Exploratory experiment designed to discover new patterns targeting G3BP1 in in vitro protein analysis systems. Primary outcome: characterization of IDR contributions to LLPS regulation
This experiment focused on characterizing the three distinct intrinsically disordered regions (IDRs) within G3BP1 and their role in regulating liquid-liquid phase separation propensity. The researchers examined how these IDRs contribute to G3BP1's function as a molecular switch and investigated the fine-tuning effects of phosphorylation within these regions. The study likely involved structural analysis, biochemical assays to measure phase separation properties, and phosphorylation mapping to understand how post-translational modifications modulate G3BP1's behavior in stress granule assembly.
Protein constructs: Recombinant human G3BP1 full-length (FL, aa 1-466) and IDR deletion mutants: ΔAcidic (Δ1-139), ΔNTR (Δ1-231), ΔC-term (Δ366-466). IDR1 (Acidic, 1-139), IDR2 (NTF2-like to RRM spacer, 140-231), IDR3 (C-terminal, 366-466). Express as His-SUMO fusions in E. coli, purify by Ni-NTA + SUMO protease cleavage. Phosphorylation sites: S149, T192 in IDR2; S232 in IDR3. Generate phosphomimetics (S→E, T→E) and non-phosphorylatable mutants (S/T→A). Disorder prediction: Run PONDR-FIT, IUPred2A, AlphaFold2 to confirm IDR regions. Circular dichroism: Measure secondary structure (far-UV CD, 190-260 nm) for FL and mutants. IDRs should show random coil signature (minimum at ~200 nm).
...Quantitative predictions: (1) CD spectra: FL G3BP1 shows mixed α/β (NTF2, RRM domains) + random coil (IDRs). ΔAcidic, ΔNTR show increased α-helical content (loss of disordered signal). (2) Phase separation: FL requires 8-12 μM protein + 15 ng/μL RNA for droplets. ΔAcidic and ΔNTR: threshold increases to 15-20 μM (reduced propensity). ΔC-term: no LLPS up to 50 μM (IDR3 essential). (3) Salt sensitivity: FL critical salt ~200 mM. ΔAcidic: critical salt increases to 250-300 mM (less charge-driven). (4) Phosphorylation: S149E or T192E reduces LLPS propensity 40-60% (higher concentration required).
...Primary: FL G3BP1 undergoes LLPS (OD600 >0.3 at 10 μM + 20 ng/μL RNA, p<0.001 vs. no RNA). ΔC-term abolishes LLPS (OD600 <0.1 under same conditions, p<0.001 vs. FL). Secondary: (1) ΔAcidic and ΔNTR increase critical concentration >1.5-fold (p<0.01). (2) Phosphomimetics reduce LLPS: S149E or T192E shifts critical concentration >40% higher (p<0.01). (3) CD confirms disorder: IDR peptides show <10% α-helix + β-sheet, >80% random coil. (4) SAXS: Kratky plot shows extended coil (no plateau) for FL, more compact for ΔC-term. (5) Phosphorylation site occupancy >80% by mass spec.
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