RASA4B

Model summary

A calcium-regulated small GTPase-activating scaffold that cycles between the cytosol and the plasma membrane. Tandem calcium-sensitive membrane-targeting modules recruit the protein to anionic lipid patches, where a central catalytic core accelerates GTP hydrolysis on membrane-proximal small GTPases. A C-terminal lipid- and zinc-binding tail stabilizes membrane association and tunes signaling specificity. Through this spatially controlled inactivation of GTPases, it dampens downstream pathways that coordinate cytoskeletal remodeling and receptor-driven signaling during cellular responses to calcium.

Predicted Gene Ontology terms

1. 01

   I begin by dissecting the domain architecture.

   The N-terminus carries two C2 modules: IPR000008 (C2 domain, residues 1–105) followed by IPR000008 (C2 domain, residues 116–234), encompassed by IPR035892 (C2 domain superfamily, residues 2–266). Tandem C2 domains create a calcium-regulated membrane-targeting clamp; Ca2+ coordination in C2 folds promotes electrostatic engagement with acidic phospholipids, thereby gating reversible association with membranes. Immediately downstream lies a central catalytic core defined by IPR001936 (Ras GTPase-activating domain, residues 244–605) and its broader scaffold IPR008936 (Rho GTPase activation protein homologous superfamily, residues 264–608), with IPR039360 (Ras GTPase-activating protein family, residues 21–774) and the catalytic hallmark IPR023152 (Ras GTPase-activating protein, conserved site, residues 468–482). This architecture encodes a GAP module that accelerates GTP hydrolysis on small GTPases by stabilizing the transition state and positioning catalytic residues. The C-terminus transitions into lipid-binding and signaling modules: IPR011993 (PH-like domain superfamily, residues 562–713) and IPR001849 (Pleckstrin homology domain, residues 566–675), with a lineage-specific signature IPR037777 (RASA4, PH domain, residues 542–681). The extreme C-terminus contains IPR001562 (Zinc finger, Btk motif, residues 675–711), a PH-associated zinc-binding element that fine-tunes membrane and protein interactions. Together, the ordered layout—tandem C2 membrane clamp, central RasGAP engine, and C-terminal PH/Btk membrane-sensing tail—creates a Ca2+-tunable, membrane-recruitable GTPase-activating scaffold.

2. 02

   From this architecture, the molecular function resolves as GO:0005096 GTPase activator activity.

   The RasGAP domain (IPR001936) and its conserved catalytic site (IPR023152) cause stimulation of GTP hydrolysis on Ras-family and related small GTPases, turning them off. The tandem C2 and PH/Btk modules do not catalyze chemistry themselves but enforce spatial control: they dock the GAP core at specific membrane microdomains where substrates reside.

3. 03

   This catalytic shutoff propagates to biological process.

   By accelerating GTP hydrolysis on small GTPases, the protein executes GO:0034260 negative regulation of GTPase activity—switching off pathways that drive actin remodeling, vesicle trafficking, and MAPK signaling. The C2 clamp’s Ca2+-dependent membrane affinity couples the GAP’s action to calcium signals, embedding the protein in GO:0071277 cellular response to calcium ion. Calcium spikes recruit the tandem C2 domains and the PH/Btk tail, synchronizing GTPase inactivation with receptor-proximal signaling and cytoskeletal responses.

4. 04

   Cellular location follows directly from the targeting modules.

   The tandem C2 domains and PH/Btk tail drive reversible membrane association, while the absence of transmembrane helices supports a soluble baseline. This yields a dynamic partition between GO:0005829 cytosol and GO:0005886 plasma membrane. In resting conditions, the protein remains cytosolic; upon Ca2+ elevation and phosphoinositide production, it accumulates at the plasma membrane to inactivate membrane-proximal GTPases.

5. 05

   Mechanistically, I hypothesize a Ca2+-gated membrane recruitment cycle.

   Calcium binding to the N-terminal C2 tandem exposes lipid-binding surfaces, docking the protein at anionic membrane patches. The central RasGAP domain then engages membrane-anchored small GTPases, while the C-terminal PH/Btk module stabilizes the complex and biases localization toward phosphoinositide-rich nanodomains. Likely interaction partners include membrane-proximal small GTPases and their effectors: Ras-family substrates at the cell surface, Src-family kinases and adaptor complexes that generate PI3K products, and actin-regulatory assemblies. This ensemble positions the GAP to terminate signaling pulses with high spatial fidelity at the plasma membrane while remaining soluble in the cytosol when signaling subsides.