B-isox MS Phase Separation Protein Screening Service

Why Use B-isox MS for Phase Separation Protein Screening

Identifying candidate phase separation-associated proteins from complex biological samples presents a challenge that conventional proteomics workflows are not designed to address. The proteins most relevant to condensate formation — those with extended intrinsically disordered regions (IDRs), low-complexity domains (LCDs), and prion-like sequence features — are often underrepresented in standard MS datasets due to poor digestion efficiency with trypsin alone, low abundance, and unusual sequence composition not captured by standard search parameters.

The B-isox enrichment principle

Biotinylated isoxazole (B-isox) is a small molecule that forms organized microcrystalline assemblies at low temperature under defined buffer conditions. These microcrystals present a surface that preferentially interacts with proteins containing sequence features associated with phase separation propensity — particularly intrinsic disorder, low-complexity regions, and repetitive sequence motifs that support multivalent interactions. Proteins that co-precipitate with the B-isox assemblies can be released, digested, and identified by LC-MS/MS, producing an enrichment-based profile of candidate phase separation-associated proteins.

What makes this approach distinct

• No prior candidate knowledge required. Unlike antibody-based enrichment or targeted assays, B-isox MS captures proteins across the abundance range that share biophysical features associated with condensate formation, without requiring prior knowledge of which candidates to expect.
• No genetic modification needed. Unlike proximity labeling (BioID, APEX), B-isox MS works directly with native cell and tissue samples without requiring transfection, fusion protein expression, or genetic tagging.
• Proteome-wide scope. Unlike in vitro droplet assays that test one purified protein at a time, B-isox MS surveys the entire proteome in a single experiment.

Limitations and appropriate use

B-isox preferentially enriches proteins whose phase separation-associated sequence features are compatible with its binding mechanism. Proteins that phase separate primarily through folded domain interactions, nucleic acid scaffolding, or modification-dependent mechanisms may not be efficiently captured. The enrichment profile should be treated as a discovery-level survey identifying candidates that require orthogonal confirmation, not as a definitive inventory of all phase separation-competent proteins.

Applications in Phase Separation Research

The B-isox MS workflow is applicable across several research contexts where identifying candidate phase separation-associated proteins is a priority.

Drug perturbation and target discovery

Compare the B-isox-enriched proteome between drug-treated and vehicle control groups to identify proteins whose association with phase separation-associated features is altered by pharmacological intervention. This approach is directly applicable to programs targeting condensate-modulating compounds, BRD4 inhibitors, RNA helicase inhibitors, and stress granule modulators. By quantifying which proteins shift in and out of the B-isox-enriched fraction upon treatment, the assay can identify both direct targets and downstream effectors of condensate-disrupting or condensate-promoting drugs.

Stress response profiling

Oxidative stress, heat shock, osmotic stress, ER stress, and hypoxia trigger rapid changes in stress granule and condensate composition. B-isox MS profiling across stress conditions maps which proteins are recruited to or released from condensate-associated states, providing a proteome-wide view of stress-induced condensate remodeling. This application is particularly valuable for identifying stress-specific phase separation-associated proteins and for characterizing the dynamic proteome of stress granules across time courses and stressor doses.

Disease model characterization

Compare B-isox-enriched proteomes from disease model samples versus matched controls to identify proteins differentially enriched under pathological conditions. Applications include: cancer vs. normal tissue to identify tumor-specific condensate proteome changes; neurodegeneration models (ALS, FTD, Huntington's) to map the aggregation-prone proteome; viral infection models to identify host and viral proteins redistributing into condensates; and metabolic disease models to assess how nutrient state alters the condensate-associated proteome.

Mutant and genetic perturbation studies

Compare B-isox MS profiles between wild-type and mutant cell lines, knockout models, or patient-derived mutations to identify proteins whose B-isox enrichment status is altered by specific genetic perturbations. This is directly relevant for mutations in known phase separation regulators (FUS, TDP-43, hnRNPA1, G3BP1, UBQLN2) and for identifying downstream effects on the broader condensate-associated proteome.

B-isox MS Service Workflow: End-to-End Pipeline

We operate a standardized, modular pipeline that converts research samples into a prioritized list of candidate phase separation-associated proteins. Each step is quality-controlled and customizable.

Data Analysis and Deliverables

Each project delivers a complete data package designed to support candidate selection, downstream experimental planning, and publication-ready reporting.

Raw and processed MS data

• Raw instrument files (.raw format), MaxQuant-processed identification tables, LFQ intensity matrices, and QC reports
• All intermediate files included to support auditability and custom downstream analysis

Differential enrichment analysis

• Fold change and statistical significance (t-test, permutation-based FDR) for each protein between comparison groups
• Visualized as volcano plots, heatmaps with hierarchical clustering, and PCA scores plots

Disorder and phase separation prediction profiles

• IDR prediction profiles (VSL2, VL3) across the full amino acid sequence with disordered regions annotated
• FuzDrop and catGRANULE scores reported per protein and per region
• Regions exceeding prediction thresholds flagged as computationally predicted to have phase separation-associated features

Database cross-reference and functional annotation

• Each candidate annotated with match status against DrLLPS, PhaSepDB, and literature-curated LLPS reference lists
• GO enrichment, KEGG pathway analysis, and STRING protein-protein interaction networks

Prioritized candidate ranking table

• Multi-dimensional ranking integrating: protein ID, enrichment FC, adjusted p-value, IDR score, FuzDrop score, catGRANULE score, database match, and composite priority level (High/Medium/Informational)
• All priority levels labeled as computational predictions requiring orthogonal validation

Publication-ready figures

• All standard visualizations provided as vector PDF and 300 dpi PNG
• Formatted for direct use in manuscripts and presentations

Candidate Prioritization Strategy

The ranked candidate list integrates multiple independent evidence lines into a structured prioritization framework. Each candidate is assessed across five tiers:

Evidence tier 1 — Quantitative enrichment significance. Proteins with statistically significant enrichment (adjusted p-value ≤ 0.05, |FC| ≥ 1.5) receive baseline high priority. Proteins enriched below significance thresholds are flagged as lower confidence.

Evidence tier 2 — Intrinsic disorder and low-complexity features. Candidates with strong IDR predictions (VSL2 or VL3 ≥ 0.5 over ≥30 consecutive residues), extended LCDs, and multivalent interaction potential receive elevated priority. Scoring includes fraction disordered, longest disordered segment length, and amino acid composition bias.

Evidence tier 3 — Phase separation propensity predictions. FuzDrop scores above 0.6 and catGRANULE scores above 0.5 are supporting evidence. Proteins scoring above threshold on both models receive additional weight.

Evidence tier 4 — Database and literature cross-reference. Candidates matching DrLLPS, PhaSepDB, or published LLPS lists receive additional priority, aligning with independently validated phase separation-associated proteins.

Evidence tier 5 — Network centrality and functional context. Hub proteins in STRING networks or proteins in phase separation-enriched categories (RNA binding, translation, stress granule assembly) receive prioritization.

Integrated ranking: High — multi-dimensional support across ≥3 evidence tiers; Medium — 2 tiers; Informational — single tier or below-threshold scores. All priority levels are computational predictions requiring orthogonal experimental validation.

B-isox MS vs Alternative Approaches

Recommended combined workflow: B-isox MS for proteome-wide discovery and prioritization → targeted validation (mutant/drug perturbation in cells) → in vitro droplet assay and biophysical characterization for high-priority candidates.

Sample Types and Compatibility

Quality requirements

• Protein extracts should show no visible degradation on SDS-PAGE
• For multi-condition projects: ≥3 biological replicates per condition recommended
• Pilot projects (≤2 replicates) proceed with reduced statistical power
• FFPE tissue and RNAlater-preserved samples are not recommended — these treatments alter protein biophysical properties and are incompatible with B-isox enrichment

Storage and shipping

• Store at −80°C; ship on dry ice (≥5 kg, sufficient for 72-hour transit)
• Include sample manifest with: sample IDs, condition labels, protein concentration (if pre-extracted), collection date, and treatment details

Case Study — B-isox MS Reveals Stress-Specific Remodeling of the Phase Separation-Associated Proteome

Kato M, Han TW, Xie S, et al. Cell-free formation of RNA granules: low complexity sequence domains form dynamic fibers within hydrogels. Cell. 2012;149(4):753-767. doi:10.1016/j.cell.2012.04.017

• Background
• Methods
• Results
• Conclusions

The discovery that low-complexity sequence domains (LCDs) in RNA-binding proteins can form amyloid-like fibers under physiological conditions raised a fundamental question: which proteins in the proteome contain such domains, and do these domains mediate the assembly of RNA granules and other membraneless organelles? Prior to this work, the set of proteins with LCDs capable of forming such assemblies was unknown. The authors developed the B-isox precipitation method specifically to address this question — to enrich proteins with LCDs from complex cell lysates and identify them by mass spectrometry.

HeLa cell lysates were incubated with B-isox under controlled conditions. The B-isox-precipitated protein fraction was isolated by centrifugation, separated by SDS-PAGE, and gel slices subjected to in-gel tryptic digestion followed by LC-MS/MS analysis on an LTQ Orbitrap mass spectrometer. Identified proteins were analyzed for LCD content, RNA-binding domains, and sequence features predictive of granule association.

The B-isox precipitation identified a specific set of RNA-binding proteins enriched in LCDs, including known stress granule components (G3BP1, TIA1, TIAR, FUS, TDP-43, hnRNPA1, hnRNPA2B1), RNA helicases (DDX family members), and splicing factors. The LCDs from several of these proteins were shown to form hydrogels that trap specific RNA species, establishing a mechanistic link between LCD-mediated assembly and RNA granule formation. The study demonstrated that B-isox enrichment specifically captures proteins with phase separation-associated LCD features while depleting structural proteins and metabolic enzymes, validating the selectivity of the approach.

This landmark study established the B-isox MS methodology that forms the basis of our screening service. The analytical pipeline — B-isox precipitation, SDS-PAGE, in-gel digestion, LC-MS/MS, and sequence feature analysis — has been extended and refined in our service to include quantitative comparison across conditions, multi-dimensional scoring (IDR prediction, FuzDrop, catGRANULE), database cross-referencing, and systematic candidate prioritization.

FAQs About B-isox MS Phase Separation Screening

Learn about other Q&A.

Demo

Representative data visualization outputs from a B-isox MS project. The table below illustrates the format and content of the prioritized candidate ranking deliverable.

*The table shows example report format only. Actual results depend on sample type, experimental design, and LC-MS/MS data quality. All priority classifications are computational predictions requiring orthogonal validation.

Volcano Plot

Differential enrichment of B-isox captured proteins between conditions. Significantly enriched candidates highlighted.

IDR Prediction Profile

VSL2 and VL3 disorder scores across the amino acid sequence of a candidate protein, with predicted disordered regions annotated.

Functional Enrichment

KEGG pathway and GO enrichment analysis of B-isox-enriched proteins, highlighting RNA processing and stress granule categories.

References

1. Kato M, Han TW, Xie S, et al. Cell-free formation of RNA granules: low complexity sequence domains form dynamic fibers within hydrogels. Cell. 2012;149(4):753-767. doi:10.1016/j.cell.2012.04.017
2. Han TW, Kato M, Xie S, et al. Cell-free formation of RNA granules: bound RNAs identify features and components of cellular assemblies. Cell. 2012;149(4):768-779. doi:10.1016/j.cell.2012.04.016
3. Molliex A, Temirov J, Lee J, et al. Phase separation by low complexity domains promotes stress granule assembly and drives pathological fibrillization. Cell. 2015;163(1):123-133. doi:10.1016/j.cell.2015.09.015
4. Alberti S, Gladfelter A, Mittag T. Considerations and challenges in studying liquid-liquid phase separation and biomolecular condensates. Cell. 2019;176(3):419-434. doi:10.1016/j.cell.2018.12.035

*This service is for research use only (RUO). Results are intended to support the discovery and prioritization of candidate phase separation-associated proteins. Candidate proteins and predicted phase separation-prone regions should be validated by orthogonal assays. This service is not intended for clinical diagnosis, treatment decision-making, or therapeutic efficacy evaluation.