Exploratory experiment designed to discover new patterns targeting G3BP1 in in vitro biochemical assays, cultured cells. Primary outcome: demonstration of TRIM21-mediated G3BP1 ubiquitination
Investigation of the molecular mechanism by which TRIM21 regulates stress granule homeostasis through direct interaction with and ubiquitination of G3BP1, a core stress granule protein. The study demonstrated that TRIM21 catalyzes K63-linked ubiquitination of G3BP1 and that this modification can effectively inhibit liquid-liquid phase separation in vitro, providing mechanistic insight into how ubiquitination controls stress granule assembly.
Materials: Recombinant TRIM21 E3 ligase, purified G3BP1 protein, E1/E2 enzymes (UBE1, UbcH5c), K63-only ubiquitin chains, anti-G3BP1 antibody, anti-K63-ubiquitin antibody. Methods: (1) Co-IP: Transfect HEK293T cells with FLAG-TRIM21 and HA-G3BP1, lyse after 48h, immunoprecipitate with anti-FLAG beads, detect with anti-HA. n=4 biological replicates. (2) In vitro ubiquitination: Combine 1μM G3BP1, 100nM TRIM21, 100nM E1, 500nM E2, 10μM K63-ubiquitin in reaction buffer at 37°C for 2h. Detect ubiquitin chains by Western blot with anti-K63-Ub antibody. n=3 technical replicates. (3) LLPS assay: Mix 10μM G3BP1 (±ubiquitination) with RNA (polyU, 25ng/μL) in phase separation buffer, image droplet formation by DIC microscopy at 1, 5, 15, 30 min. Quantify droplet number and size using ImageJ.
...Quantitative predictions: (1) TRIM21 co-IPs with G3BP1 (>5-fold enrichment over IgG control, p<0.01). (2) K63-ubiquitin signal on G3BP1 increases 10-20 fold with TRIM21 vs. without. (3) Ubiquitinated G3BP1 shows 60-80% reduction in droplet formation compared to unmodified G3BP1. (4) TRIM21-C16A shows no ubiquitination or LLPS inhibition. (5) Time course: droplet formation plateaus at 15 min for control G3BP1, but ubiquitinated G3BP1 shows <20% of control droplet density at all timepoints.
Primary: Detection of K63-linked ubiquitin chains on G3BP1 by Western blot (MW shift >30 kDa, signal >10x background). Secondary: Statistically significant reduction (p<0.05, two-tailed t-test) in LLPS droplet density (droplets/field) for ubiquitinated vs control G3BP1 (>50% reduction required). Tertiary: Catalytic mutant TRIM21-C16A shows <10% of WT ubiquitination activity.
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