments:

  • Phosphatase treatment to remove phosphorylation and eliminate signal

  • Pre-treatment with kinase inhibitors (CDK9 inhibitors like SNS-032 or DRB) to reduce phosphorylation

• Molecular approaches:

  • siRNA-mediated knockdown of the kinase (CDK9) to reduce phosphorylation

  • Competition assays with the immunizing phosphopeptide

  • Cross-validation with multiple antibodies from different sources

• Specificity controls:

  • Confirm that the antibody detects endogenous KAT5 only when phosphorylated at S90

  • Test cross-reactivity with other phosphorylated proteins

  • Peptide competition assays using phosphorylated vs. non-phosphorylated peptides

The search results indicate that commercial phospho-KAT5 (S90) antibodies have been validated through affinity purification using specific phosphopeptides and demonstrated to detect KAT5 only when phosphorylated at S90 .

What considerations should guide experimental design with phospho-KAT5 (S90) antibodies?

When designing experiments with phospho-KAT5 (S90) antibodies, researchers should consider:

• Sample preparation:

  • Include phosphatase inhibitors in lysis buffers to preserve phosphorylation status

  • For nuclear proteins, use appropriate fractionation methods to extract chromatin-bound KAT5

  • For tissue samples, optimal fixation and antigen retrieval methods are critical

• Antibody parameters:

  • Use validated antibody dilutions for specific applications (IHC: 1:50-1:300, IF: 1:50-200, ELISA: 1:5000)

  • Confirm species reactivity (human, mouse, rat)

  • Verify antibody format (liquid in PBS with glycerol, BSA, sodium azide)

• Controls:

  • Include phosphorylation-deficient mutants (S90A) as negative controls

  • Consider treatments that alter phosphorylation status (kinase inhibitors, phosphatase inhibitors)

  • Include loading controls and phosphorylation-independent KAT5 antibodies

• Storage and handling:

  • Store antibodies at -20°C for long-term storage

  • For frequent use, short-term storage at 4°C (up to one month)

  • Avoid repeated freeze-thaw cycles

• Application-specific considerations:

  • For IHC: Heat-mediated antigen retrieval with citrate buffer

  • For nuclear proteins: Consider chromatin immunoprecipitation protocols

  • For co-immunoprecipitation: Optimize buffer conditions to maintain interactions

How can phospho-mutants (S90A) be utilized to study KAT5 function?

Phospho-mutants like S90A (serine-to-alanine substitution that prevents phosphorylation) are powerful tools for studying KAT5 function:

• Functional studies:

  • Cell proliferation assays to assess growth effects (e.g., xCELLigence real-time cell analysis)

  • DNA repair assays to evaluate impact on damage response

  • Transcriptional reporter assays to measure effects on gene expression

• Interaction studies:

  • Co-immunoprecipitation to compare binding partners of wild-type vs. S90A mutant

  • Chromatin immunoprecipitation to assess recruitment to specific genomic loci

  • Protein-protein interaction assays to evaluate histone H3 binding

• Localization studies:

  • Immunofluorescence to visualize subcellular localization

  • Biochemical fractionation to compare chromatin association

  • Live-cell imaging with fluorescently-tagged constructs

• Controls and validation:

  • Negative controls for phospho-specific antibodies

  • Rescue experiments in KAT5-depleted cells

  • Comparison with other phospho-mutants (e.g., S86A) to distinguish site-specific effects

Research has demonstrated that TIP60 S90A exhibits reduced proliferation compared to wild-type TIP60, indicating the importance of this phosphorylation site for normal cell growth . Interestingly, S86A mutation had an even more pronounced effect on proliferation, suggesting differential regulation by these two phosphorylation sites .

What are best practices for immunoprecipitation of phospho-KAT5 (S90)?

Successful immunoprecipitation of phosphorylated KAT5 requires careful attention to preserving the phosphorylation state and maintaining protein interactions:

• Cell lysis and buffer composition:

  • Include phosphatase inhibitors (e.g., sodium orthovanadate, sodium fluoride, phosphatase inhibitor cocktails)

  • Use protease inhibitors to prevent degradation

  • For chromatin-bound proteins, consider nuclear extraction followed by sonication

  • Buffer example: 20 mM Tris–HCl, pH 7.5, 150 mM NaCl, 1% Triton X-100, 5 mM EDTA, protease inhibitors, phosphatase inhibitors

• Immunoprecipitation strategy:

  • For tagged constructs: Use FLAG-tagged TIP60 and anti-FLAG affinity gel

  • For endogenous protein: Use phospho-specific antibodies or pan-KAT5 antibodies

  • Pre-clear lysates to reduce non-specific binding

  • Optimize antibody amounts and incubation conditions (e.g., 2h incubation at 4°C)

• Washing and elution:

  • Multiple wash steps with appropriate stringency (e.g., TBS with varying NaCl concentrations)

  • For FLAG-tagged proteins: Elution with 3× FLAG peptide (e.g., 150 ng/μl)

  • For other IPs: Elution with Laemmli buffer or acidic glycine

• Nuclear fractionation protocol:

  • Cytosol lysis buffer: 10 mM HEPES, 10 mM KCl, 1.5 mM MgCl₂, 0.34 mM sucrose, 10% glycerol, protease/phosphatase inhibitors, 0.1% Triton X-100

  • Nuclear extraction buffer: BC100 buffer (20 mM Tris pH8, 100 mM NaCl, 0.2 mM EDTA, 10% glycerol, 0.5% Triton X-100, inhibitors)

  • Sonication to release chromatin-bound proteins (e.g., using a Bioruptor)

• Verification and analysis:

  • Western blot with phospho-specific antibodies to confirm phosphorylation status

  • Mass spectrometry to identify interaction partners and modification sites

  • In vitro kinase assays to assess phosphorylation dynamics

These detailed protocols enable researchers to effectively study KAT5 phosphorylation status and its impact on protein interactions in various cellular contexts.

How does KAT5 S90 phosphorylation contribute to cancer biology?

While the search results don't provide direct evidence linking S90 phosphorylation specifically to cancer progression, we can infer potential implications based on KAT5's known roles in cancer-related processes:

KAT5 functions in several pathways relevant to cancer:

• DNA repair, particularly double-strand breaks

• Cell cycle regulation and proliferation

• Transcriptional activation of specific gene sets

• Interaction with tumor suppressors like p53

Given that S90 phosphorylation regulates TIP60's recruitment to chromatin and affects cell proliferation , alterations in this phosphorylation could potentially impact cancer development or progression. Research has shown that phosphorylation-deficient TIP60 S90A mutants exhibit reduced proliferation , suggesting that enhanced S90 phosphorylation might promote cell proliferation—a hallmark of cancer.

The search results also mention that O-GlcNAc modified-TIP60/KAT5 is required for PCK1 deficiency-induced metastasis , indicating that post-translational modifications of KAT5 can influence cancer metastasis. Understanding the interplay between different modifications, including S90 phosphorylation, could provide insights into KAT5's role in cancer biology.

How do different post-translational modifications of KAT5 interact?

KAT5 undergoes multiple post-translational modifications that likely form a complex regulatory network:

• Phosphorylation at multiple sites:

  • S90 phosphorylation by CDK9 regulates chromatin recruitment through histone H3 binding

  • S86 phosphorylation by GSK3 under starvation conditions activates acetyltransferase activity toward autophagy regulators

  • These sites may interact functionally, as mutations at either site affect cell proliferation

• O-GlcNAcylation:

  • KAT5 can be O-GlcNAcylated by OGT

  • This modification is implicated in PCK1 deficiency-induced metastasis

  • Potential crosstalk with phosphorylation sites is possible, as O-GlcNAcylation and phosphorylation often compete for similar residues

• Ubiquitination:

  • KAT5 can be targeted by p300/CBP-associated E4-type ubiquitin ligase activity

  • This modification likely regulates KAT5 protein stability

The interaction between these modifications could involve:

• Sequential modification: One modification enabling or blocking another

• Competition for sites: Modifications competing for the same or nearby residues

• Allosteric effects: Modifications at one site affecting protein conformation and accessibility of other sites

• Differential regulation: Different modifications responding to distinct cellular signals

Understanding this complex interplay will require sophisticated approaches combining mass spectrometry, mutational analysis, and functional studies.

What is the role of KAT5 S90 phosphorylation in cellular stress responses?

While the search results don't directly address the role of S90 phosphorylation in stress responses, KAT5's functions in various stress-related pathways suggest potential involvement:

• DNA damage response:

  • KAT5 is crucial for DNA double-strand break repair

  • S90 phosphorylation regulates KAT5's chromatin recruitment , which would be necessary for accessing DNA damage sites

  • Phosphorylation could potentially be modulated in response to DNA damaging agents

• Metabolic stress:

  • KAT5 S86 is phosphorylated by GSK3 under starvation conditions

  • This activates KAT5's acetyltransferase activity toward autophagy regulators

  • S90 phosphorylation might similarly respond to metabolic stress signals

• Replication stress:

  • KAT5's role in chromatin modification is important during DNA replication

  • S90 phosphorylation by CDK9 could coordinate KAT5 activity with the transcription and replication machinery

• Cellular senescence:

  • KAT5 is involved in transcriptional programs associated with replicative senescence

  • Phosphorylation status might change during senescence induction

Future research could investigate how S90 phosphorylation is dynamically regulated in response to various stressors, potentially revealing specific roles in stress signaling pathways and adaptive responses.

How evolutionarily conserved is KAT5 S90 phosphorylation?

• The search results indicate that phospho-KAT5 (S90) antibodies react with human, mouse, and rat KAT5 , suggesting conservation of this phosphorylation site across at least these mammalian species.

• KAT5/Tip60 itself is highly conserved from yeast to humans, with homologs in model organisms including:

  • Yeast: Esa1

  • Drosophila: dTip60

  • Mouse: mTip60

  • Human: hTip60/KAT5

• Conservation of the phosphorylation site would suggest functional importance across species.

To fully address this question, researchers could:

• Perform sequence alignments to identify conservation of S90 and surrounding residues

• Examine phospho-proteomics data from multiple species

• Test whether CDK9 can phosphorylate KAT5 homologs from different species

• Investigate functional conservation by testing whether phospho-site mutants of KAT5 homologs show similar phenotypes across species

The high degree of functional conservation of KAT5 across species suggests that important regulatory mechanisms, potentially including S90 phosphorylation, may also be conserved.

Can phospho-KAT5 (S90) serve as a biomarker in disease states?

The potential of phospho-KAT5 (S90) as a biomarker in disease states is not directly addressed in the search results, but we can consider possibilities based on KAT5's functions:

• Cancer biomarker potential:

  • KAT5 is involved in DNA repair, cell proliferation, and transcriptional regulation—all processes relevant to cancer

  • S90 phosphorylation affects cell proliferation , suggesting potential dysregulation in cancer

  • Altered phosphorylation status could potentially correlate with cancer progression or treatment response

  • The availability of specific phospho-KAT5 (S90) antibodies would facilitate biomarker studies

• Neurodegenerative diseases:

  • KAT5 shows higher expression in brain tissue

  • Epigenetic dysregulation is implicated in neurodegenerative diseases

  • Phosphorylation status could potentially be altered in conditions like Alzheimer's or Parkinson's disease

• Technical considerations for biomarker development:

  • Tissue-based assays (IHC) using phospho-specific antibodies would be feasible with existing reagents

  • Preservation of phosphorylation status in clinical samples would be critical

  • Validation in large patient cohorts would be necessary to establish clinical utility

• Therapeutic implications:

  • If aberrant S90 phosphorylation contributes to disease pathogenesis, targeting CDK9 could be a therapeutic approach

  • Monitoring phospho-KAT5 (S90) levels could potentially serve as a pharmacodynamic biomarker for CDK9 inhibitors

Future research should explore correlations between phospho-KAT5 (S90) levels and disease states, particularly in cancers and conditions involving DNA repair defects or epigenetic dysregulation.

What are the key unresolved questions regarding KAT5 S90 phosphorylation?

Despite significant progress in understanding KAT5 S90 phosphorylation, several important questions remain:

• Regulatory mechanisms:

  • What upstream signals regulate CDK9-mediated phosphorylation of KAT5 at S90?

  • What phosphatases dephosphorylate KAT5 at S90?

  • How is the balance between phosphorylation and dephosphorylation dynamically regulated?

• Functional implications:

  • How does S90 phosphorylation specifically affect KAT5's acetyltransferase activity toward different substrates?

  • Does S90 phosphorylation influence KAT5's ability to catalyze other acyl modifications (crotonylation, lactylation, etc.)?

  • What is the functional interplay between S90 and S86 phosphorylation?

• Structural basis:

  • How does S90 phosphorylation structurally enhance histone H3 binding?

  • Does phosphorylation induce conformational changes in KAT5?

• Disease relevance:

  • Is S90 phosphorylation dysregulated in specific diseases?

  • Could targeting this phosphorylation be therapeutically beneficial?

• Compensation mechanisms:

  • What are the molecular mechanisms behind the observed compensation for loss of S90 phosphorylation?

  • Which kinase can phosphorylate S90 when CDK9 is inhibited?

Addressing these questions will require integrated approaches combining structural biology, biochemistry, cell biology, and disease models.

What emerging technologies might advance phospho-KAT5 (S90) research?

Several cutting-edge technologies could significantly advance our understanding of KAT5 S90 phosphorylation:

• Proximity labeling proteomics:

  • BioID or TurboID fused to wild-type or S90A mutant KAT5 to identify differential interactors

  • APEX2-based approaches to map the local environment of phosphorylated vs. unphosphorylated KAT5

• Advanced imaging techniques:

  • Super-resolution microscopy to visualize phospho-KAT5 localization at DNA damage sites

  • Live-cell FRET biosensors to monitor KAT5 phosphorylation dynamics in real time

  • Correlative light and electron microscopy (CLEM) to map phospho-KAT5 to specific nuclear structures

• Single-cell analysis:

  • Mass cytometry (CyTOF) with phospho-specific antibodies to analyze heterogeneity in KAT5 phosphorylation

  • Single-cell proteomics to correlate phosphorylation status with cellular phenotypes

• Genome engineering:

  • CRISPR knock-in of specific phosphorylation site mutations

  • Base editing to generate precise S90 modifications

  • Optogenetic or chemically-inducible CDK9 activation to study temporal dynamics

• Structural biology:

  • Cryo-EM structures of KAT5 complexes with and without S90 phosphorylation

  • Hydrogen-deuterium exchange mass spectrometry to detect phosphorylation-induced conformational changes

  • AlphaFold and other AI-based structure prediction to model phosphorylation effects