RASA4 (CAPRI): A Calcium-Regulated Ras GTPase-Activating Protein

Introduction and Summary

RASA4, also known as CAPRI (Calcium-Promoted Ras Inactivator), GAPL, or KIAA0538, is a member of the GAP1 family of Ras GTPase-activating proteins (RasGAPs) in humans. The protein was first identified and characterized in 2001 by Lockyer and colleagues as a calcium-dependent negative regulator of the Ras-MAPK signaling pathway [lockyer-2001-capri-discovery-abstract]. RASA4/CAPRI functions as a molecular switch that links calcium signaling to the inactivation of small GTPases, primarily Ras and Rap1, at the plasma membrane. Its primary biochemical activity is to accelerate the intrinsic GTPase activity of Ras proteins, converting them from the active GTP-bound state to the inactive GDP-bound state, thereby terminating Ras-mediated signaling [scheffzek-2019-rasgap-review-abstract].

The defining feature of RASA4/CAPRI is its regulation by intracellular calcium. Upon elevation of cytosolic calcium levels, CAPRI rapidly translocates from the cytoplasm to the plasma membrane, where it exerts its GAP activity on membrane-localized Ras and Rap1 [lockyer-2001-capri-discovery-abstract]. This calcium-dependent translocation mechanism positions CAPRI as a critical integrator of calcium and Ras signaling pathways, two fundamental signaling systems that control cell proliferation, differentiation, and numerous other cellular processes.

Domain Architecture and Structure

RASA4/CAPRI possesses a modular domain architecture characteristic of the GAP1 family, which also includes GAP1(IP4BP), GAP1(m), and RASAL [kupzig-2006-gap1-dual-activity-abstract]. The protein is organized with two amino-terminal C2 domains (C2A and C2B), a central GAP-related domain (GRD) containing the catalytic RasGAP activity, and a carboxyl-terminal pleckstrin homology (PH) domain followed by a Bruton's tyrosine kinase (Btk) motif [liu-2005-capri-rasal-temporal-fulltext].

The tandem C2 domains are the primary calcium-sensing modules of CAPRI. C2 domains are approximately 130-residue motifs that bind calcium ions and phospholipids, mediating calcium-dependent membrane association [liu-2005-capri-rasal-temporal-fulltext]. Studies using domain deletion and chimeric constructs demonstrated that the C2A and C2B domains together are both necessary and sufficient for sensing increases in cytosolic calcium, driving the initial translocation of CAPRI to membranes [liu-2005-capri-rasal-temporal-fulltext]. However, the sustained association of CAPRI with the plasma membrane, which distinguishes it from the related protein RASAL, requires the PH domain. A domain-breaking mutation (W664A) in the PH domain abolished sustained membrane translocation while preserving the initial calcium-triggered response [liu-2005-capri-rasal-temporal-fulltext].

The central GRD contains the catalytic machinery for GAP activity, including the critical "arginine finger" residue that is inserted into the active site of Ras to stabilize the transition state for GTP hydrolysis [scheffzek-2019-rasgap-review-abstract]. Mutagenesis of this arginine finger (R473S in CAPRI) eliminates both RasGAP and RapGAP activities, confirming that both substrates utilize the same catalytic mechanism [kupzig-2006-gap1-dual-activity-abstract].

Biochemical Function: Dual RasGAP and RapGAP Activity

A fundamental discovery regarding CAPRI was that it possesses dual specificity, acting as a GTPase-activating protein for both Ras and the related GTPase Rap1 [kupzig-2006-gap1-dual-activity-abstract]. This dual activity is shared by other GAP1 family members including GAP1(IP4BP) and RASAL, but not by GAP1(m), which appears to be Ras-specific [kupzig-2006-gap1-dual-activity-abstract]. Kinetic analyses of GAP1(IP4BP), which is biochemically similar to CAPRI, revealed Km values of 213 μM for Ras and 42 μM for Rap, with estimated kcat values of 48 s⁻¹ and 16 s⁻¹ respectively [kupzig-2006-gap1-dual-activity-abstract]. Importantly, regions outside the isolated GRD are required for Rap GAP activity; the GRD alone retains RasGAP activity but has very low RapGAP activity [kupzig-2006-gap1-dual-activity-abstract].

A sophisticated mechanism for switching between RasGAP and RapGAP activities was elucidated by Dai and colleagues in 2011 [dai-2011-capri-dimerization-abstract]. They discovered that CAPRI forms calcium-dependent homodimers both in vitro and in living cells. The dimerization site was mapped to a C-terminal helix motif that forms a hydrophobic interface. Critically, monomeric and dimeric CAPRI exhibit different substrate preferences: monomeric CAPRI displays stronger RasGAP activity, while the dimeric form has stronger RapGAP activity [dai-2011-capri-dimerization-abstract]. This calcium-dependent switch in oligomeric state provides a mechanism for coordinating the regulation of Ras and Rap1 signaling pathways in response to the amplitude of calcium signals.

Calcium-Dependent Regulation and Temporal Signal Processing

One of the most distinctive features of CAPRI is its behavior as a "low-pass filter" for calcium signals, contrasting sharply with RASAL, which tracks calcium oscillations in real time [liu-2005-capri-rasal-temporal-fulltext]. When cells are stimulated with agonists that induce calcium oscillations, RASAL undergoes synchronous, oscillatory associations with the plasma membrane, deactivating Ras during each period of membrane association [walker-2004-rasal-discovery-abstract]. In contrast, CAPRI displays a long-lasting, sustained translocation to the plasma membrane that is refractory to ongoing calcium oscillations [liu-2005-capri-rasal-temporal-fulltext].

Quantitative analysis revealed that CAPRI has a half-maximal dissociation time of greater than 280 seconds from the plasma membrane, compared to only 17 seconds for RASAL and 13 seconds for PKCγ, a conventional protein kinase C [liu-2005-capri-rasal-temporal-fulltext]. This sustained membrane association is not simply a consequence of prolonged calcium elevation; experiments performed in calcium-free conditions showed that CAPRI dissociation from the membrane occurs much more slowly than the decline in cytosolic calcium, indicating that predominantly calcium-independent interactions maintain CAPRI at the plasma membrane after the initial calcium-triggered translocation [liu-2005-capri-rasal-temporal-fulltext]. The PH domain is critical for this sustained association, likely through interactions with membrane lipids or protein partners that become available following receptor activation.

The functional consequence of this temporal filtering is that CAPRI converts different intensities of stimulation into different durations of Ras inactivation, rather than encoding information in the frequency domain as RASAL does [liu-2005-capri-rasal-temporal-fulltext]. As summarized in a commentary by Muallem, CAPRI acts as an integrator that extends the duration of calcium's effect on Ras activity, while RASAL functions as a linear decoder that preserves calcium frequency information [muallem-2005-decoding-ca-commentary].

Tissue Expression and Subcellular Localization

According to the Human Protein Atlas, RASA4 shows widespread tissue expression with particularly high RNA levels in skeletal muscle (223 nTPM), tongue, and endometrium [human-protein-atlas-rasa4]. Moderate expression is observed across brain regions including the amygdala, hippocampal formation, and cerebral cortex, as well as in kidney. Protein-level detection has been confirmed in colon (peripheral nerve/ganglion), kidney tubular cells, and testis Leydig cells, though the Human Protein Atlas notes that antibody-based detection shows variable reliability due to potential cross-reactivity [human-protein-atlas-rasa4].

At the subcellular level, CAPRI is predominantly cytoplasmic at rest, lacking phosphoinositide binding activity that would otherwise target it to membranes. Immunofluorescence studies in the Human Protein Atlas indicate localization to vesicles and cell junctions in cultured cell lines, with notable single-cell variation in expression levels [human-protein-atlas-rasa4]. However, the functional literature establishes that CAPRI exerts its primary function at the plasma membrane, where it acts on Ras pools localized to this compartment. Pivotal work by Bivona and colleagues demonstrated that Ras is activated on and signals from both the plasma membrane and the Golgi apparatus, with distinct mechanisms regulating each compartment [bivona-2003-ras-golgi-abstract]. While calcium positively regulates Ras at the Golgi through the guanine nucleotide exchange factor RasGRP1, it negatively regulates Ras at the plasma membrane through CAPRI [bivona-2003-ras-golgi-abstract]. In Jurkat T cells stimulated through the T-cell receptor, Ras activation was found to be restricted to the Golgi apparatus specifically because CAPRI inactivated plasma membrane Ras, demonstrating an unambiguous physiological role for compartmentalized Ras signaling [bivona-2003-ras-golgi-abstract].

Using live-cell imaging with a GFP-tagged Ras-binding domain (RBD) reporter, Liu and colleagues directly visualized calcium-triggered Ras deactivation by CAPRI in real time [liu-2005-capri-rasal-temporal-fulltext]. Upon receptor stimulation, CAPRI translocated to the plasma membrane, which correlated precisely with dissociation of the GFP-RBD reporter from the membrane, indicating loss of active Ras-GTP. Importantly, CAPRI showed no detectable GAP activity at resting calcium levels, suggesting that it exists in an inactive or "closed" conformation in the cytosol and is acutely activated upon membrane recruitment [liu-2005-capri-rasal-temporal-fulltext].

Biological Functions

Regulation of Neutrophil Chemotaxis

A significant recent advance in understanding CAPRI function came from studies of neutrophil chemotaxis by Xu and colleagues [xu-2021-capri-neutrophil-chemotaxis-abstract]. Neutrophils navigate through enormous concentration ranges of chemoattractant gradients (10⁻¹¹ to 10⁻⁶ M) through a process called adaptation, whereby cells remain sensitive to stronger stimuli despite continued exposure to existing chemoattractants. Using CRISPR-generated CAPRI knockout neutrophil-like cells, they demonstrated that CAPRI controls GPCR-stimulated Ras adaptation [xu-2021-capri-neutrophil-chemotaxis-abstract].

CAPRI-deficient cells displayed nonadaptive Ras activation in response to chemoattractants, with significantly increased phosphorylation of downstream effectors including AKT, GSK-3α/β, and cofilin, and excessive actin polymerization [xu-2021-capri-neutrophil-chemotaxis-abstract]. Phenotypically, CAPRI knockout cells showed defective chemotaxis in response to high-concentration gradients but, interestingly, exhibited improved chemotaxis in low or subsensitive concentration gradients due to their enhanced sensitivity [xu-2021-capri-neutrophil-chemotaxis-abstract]. This work established that CAPRI functions to lower cellular sensitivity to chemoattractants, enabling neutrophils to chemotax through a wider concentration range.

More recent work by the same group in 2025 further elucidated the upstream pathway controlling CAPRI activity in neutrophils [xu-2025-plcg2-capri-abstract]. They demonstrated that phospholipase Cγ2 (PLCγ2) is essential for both spontaneous and chemoattractant-induced calcium oscillations. PLCγ2-knockout cells exhibited impaired calcium oscillations and diminished CAPRI membrane targeting, resulting in elevated Ras activation and enhanced downstream signaling. This study established a PLCγ2 → calcium oscillation → CAPRI → Ras inactivation signaling axis that controls neutrophil sensitivity to chemoattractants [xu-2025-plcg2-capri-abstract].

Fc Receptor-Mediated Phagocytosis

Zhang and colleagues discovered an unexpected adaptor function for CAPRI in Fc gamma receptor-mediated phagocytosis, independent of its GAP activity [zhang-2005-capri-phagocytosis-abstract]. CAPRI-deficient macrophages showed impaired FcγR-mediated phagocytosis and oxidative burst, along with defective activation of the Rho GTPases Cdc42 and Rac1 [zhang-2005-capri-phagocytosis-abstract]. Remarkably, CAPRI was found to interact constitutively with both Cdc42 and Rac1, serving as an adaptor protein that recruits these GTPases to phagocytic cups during FcγR-mediated phagocytosis [zhang-2005-capri-phagocytosis-abstract]. CAPRI-deficient mice exhibited impaired innate immune responses to bacterial infection, demonstrating the physiological importance of this adaptor function [zhang-2005-capri-phagocytosis-abstract]. This study revealed that CAPRI provides a previously unknown link between Fc receptors and Rho GTPase activation essential for innate immunity.

T Cell Adhesion

In T lymphocytes, CAPRI was found to mediate CD28-dependent inhibition of cell adhesion [strazza-2015-cd28-capri-abstract]. CD28 signaling induces calcium influx, which triggers CAPRI translocation to the plasma membrane via its C2 domains. At the membrane, CAPRI inhibits Rap1 activity through its RapGAP function, leading to decreased LFA-1-mediated adhesion to antigen-presenting cells [strazza-2015-cd28-capri-abstract]. This pathway provides a mechanism for CD28 co-stimulation to modulate T cell adhesion dynamics during immune responses.

Splice Variants and RANKL Regulation

An alternatively spliced variant of CAPRI, termed deltaCAPRI, was identified in primary osteoblasts [hikita-2005-deltacapri-rankl-abstract]. This variant lacks one exon within the GAP-related domain and, in contrast to full-length CAPRI, acts to activate rather than inactivate the Ras pathway [hikita-2005-deltacapri-rankl-abstract]. DeltaCAPRI promotes the shedding of RANKL (receptor activator of NF-κB ligand), a critical regulator of osteoclastogenesis, by upregulating expression of MMP14 through Ras signaling [hikita-2005-deltacapri-rankl-abstract]. This finding highlights the importance of alternative splicing in generating functionally distinct CAPRI isoforms with opposing effects on Ras signaling.

Role in Disease

Cancer

RasGAPs have emerged as an expanding class of tumor suppressors, since loss of GAP function provides an alternative mechanism for aberrant Ras activation beyond direct Ras mutations [maertens-2014-rasgap-cancer-review-abstract]. While NF1 (neurofibromin) is the best-characterized tumor suppressor RasGAP, other family members including RASAL are epigenetically silenced in various cancers [jin-2007-rasal-epigenetic-abstract]. RASA4 hypermethylation has been reported in juvenile myelomonocytic leukemia, where it correlates with poor prognosis and higher relapse risk after stem cell transplantation [poetsch-2014-rasa4-jmml-abstract]. In cervical cancer, reduced RASA4 expression correlates with tumor progression, and ectopic RASA4 expression inhibits proliferation of cervical cancer cells through effects on HIF-α signaling [chen-2021-rasa4-cervical-cancer-abstract].

Open Questions

Several important questions about RASA4/CAPRI remain to be addressed:

1. Structural basis of dual specificity: While it is established that CAPRI has both RasGAP and RapGAP activities, the structural details of how Rap1 engages the catalytic domain, and how the oligomeric state influences substrate preference, remain incompletely understood.

2. PH domain ligands: The identity of the membrane ligands (lipids and/or proteins) that engage the PH domain and maintain CAPRI at the plasma membrane after the initial calcium-triggered translocation is not fully defined.

3. Tissue-specific functions: CAPRI is expressed in multiple cell types, but its specific functions in different tissues and developmental contexts are not comprehensively characterized.

4. Coordination with other calcium sensors: How CAPRI integrates with the broader network of calcium-sensing proteins, including conventional PKCs, to coordinate cellular responses to calcium signals remains an area of active investigation.

5. Therapeutic targeting: Whether loss of CAPRI function contributes to cancer progression broadly, and whether CAPRI activity can be therapeutically modulated, represents an important translational question.

6. Interaction with Rho GTPases: The molecular basis of CAPRI's constitutive interaction with Cdc42 and Rac1, and how this adaptor function is coordinated with its GAP activities, requires further investigation.

References

• lockyer-2001-capri-discovery-abstract: Lockyer PJ, Kupzig S, Cullen PJ. CAPRI regulates Ca(2+)-dependent inactivation of the Ras-MAPK pathway. Curr Biol. 2001;11(12):981-6. PMID: 11448776. DOI: https://doi.org/10.1016/s0960-9822(01)00261-5

• liu-2005-capri-rasal-temporal-fulltext: Liu Q, Walker SA, Gao D, et al. CAPRI and RASAL impose different modes of information processing on Ras due to contrasting temporal filtering of Ca2+. J Cell Biol. 2005;170(2):183-90. PMID: 16009725. PMCID: PMC1351313. DOI: https://doi.org/10.1083/jcb.200504167

• kupzig-2006-gap1-dual-activity-abstract: Kupzig S, Deaconescu D, Bouyoucef D, et al. GAP1 family members constitute bifunctional Ras and Rap GTPase-activating proteins. J Biol Chem. 2006;281(15):9891-900. PMID: 16431904. PMCID: PMC1904491. DOI: https://doi.org/10.1074/jbc.M512802200

• dai-2011-capri-dimerization-abstract: Dai Y, Walker SA, de Vet E, et al. Ca2+-dependent monomer and dimer formation switches CAPRI Protein between Ras GTPase-activating protein (GAP) and RapGAP activities. J Biol Chem. 2011;286(22):19905-16. PMID: 21460216. PMCID: PMC3103366. DOI: https://doi.org/10.1074/jbc.M110.201301

• zhang-2005-capri-phagocytosis-abstract: Zhang J, Guo J, Dzhagalov I, He YW. An essential function for the calcium-promoted Ras inactivator in Fcgamma receptor-mediated phagocytosis. Nat Immunol. 2005;6(9):911-9. PMID: 16041389. PMCID: PMC1464573. DOI: https://doi.org/10.1038/ni1232

• bivona-2003-ras-golgi-abstract: Bivona TG, Pérez De Castro I, Ahearn IM, et al. Phospholipase Cgamma activates Ras on the Golgi apparatus by means of RasGRP1. Nature. 2003;424(6949):694-8. PMID: 12845332. DOI: https://doi.org/10.1038/nature01806

• xu-2021-capri-neutrophil-chemotaxis-abstract: Xu X, Wen X, Moosa A, Bhimani S, Jin T. Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients. Proc Natl Acad Sci USA. 2021;118(43):e2002162118. PMID: 34675073. PMCID: PMC8639426. DOI: https://doi.org/10.1073/pnas.2002162118

• strazza-2015-cd28-capri-abstract: Strazza M, Azoulay-Alfaguter I, Dun B, et al. CD28 inhibits T cell adhesion by recruiting CAPRI to the plasma membrane. J Immunol. 2015;194(6):2871-7. PMID: 25637021. DOI: https://doi.org/10.4049/jimmunol.1401492

• walker-2004-rasal-discovery-abstract: Walker SA, Kupzig S, Bouyoucef D, et al. Identification of a Ras GTPase-activating protein regulated by receptor-mediated Ca2+ oscillations. EMBO J. 2004;23(8):1749-60. PMID: 15057271. PMCID: PMC394250. DOI: https://doi.org/10.1038/sj.emboj.7600197

• scheffzek-2019-rasgap-review-abstract: Scheffzek K, Shivalingaiah G. Ras-Specific GTPase-Activating Proteins-Structures, Mechanisms, and Interactions. Cold Spring Harb Perspect Med. 2019;9(3):a031500. PMID: 30104198. PMCID: PMC6396337. DOI: https://doi.org/10.1101/cshperspect.a031500

• maertens-2014-rasgap-cancer-review-abstract: Maertens O, Cichowski K. An expanding role for RAS GTPase activating proteins (RAS GAPs) in cancer. Adv Biol Regul. 2014;55:1-14. PMID: 24814062. DOI: https://doi.org/10.1016/j.jbior.2014.04.002

• muallem-2005-decoding-ca-commentary: Muallem S. Decoding Ca2+ signals: a question of timing. J Cell Biol. 2005;170(2):173-5. PMID: 16027217. PMCID: PMC2171404. DOI: https://doi.org/10.1083/jcb.200506047

• hikita-2005-deltacapri-rankl-abstract: Hikita A, Kadono Y, Chikuda H, et al. Identification of an alternatively spliced variant of Ca2+-promoted Ras inactivator as a possible regulator of RANKL shedding. J Biol Chem. 2005;280(50):41700-6. PMID: 16234249. DOI: https://doi.org/10.1074/jbc.M507000200

• jin-2007-rasal-epigenetic-abstract: Jin H, Wang X, Ying J, et al. Epigenetic silencing of a Ca(2+)-regulated Ras GTPase-activating protein RASAL defines a new mechanism of Ras activation in human cancers. Proc Natl Acad Sci USA. 2007;104(30):12353-8. PMID: 17640920. PMCID: PMC1941473. DOI: https://doi.org/10.1073/pnas.0700153104

• poetsch-2014-rasa4-jmml-abstract: Poetsch AR, Lipka DB, Witte T, et al. RASA4 undergoes DNA hypermethylation in resistant juvenile myelomonocytic leukemia. Epigenetics. 2014;9(9):1252-60. PMID: 25147919. PMCID: PMC4169017. DOI: https://doi.org/10.4161/epi.29941

• chen-2021-rasa4-cervical-cancer-abstract: Chen J, Huang J, Huang Q, et al. RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells. Bioengineered. 2021;12(2):10723-10733. PMID: 34752201. PMCID: PMC8809920. DOI: https://doi.org/10.1080/21655979.2021.2002499

• xu-2025-plcg2-capri-abstract: Xu X, Kim WS, Lee A, Jin T. PLCγ2 controls neutrophil-like cell sensitivity through calcium oscillation and gates chemoattractant concentration range for chemotaxis. Front Immunol. 2025;16:1633390. PMID: 40821808. PMCID: PMC12354650. DOI: https://doi.org/10.3389/fimmu.2025.1633390

• human-protein-atlas-rasa4: The Human Protein Atlas. RASA4 - Ras GTPase-activating protein 4. URL: https://www.proteinatlas.org/ENSG00000105808-RASA4. Accessed January 2026.

Citations

1. bivona-2003-ras-golgi-abstract.md
2. chen-2021-rasa4-cervical-cancer-abstract.md
3. dai-2011-capri-dimerization-abstract.md
4. hikita-2005-deltacapri-rankl-abstract.md
5. human-protein-atlas-rasa4.md
6. jin-2007-rasal-epigenetic-abstract.md
7. kupzig-2006-gap1-dual-activity-abstract.md
8. liu-2005-capri-rasal-temporal-fulltext.md
9. lockyer-2001-capri-discovery-abstract.md
10. maertens-2014-rasgap-cancer-review-abstract.md
11. muallem-2005-decoding-ca-commentary.md
12. poetsch-2014-rasa4-jmml-abstract.md
13. scheffzek-2019-rasgap-review-abstract.md
14. strazza-2015-cd28-capri-abstract.md
15. walker-2004-rasal-discovery-abstract.md
16. xu-2021-capri-neutrophil-chemotaxis-abstract.md
17. xu-2025-plcg2-capri-abstract.md
18. zhang-2005-capri-phagocytosis-abstract.md

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Research report: Human RASA4 (CAPRI) – calcium-regulated Ras GTPase-activating protein (UniProt O43374)

Verification of identity and domain architecture
- Identity: The human gene RASA4 encodes the Ras GTPase-activating protein commonly known as CAPRI (calcium‑promoted Ras inactivator), matching the provided UniProt O43374 entry and synonyms. Reviews of RasGAP families explicitly list CAPRI as RASA4 and as a calcium-regulated Ras inhibitor (CAPRI/RASA4) (king2013nonredundantfunctionsfor pages 1-2, grewal2006molecularmechanismsinvolved pages 2-4). URL: https://doi.org/10.1126/scisignal.2003669 (2013-02) (king2013nonredundantfunctionsfor pages 1-2); https://doi.org/10.1002/bies.20503 (2006-12) (grewal2006molecularmechanismsinvolved pages 2-4)
- Domains and family: RASA4 belongs to the GAP1 family of RasGAPs and is a modular protein with N‑terminal C2 domains, a PH domain, and a C‑terminal catalytic RasGAP domain. C2 domains mediate calcium-dependent membrane association; PH domains support lipid binding and recruitment (notably PI(3,4,5)P3); the catalytic RasGAP module accelerates GTP hydrolysis on Ras by transition‑state stabilization (scheffzek2019rasspecificgtpaseactivatingproteinsstructures pages 1-3, grewal2006molecularmechanismsinvolved pages 2-4, king2013nonredundantfunctionsfor pages 1-2). URLs: https://doi.org/10.1101/cshperspect.a031500 (2019-08) (scheffzek2019rasspecificgtpaseactivatingproteinsstructures pages 1-3); https://doi.org/10.1002/bies.20503 (2006-12) (grewal2006molecularmechanismsinvolved pages 2-4); https://doi.org/10.1126/scisignal.2003669 (2013-02) (king2013nonredundantfunctionsfor pages 1-2)

Table: Compact, evidence-linked summary of human RASA4/CAPRI covering identity, domains, regulation, catalytic activity, localization, key cell contexts, pathway roles, and knockdown phenotypes; useful as a quick reference with direct citations to the gathered evidence (pqac IDs).

Key concepts and definitions
- Primary biochemical function: RASA4 is a Ras-specific GAP that accelerates hydrolysis of GTP bound to Ras, thereby inactivating Ras signaling. The GAP mechanism involves stabilization of the GTP hydrolysis transition state by the conserved helical RasGAP catalytic domain (scheffzek2019rasspecificgtpaseactivatingproteinsstructures pages 1-3). URL: https://doi.org/10.1101/cshperspect.a031500 (2019-08) (scheffzek2019rasspecificgtpaseactivatingproteinsstructures pages 1-3)
- Regulation by Ca2+ and lipids: RASA4 is a calcium-regulated RasGAP. Elevations in intracellular Ca2+ prompt C2‑domain–dependent membrane translocation; PH domain interactions with PIP3 further facilitate membrane targeting. Membrane localization both brings RASA4 to Ras at the plasma membrane and is required for catalytic activation of the GAP domain (grewal2006molecularmechanismsinvolved pages 2-4, king2013nonredundantfunctionsfor pages 1-2, xu2022rasinhibitorsgate pages 10-12, xu2020rasinhibitorcapri pages 6-10). URLs: https://doi.org/10.1002/bies.20503 (2006-12) (grewal2006molecularmechanismsinvolved pages 2-4); https://doi.org/10.1126/scisignal.2003669 (2013-02) (king2013nonredundantfunctionsfor pages 1-2); https://doi.org/10.3389/fimmu.2022.1020117 (2022-10-18) (xu2022rasinhibitorsgate pages 10-12); https://doi.org/10.1101/2020.04.23.058131 (2020-04-24) (xu2020rasinhibitorcapri pages 6-10)
- Substrate specificity: RASA4 acts on Ras isoforms; direct association with N‑Ras was observed in chemoattractant‑stimulated neutrophil‑like cells. Some GAP1 family members (including RASA4) can show in vitro activity toward Rap1, although the primary physiological role of RASA4 is as a RasGAP in the contexts studied (xu2020rasinhibitorcapri pages 6-10, king2013nonredundantfunctionsfor pages 1-2). URLs: https://doi.org/10.1101/2020.04.23.058131 (2020-04-24) (xu2020rasinhibitorcapri pages 6-10); https://doi.org/10.1126/scisignal.2003669 (2013-02) (king2013nonredundantfunctionsfor pages 1-2)

Recent developments and latest research (emphasis 2022–2024)
- Chemotaxis adaptation gatekeeper in immune cells: A 2022 review synthesizes that CAPRI (RASA4) in human neutrophils controls GPCR‑mediated Ras adaptation and cell sensitivity; CAPRI is highly expressed in HL60 and human neutrophils, translocates to the leading edge in response to chemoattractants, and its recruitment depends on Ca2+ and PIP3 but not on GAP catalytic activity. CAPRI loss causes elevated basal Ras, nonadaptive Ras/Rap1 activation, excessive actin polymerization, and shifts chemotaxis to lower‑concentration ranges (xu2022rasinhibitorsgate pages 10-12). URL: https://doi.org/10.3389/fimmu.2022.1020117 (2022-10-18) (xu2022rasinhibitorsgate pages 10-12)
- Mechanistic details from cell studies consistent with the above: CAPRI translocation is GPCR/Gi dependent; ΔC2 and ΔPH mutants are defective in membrane targeting; loss of CAPRI sustains PI3K activity (PIP3 accumulation), leads to persistent AKT phosphorylation (T308/T473) and cofilin changes. CAPRI associates with N‑Ras and colocalizes with active Ras and F‑actin at the leading edge (xu2020rasinhibitorcapri pages 6-10). URL: https://doi.org/10.1101/2020.04.23.058131 (2020-04-24) (xu2020rasinhibitorcapri pages 6-10)

Cellular and subcellular localization
- Localization dynamics: RASA4 is largely cytosolic at rest and translocates to the plasma membrane/leading edge upon calcium influx or GPCR stimulation. C2 domains sense Ca2+ and bind phospholipids; PH domain binding to PIP3 further promotes recruitment. Membrane association is necessary for GAP activity and for interaction with Ras (grewal2006molecularmechanismsinvolved pages 2-4, king2013nonredundantfunctionsfor pages 1-2, xu2020rasinhibitorcapri pages 6-10, xu2022rasinhibitorsgate pages 10-12). URLs: https://doi.org/10.1002/bies.20503 (2006-12) (grewal2006molecularmechanismsinvolved pages 2-4); https://doi.org/10.1126/scisignal.2003669 (2013-02) (king2013nonredundantfunctionsfor pages 1-2); https://doi.org/10.1101/2020.04.23.058131 (2020-04-24) (xu2020rasinhibitorcapri pages 6-10); https://doi.org/10.3389/fimmu.2022.1020117 (2022-10-18) (xu2022rasinhibitorsgate pages 10-12)

Pathways and precise roles
- Ras–MAPK/ERK: By inactivating Ras at the plasma membrane, RASA4 limits MAPK/ERK pathway activation following receptor stimulation. Early work and reviews place CAPRI as a Ca2+-dependent attenuator of the Ras–MAPK cascade (grewal2006molecularmechanismsinvolved pages 2-4). URL: https://doi.org/10.1002/bies.20503 (2006-12) (grewal2006molecularmechanismsinvolved pages 2-4)
- GPCR chemotaxis and PI3K–AKT–GSK3: In neutrophil-like cells, CAPRI restrains Ras-driven PI3K signaling, thereby modulating PIP3, AKT phosphorylation, GSK3 regulation, cofilin phosphorylation, actin polymerization, and adaptation to chemoattractant gradients (xu2020rasinhibitorcapri pages 6-10, xu2022rasinhibitorsgate pages 10-12). URLs: https://doi.org/10.1101/2020.04.23.058131 (2020-04-24) (xu2020rasinhibitorcapri pages 6-10); https://doi.org/10.3389/fimmu.2022.1020117 (2022-10-18) (xu2022rasinhibitorsgate pages 10-12)

Immune-cell contexts and applications
- Neutrophils/HL60: CAPRI is required for proper adaptation to high chemoattractant concentrations. Knockdown results: elevated basal Ras, prolonged Ras activation after stimulation, increased PIP3 and sustained AKT phosphorylation, excessive actin polymerization; chemotaxis impaired at high concentration but improved at low/subsensitive ranges, indicating CAPRI gates the usable concentration window for chemotaxis (xu2020rasinhibitorcapri pages 6-10, xu2022rasinhibitorsgate pages 10-12). URLs: https://doi.org/10.1101/2020.04.23.058131 (2020-04-24) (xu2020rasinhibitorcapri pages 6-10); https://doi.org/10.3389/fimmu.2022.1020117 (2022-10-18) (xu2022rasinhibitorsgate pages 10-12)
- T cells and lymphoid expression: RASA4 is enriched in lymphoid organs (spleen, lymph nodes), and multiple RasGAPs co-exist in T cells. Stimulus-dependent recruitment to the plasma membrane has been described (e.g., CD28-dependent), supporting a role in tuning Ras/ERK during T‑cell activation and adhesion; RASA4 is discussed alongside RASA2/3 in T‑cell signaling frameworks (king2013nonredundantfunctionsfor pages 2-3, xu2022rasinhibitorsgate pages 14-14). URLs: https://doi.org/10.1126/scisignal.2003669 (2013-02) (king2013nonredundantfunctionsfor pages 2-3); https://doi.org/10.3389/fimmu.2022.1020117 (2022-10-18) (xu2022rasinhibitorsgate pages 14-14)
- Mast cells: CAPRI shuttling to the plasma membrane in mast cells has been linked to regulation of activation in response to calcium‑mobilizing stimuli, consistent with its general Ca2+-dependent inhibitory action on Ras–MAPK (grewal2006molecularmechanismsinvolved pages 2-4). URL: https://doi.org/10.1002/bies.20503 (2006-12) (grewal2006molecularmechanismsinvolved pages 2-4)

Statistics and quantitative findings (from recent cell studies)
- In neutrophil-like HL60 cells lacking CAPRI, investigators report increased basal Ras activity and stronger, prolonged Ras responses after chemoattractant; signaling readouts include sustained phosphorylation of AKT at T308/T473 and persistent PIP3 accumulation, consistent with failure of adaptation. These effects co-occur with excessive actin polymerization and altered chemotaxis performance across gradient ranges. While precise fold-changes vary by experiment, the qualitative direction and persistence of signaling changes are consistent across studies (xu2020rasinhibitorcapri pages 6-10, xu2022rasinhibitorsgate pages 10-12). URLs: https://doi.org/10.1101/2020.04.23.058131 (2020-04-24) (xu2020rasinhibitorcapri pages 6-10); https://doi.org/10.3389/fimmu.2022.1020117 (2022-10-18) (xu2022rasinhibitorsgate pages 10-12)

Expert opinions and authoritative reviews
- Family-level perspective: Authoritative reviews of RasGAP structures and mechanisms emphasize that accessory domains, including C2 and PH domains in GAP1-family members like RASA4, dictate subcellular targeting and regulatory control of catalytic activity, and that membrane recruitment is central to function. These reviews also note that some RasGAPs can affect Rap, but RASA4’s physiological role is best supported as a calcium-regulated RasGAP at membranes (scheffzek2019rasspecificgtpaseactivatingproteinsstructures pages 1-3, king2013nonredundantfunctionsfor pages 1-2). URLs: https://doi.org/10.1101/cshperspect.a031500 (2019-08) (scheffzek2019rasspecificgtpaseactivatingproteinsstructures pages 1-3); https://doi.org/10.1126/scisignal.2003669 (2013-02) (king2013nonredundantfunctionsfor pages 1-2)

Disease and translational context
- Cancer and disease associations: At the level of principle, loss of function or downregulation of RasGAPs can contribute to aberrant Ras signaling in cancers; reviews place CAPRI/RASA4 within the cytoplasmic GAPs that modulate oncogenic cascades. However, compared with NF1 or RASA1, specific recurrent genetic alterations in RASA4 in human tumors are less well documented, and recent targeted 2023–2024 reports are limited in the present evidence set (scheffzek2019rasspecificgtpaseactivatingproteinsstructures pages 1-3). URL: https://doi.org/10.1101/cshperspect.a031500 (2019-08) (scheffzek2019rasspecificgtpaseactivatingproteinsstructures pages 1-3)

Notes on gene symbol ambiguity and paralogs
- The human genome encodes a close paralog, RASA4B, which appears in some databases. The present report is restricted to RASA4 (UniProt O43374) and does not conflate with RASA4B. Evidence summarized above explicitly refers to CAPRI/RASA4 in human immune cells and to the GAP1 family mechanisms (king2013nonredundantfunctionsfor pages 1-2). URL: https://doi.org/10.1126/scisignal.2003669 (2013-02) (king2013nonredundantfunctionsfor pages 1-2)

Conclusions
- RASA4 (CAPRI) is a calcium-regulated, membrane-recruited RasGAP that inactivates Ras at the plasma membrane to tune receptor-driven signaling. Its C2 and PH domains coordinate Ca2+ and phosphoinositide inputs to control translocation and catalytic activation, and in immune cells it functions as a gatekeeper of GPCR-mediated Ras adaptation, thereby governing PI3K–AKT signaling, actin dynamics, and chemotactic range. Recent work (2022) consolidates RASA4’s role in neutrophil chemotaxis and signaling adaptation; additional cell-type–specific roles in T cells and mast cells are consistent with its regulatory logic and membrane recruitment mechanisms (xu2022rasinhibitorsgate pages 10-12, xu2020rasinhibitorcapri pages 6-10, grewal2006molecularmechanismsinvolved pages 2-4, king2013nonredundantfunctionsfor pages 1-2, scheffzek2019rasspecificgtpaseactivatingproteinsstructures pages 1-3). URLs: https://doi.org/10.3389/fimmu.2022.1020117 (2022-10-18) (xu2022rasinhibitorsgate pages 10-12); https://doi.org/10.1101/2020.04.23.058131 (2020-04-24) (xu2020rasinhibitorcapri pages 6-10); https://doi.org/10.1002/bies.20503 (2006-12) (grewal2006molecularmechanismsinvolved pages 2-4); https://doi.org/10.1126/scisignal.2003669 (2013-02) (king2013nonredundantfunctionsfor pages 1-2); https://doi.org/10.1101/cshperspect.a031500 (2019-08) (scheffzek2019rasspecificgtpaseactivatingproteinsstructures pages 1-3)

References

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2. (grewal2006molecularmechanismsinvolved pages 2-4): Thomas Grewal and Carlos Enrich. Molecular mechanisms involved in ras inactivation: the annexin a6–p120gap complex. BioEssays, 28:1211-1220, Dec 2006. URL: https://doi.org/10.1002/bies.20503, doi:10.1002/bies.20503. This article has 74 citations and is from a peer-reviewed journal.

3. (scheffzek2019rasspecificgtpaseactivatingproteinsstructures pages 1-3): Klaus Scheffzek and Giridhar Shivalingaiah. Ras-specific gtpase-activating proteins-structures, mechanisms, and interactions. Cold Spring Harbor perspectives in medicine, 9 3:a031500, Aug 2019. URL: https://doi.org/10.1101/cshperspect.a031500, doi:10.1101/cshperspect.a031500. This article has 104 citations and is from a peer-reviewed journal.

4. (xu2020rasinhibitorcapri pages 6-10): Xuehua Xu, Xi Wen, Amer Moosa, Smit Bhimani, and Tian Jin. Ras inhibitor capri enables neutrophils to chemotax through a higher-concentration range of gradients. bioRxiv, Apr 2020. URL: https://doi.org/10.1101/2020.04.23.058131, doi:10.1101/2020.04.23.058131. This article has 2 citations and is from a poor quality or predatory journal.

5. (xu2022rasinhibitorsgate pages 10-12): Xuehua Xu and Tian Jin. Ras inhibitors gate chemoattractant concentration range for chemotaxis through controlling gpcr-mediated adaptation and cell sensitivity. Frontiers in Immunology, Oct 2022. URL: https://doi.org/10.3389/fimmu.2022.1020117, doi:10.3389/fimmu.2022.1020117. This article has 3 citations and is from a peer-reviewed journal.

6. (xu2022rasinhibitorsgate pages 14-14): Xuehua Xu and Tian Jin. Ras inhibitors gate chemoattractant concentration range for chemotaxis through controlling gpcr-mediated adaptation and cell sensitivity. Frontiers in Immunology, Oct 2022. URL: https://doi.org/10.3389/fimmu.2022.1020117, doi:10.3389/fimmu.2022.1020117. This article has 3 citations and is from a peer-reviewed journal.

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Citations

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2. grewal2006molecularmechanismsinvolved pages 2-4
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6. king2013nonredundantfunctionsfor pages 2-3
7. xu2022rasinhibitorsgate pages 14-14
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9. https://doi.org/10.1002/bies.20503
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12. https://doi.org/10.1101/2020.04.23.058131
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Overview of RASA4 (CAPRI) and Its Function in Ras Signaling

RASA4, also known as RAS p21 protein activator 4 or CAPRI (Calcium-Promoted Ras Inactivator), is a member of the GAP1 family of GTPase-activating proteins that down-regulate Ras signaling in a calcium-dependent manner (www.ncbi.nlm.nih.gov). Like other Ras GAPs, RASA4 accelerates the intrinsic GTPase activity of Ras, converting Ras from its active GTP-bound form to an inactive GDP-bound form (www.ncbi.nlm.nih.gov). This terminates Ras-mediated signaling, notably the Ras–Raf–MEK–ERK (MAPK) pathway which controls gene expression, cell growth, and differentiation (www.ncbi.nlm.nih.gov). RASA4 is therefore a negative regulator of Ras signaling that links intracellular calcium signals to Ras inactivation (pharos.nih.gov). It is widely expressed across tissues (with especially high mRNA levels in immune organs like spleen and lymph nodes) (pmc.ncbi.nlm.nih.gov), suggesting a broad role in cellular signaling. Importantly, RASA4 was first characterized in 2001 as a calcium-regulated Ras GAP that localizes to the cytosol at rest and translocates to the plasma membrane upon Ca^2+ elevation (pmc.ncbi.nlm.nih.gov). Below, we discuss RASA4’s structure, biochemical activity, cellular localization, and involvement in key biological processes, citing recent research (including 2021–2024 studies) for the latest insights.

Domain Structure and Calcium-Regulated Activation

RASA4/CAPRI has a multi-domain architecture typical of the GAP1 subfamily of Ras GAPs (pmc.ncbi.nlm.nih.gov). It contains two tandem C2 domains at the N-terminus, a central GAP-related domain (GRD) that provides the catalytic Ras GTPase-activating function, and a C-terminal pleckstrin homology (PH) domain followed by a Bruton’s tyrosine kinase (Btk) motif (pmc.ncbi.nlm.nih.gov). The C2 domains are calcium-binding modules; notably, RASA4 (and the related RASAL1) contain the full complement of five acidic residues in each C2 domain required for Ca^2+ coordination (pmc.ncbi.nlm.nih.gov). Indeed, rises in intracellular Ca^2+ trigger RASA4’s C2 domains to bind lipid membranes, causing rapid translocation of RASA4 from the cytosol to the inner surface of the plasma membrane (pmc.ncbi.nlm.nih.gov). This Ca^2+-dependent membrane recruitment is essential for RASA4’s function – in unstimulated cells RASA4 is cytosolic, but upon a stimulus that raises Ca^2+ (such as receptor activation), RASA4 relocates to the plasma membrane where its GAP domain can access Ras proteins (pmc.ncbi.nlm.nih.gov). Interestingly, the PH domain of RASA4 appears atypical: unlike other GAP1 family members, RASA4’s PH domain does not bind phosphoinositides like PIP_2 or PIP_3 due to a Leu→Arg substitution at a key position (www.abcam.com). This suggests RASA4’s membrane targeting relies predominantly on Ca^2+/C2-domain interactions with anionic phospholipids (e.g. phosphatidylserine) rather than PH domain binding to phosphoinositide lipids. Consistent with this mechanism, deletion of the C-terminal PH-Btk module does not prevent Ca^2+-triggered translocation of RASA4 to the membrane (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov), whereas mutations that disrupt Ca^2+ binding abolish membrane recruitment (pmc.ncbi.nlm.nih.gov). Thus, RASA4 acts as a calcium sensor in cells: it remains dormant in the cytosol until a rise in Ca^2+ induces it to attach to the membrane and become active as a Ras GTPase-accelerating protein (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

GTPase-Activating Function and Substrate Specificity

Primary biochemical function: RASA4’s GRD catalyzes the hydrolysis of GTP on Ras, turning off Ras signaling. In enzymatic terms, RASA4 provides an “arginine finger” into Ras’s active site, accelerating GTP hydrolysis by several orders of magnitude (as is typical for Ras GAPs) (pmc.ncbi.nlm.nih.gov). The primary substrates are the canonical Ras family small GTPases (H-Ras, N-Ras, K-Ras), which control the MAPK cascade. By stimulating Ras’s GTPase, RASA4 prevents prolonged activation of downstream ERK/MAPK signaling that would otherwise drive transcriptional programs for proliferation and differentiation (www.ncbi.nlm.nih.gov). Importantly, emerging research shows that RASA4 is a dual-specificity GAP under certain conditions. In addition to Ras, RASA4 can target the closely related GTPase Rap1 (a regulator of cell adhesion and other processes). In vitro and cell-based assays demonstrated that RASA4 (like several GAP1 family members) has measurable GAP activity toward Rap1, albeit weaker than its RasGAP activity (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This Rap1-GAP activity is calcium-regulated. A 2006 biochemical study (Kupzig et al., J. Biol. Chem., 2006) showed that RASA4’s ability to inactivate Rap1 is low at baseline but significantly enhanced when Ca^2+ triggers RASA4’s association with the plasma membrane (pmc.ncbi.nlm.nih.gov). Subsequent work (Dai et al., 2011) revealed the mechanism: Ca^2+ not only recruits RASA4 to the membrane, but also induces RASA4 homodimerization, and this oligomeric state favors Rap1 as a substrate (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Monomeric RASA4 predominantly acts as a RasGAP, whereas Ca^2+-induced RASA4 dimers exhibit increased Rap1GAP activity (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In live-cell experiments, a dimerization-deficient mutant of RASA4 had strong Ras-inactivating activity but was impaired in turning off Rap1, confirming that dimerization is crucial for RASA4’s RapGAP function (pmc.ncbi.nlm.nih.gov). These findings position RASA4 as a versatile GAP that can coordinate Ras and Rap1 signaling depending on cellular Ca^2+ levels. In effect, RASA4 acts as a calcium-sensitive switch: at moderate Ca^2+ it mainly inactivates Ras, and at high Ca^2+ it can simultaneously dampen Rap1 signaling, thereby tuning the balance between the Ras-ERK pathway and Rap1-mediated pathways (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This dual specificity, regulated by Ca^2+ and RASA4’s conformational state, is a distinctive feature that sets RASA4 apart from many other Ras GAPs (which typically do not act on Rap GTPases).

Subcellular Localization and Activation by Calcium

Under resting conditions, RASA4 is a cytosolic protein, but its active site must access membrane-bound Ras. RASA4 solves this by moving to the membrane only when needed: upon cell stimulation that raises intracellular Ca^2+, RASA4 rapidly translocates to the inner leaflet of the plasma membrane (pmc.ncbi.nlm.nih.gov). Early work by Lockyer and colleagues (2001) showed that agonists evoking Ca^2+ mobilization (e.g. histamine in certain cell types) cause GFP-tagged CAPRI to accumulate at the plasma membrane within seconds, coincident with Ras inactivation (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This Ca^2+-triggered translocation is mediated by the two C2 domains which bind membrane phospholipids in the presence of Ca^2+ (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). The localization switch is crucial: RASA4’s GAP activity is essentially confined to when and where it attaches to the membrane. Indeed, RASA4 has little to no Ras-GAP activity in solution; it becomes an effective Ras inhibitor only in the cellular context when recruited to Ras’s membrane compartment (pmc.ncbi.nlm.nih.gov). While at the membrane, RASA4 likely interacts with Ras at the inner plasma membrane (where Ras is anchored by lipid modifications) and possibly other signaling proteins. Notably, RASA4’s membrane residence can persist as long as Ca^2+ remains elevated – for example, CAPRI tends to stay membrane-bound during sustained calcium signals, unlike some related proteins that dissociate rapidly with Ca^2+ oscillations (pmc.ncbi.nlm.nih.gov). This behavior allows RASA4 to act as a brake on Ras for the duration of a calcium signal. Once Ca^2+ levels fall, RASA4 releases from the membrane and returns to the cytosolic inactive pool. In summary, subcellular localization is tightly coupled to RASA4 function: it is a cytosolic protein that dynamically relocalizes to carry out its GAP activity at the plasma membrane in response to second messenger (Ca^2+) signals (pmc.ncbi.nlm.nih.gov).

Role in Ras/MAPK Signaling and Calcium Signaling Pathways

By linking Ca^2+ signals to Ras inactivation, RASA4 serves as a key node in crosstalk between calcium signaling pathways and the Ras/MAPK pathway. Many cell stimuli – for instance, engagement of certain G-protein coupled receptors or immune cell receptors – trigger concurrent pathways: one leading to calcium release and another activating Ras. RASA4 is thought to sense the Ca^2+ increase and shut off Ras at the appropriate time, preventing overactivation of downstream ERK/MAPK signaling (www.ncbi.nlm.nih.gov). Lockyer et al. (2001) described CAPRI as an “amplitude sensor” for intracellular Ca^2+ bursts, meaning that RASA4 can translate the intensity of Ca^2+ signals into proportional Ras inactivation (pmc.ncbi.nlm.nih.gov). In practical terms, a mild stimulus causing a small Ca^2+ transient may only weakly recruit RASA4, allowing some Ras–ERK signaling to proceed; but a strong stimulus (with high Ca^2+) robustly brings RASA4 to the membrane (and in dimeric form), swiftly terminating Ras activity (and even dampening Rap1) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This provides a negative feedback loop in signaling: for example, in growth factor or immune receptor signaling, initial Ras activation can stimulate Ca^2+ release (via PLCγ generating IP_3), and the resulting Ca^2+ then recruits CAPRI/RASA4 to turn Ras off. Such feedback ensures transient Ras activation rather than sustained signaling. The outcome is finer control of the MAPK pathway, which is critical because prolonged Ras–ERK activity can lead to inappropriate gene transcription or cell proliferation. Consistent with this role, RASA4 loss can cause hyperactive Ras signaling. Cells lacking RASA4 show prolonged Ras–ERK pathway activation when calcium is elevated, since the usual Ca^2+-induced Ras shutoff is missing (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This is evident in experimental models like CAPRI-knockdown neutrophils, which exhibit abnormally sustained Ras-GTP and downstream PI3K/Akt signaling upon stimulation (pmc.ncbi.nlm.nih.gov). Thus, in normal physiology RASA4 helps terminate Ras/MAPK signals in a timely, Ca^2+-dependent manner, preventing overactivation of pathways that drive proliferation or other responses.

Beyond the generic Ras/MAPK pathway, RASA4 also participates in specific signaling contexts. For example, RASA4 can function as an “adaptor” bridging Ras signaling to certain actin-regulating pathways. A 2005 Nature Immunology study showed that RASA4 (CAPRI) is required in Fcγ receptor-mediated phagocytosis in macrophages (pmc.ncbi.nlm.nih.gov). During phagocytosis of antibody-coated targets, calcium rises and CAPRI translocates to nascent phagocytic cups. There, RASA4 was found to interact with the Rho-family GTPases Cdc42 and Rac1 (key regulators of the actin cytoskeleton) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Although RASA4’s GAP domain is specific for Ras, it constitutively binds Cdc42/Rac1 and co-localizes with them at the membrane during phagocytosis (pmc.ncbi.nlm.nih.gov). In CAPRI knockout mice, macrophages showed impaired phagocytic engulfment and a reduced oxidative burst response, due to defective activation of Cdc42 and Rac1 at the phagocytic cup (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). These findings suggest that RASA4 serves as a scaffold or adaptor, bringing together activated Ras (to turn it off) and the actin-remodeling GTPases (to help turn them on) in the precise spatial context of the forming phagosome (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In essence, RASA4 links the Fcγ receptor’s calcium signal to coordinated regulation of two GTPase modules: it terminates local Ras signals while facilitating Rac1/Cdc42-driven actin assembly needed for particle engulfment (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This is a specialized example of how RASA4 integrates into signaling pathways beyond just Ras/MAPK, influencing cytoskeletal dynamics and innate immune responses.

Biological Functions in Immune Cells

RASA4 plays significant roles in the immune system, where stimuli often elevate Ca^2+ and require tight control of Ras and actin signaling. In innate immune cells such as neutrophils and macrophages, RASA4 has emerged as an important regulator:

• Chemotaxis and GPCR signaling in neutrophils: A recent study by Xu et al. (PNAS, 2021) identified RASA4/CAPRI as the crucial “Ras inhibitor” that enables neutrophils to properly adapt to chemoattractant gradients (pmc.ncbi.nlm.nih.gov). Neutrophils migrating toward signals (like fMLP or IL-8) experience a rapid Ras activation through GPCR pathways, which then must be attenuated (adaptation) to prevent saturation of the signaling cascade. Xu et al. showed that when CAPRI was knocked down in human neutrophil-like cells, the cells failed to turn off Ras signaling after initial stimulation – Ras-GTP remained abnormally high (non-adaptive) and downstream effectors like Akt, GSK-3β, and cofilin were hyper-phosphorylated (pmc.ncbi.nlm.nih.gov). This led to excessive F-actin polymerization and defective chemotaxis in steep chemoattractant gradients (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Essentially, without RASA4, neutrophils lost the ability to “reset” their signaling, becoming overstimulated and less directional in high signal conditions. Interestingly, the same cells showed supersensitive responses in shallow gradients – they would over-react to normally sub-threshold cues (pmc.ncbi.nlm.nih.gov). This paradoxical behavior underscores RASA4’s role in tuning signal sensitivity: with RASA4, neutrophils exhibit proper adaptation, allowing them to sense increases in chemoattractant over a wide concentration range (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). The 2021 study concluded that CAPRI is an essential inhibitory component of the chemotactic signaling network, working in tandem with excitatory signals to optimize neutrophil navigation (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This is an expert confirmation that RASA4’s calcium-triggered Ras-GAP activity has a real-world function in immune cell migration.

• Fc receptor-mediated phagocytosis in macrophages: As mentioned, RASA4 was shown to be critical for efficient phagocytosis in a 2005 mouse study (Zhang et al., Nat. Immunol. 2005). CAPRI-deficient mice (Rasa4^–/–) were viable and developed normally, with no overt issues in basal immune cell populations (pmc.ncbi.nlm.nih.gov). However, when challenged, they had a severely impaired innate immune response to infections (pmc.ncbi.nlm.nih.gov). For instance, 80% of RASA4-knockout mice succumbed to Salmonella infection within 2 days, whereas 0% of wild-type mice died in that period (pmc.ncbi.nlm.nih.gov). They also showed dramatically reduced clearance of bacterial infections like Streptococcus pneumoniae in the lungs (pmc.ncbi.nlm.nih.gov). At the cellular level, macrophages from these knockout mice could bind antibody-coated bacteria but struggled to internalize them and mount a normal respiratory burst (pmc.ncbi.nlm.nih.gov). The underlying cause was traced to failure to properly activate Cdc42 and Rac1 during phagosome formation, due to loss of CAPRI’s adaptor function (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Thus, in real-world terms, RASA4 is required for host defense, linking calcium signals from engaged Fcγ receptors to the cytoskeletal rearrangements and microbicidal responses needed to engulf and destroy pathogens. These immune phenotypes highlight that, while RASA4’s enzymatic target is Ras, its absence has pleiotropic effects on immune cell function – but notably, those effects manifest in specific processes (phagocytosis, chemotaxis) where spatially and temporally controlled Ras inactivation is critical.

It is worth noting that RASA4 is one of several Ras regulators in immune cells. For example, in T-lymphocytes, Ras activation is triggered by the Ca^2+-dependent RasGEF RasGRP1, and it’s plausible that RASA4 provides a balancing negative feedback in that context as well. Although RASA4’s role in adaptive immunity is less well characterized, its ubiquitous expression and calcium-regulation suggest it may modulate signals downstream of the T-cell receptor or other lymphocyte receptors that induce calcium influx. The pronounced phenotypes in innate immunity, however, clearly establish RASA4 as a key signaling hub in immune responses, preventing overactivation of Ras pathways and coordinating them with cytoskeletal changes via Rac/Cdc42.

Roles in Cell Proliferation and Cancer

Given Ras’s central role in cell proliferation and oncogenic transformation, RASA4 has drawn interest for its potential influence on cancer-related pathways. By turning off Ras, RASA4 can act as a tumor suppressor-like factor, especially in tumors where Ras/MAPK signaling is driving growth. Recent cancer studies have started to examine RASA4 expression and function:

• A 2021 study by Chen et al. (published in Bioengineered, Dec 2021) investigated RASA4 in cervical squamous cell carcinoma (CESC). They found that RASA4 mRNA is significantly downregulated in cervical cancer tissues compared to normal tissue (pmc.ncbi.nlm.nih.gov). Low RASA4 levels correlated with poorer patient prognosis, suggesting RASA4 loss might contribute to tumor progression (pmc.ncbi.nlm.nih.gov). Experimentally, forcing RASA4 expression in cervical cancer cell lines (HeLa and C-33A) suppressed their proliferation, while CRISPR/Cas9 knockout of RASA4 enhanced proliferation (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Mechanistically, the authors showed that RASA4 overexpression inactivates the HIF-1α signaling pathway in these cells (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Hypoxia-inducible factor 1α is often upregulated in tumors to promote angiogenesis and survival. In RASA4-overexpressing cells, HIF-1α transcriptional activity (measured by an HRE-luciferase reporter) was markedly reduced, and the levels of survivin (a pro-survival protein and HIF target) were diminished (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Conversely, RASA4 deletion led to heightened HIF-1α activity and survivin expression, effects which could be reversed by a HIF-1α inhibitor (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). These results indicate that RASA4 suppresses tumor cell growth at least in part by down-regulating HIF-1α signaling, likely through its upstream effect on Ras. (Active Ras can stabilize HIF-1α via PI3K/AKT and MAPK pathways; thus RASA4 may impede those pro-HIF signals.) The study concluded that RASA4 is a promising inhibitor of cervical cancer cell proliferation and that its low expression in tumors could be an important diagnostic or prognostic marker (pmc.ncbi.nlm.nih.gov). Indeed, RASA4 expression had a high diagnostic value for distinguishing CESC tissue (ROC AUC ~0.986 in their analysis) (pmc.ncbi.nlm.nih.gov).

• Broader cancer genomic data also hint at RASA4’s role. While RASA4 is not among the most commonly mutated genes in cancer, loss-of-function mutations do occur. For instance, a 2022 study in Diffuse Large B-cell Lymphoma (DLBCL) identified a somatic missense mutation in RASA4 (D111N) in a relapsed tumor, predicted to be damaging (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Alongside mutations in other Ras regulators (like NF1 GTPase-activating protein and RASGRP4 GEF), the RASA4 mutation was associated with enhanced ERK/MAPK activation in the relapsed lymphoma cells (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This suggests that disrupting RASA4 can contribute to aberrant Ras pathway activation in cancer, potentially conferring growth advantage or treatment resistance. Indeed, RASA4’s closest homologs (like RASA1/p120 RasGAP and NF1) are well-known tumor suppressors, and although RASA4 is calcium-regulated and cell-context-specific, it appears to have a similar brake function on mitogenic signaling. Noonan syndrome and related “RASopathy” disorders (caused by germline Ras/MAPK pathway mutations) have not implicated RASA4 to date, but somatic alterations in RASA4 might collaborate with other oncogenic events to sustain Ras activity in certain contexts.

In summary, RASA4’s activity opposes oncogenic Ras signaling, and maintaining its function is ordinarily beneficial for keeping cell proliferation in check. When RASA4 is under-expressed or mutated, cells may experience more unchecked Ras/MAPK activity, which can contribute to tumorigenesis. While RASA4 is not as famous as major tumor suppressors like NF1, ongoing research (2021–2024) highlights its potential importance in cancer biology and even as a therapeutic target: for example, strategies to boost RASA4 activity or mimic its function could, in theory, help suppress Ras-driven tumor growth. Conversely, excessive RASA4 activity might dampen necessary Ras signals, so the cell must balance this carefully – which it achieves through the calcium-dependent regulatory mechanism described above.

Conclusion and Expert Perspectives

RASA4/CAPRI emerges as a multifaceted regulator that connects calcium second-messenger signals to the control of two critical small GTPases, Ras and Rap1. Its primary role is to serve as a calcium-triggered “off-switch” for Ras, ensuring that stimuli which elevate Ca^2+ (such as immune receptor engagement or growth factors) do not lead to runaway Ras/MAPK signaling (www.ncbi.nlm.nih.gov). This function is vital across various scenarios – from a neutrophil adjusting its sensitivity in a gradient (pmc.ncbi.nlm.nih.gov), to a macrophage coordinating actin remodeling for phagocytosis (pmc.ncbi.nlm.nih.gov), to a cell in a tumor suppressing proliferative signals (pmc.ncbi.nlm.nih.gov). Structurally, RASA4 is equipped with C2 domains to sense Ca^2+ and a GAP domain to execute GTP hydrolysis on Ras (and Rap1 when dimerized) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Experts in the field highlight RASA4 as an “inhibitory component” or brake in signaling pathways: for example, Xu et al. (2021) describe CAPRI as “essential for both the sensitivity and the GPCR-mediated adaptation of human neutrophils”, acting to locally control Ras activity and prevent overstimulation (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Likewise, immunologists (Zhang et al. 2005) noted that “these results suggest that CAPRI provides a link between [FcγR signaling] and Cdc42/Rac1” (pmc.ncbi.nlm.nih.gov), underscoring its adaptor role beyond enzymatic activity. In the context of cancer, Chen et al. (2021) pointed out RASA4’s “tumor-suppressing role”, showing that restoring RASA4 can impair cancer cell proliferation by interfering with pro-growth pathways like HIF-1α (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). All these studies reinforce the concept that RASA4 is a critical regulatory node at the intersection of calcium signaling and Ras/Rap GTPase networks.

In conclusion, RASA4 (O43374) is a calcium-regulated Ras GAP that ensures appropriate termination of Ras signals and coordinates complex cellular responses. Its activity is finely tuned by intracellular Ca^2+ levels, subcellular localization, and oligomerization state, enabling it to function in processes ranging from chemotaxis and phagocytosis to growth control. Through recent advances (2021–2024), our understanding of RASA4 has expanded – revealing its dual Ras/Rap specificity, its role in immune cell signal adaptation, and its potential impact on cancer pathways. Ongoing research continues to uncover how this Calcium-Promoted Ras Inactivator contributes to normal physiology and how its dysregulation may lead to disease, solidifying RASA4’s place as an important molecular switch in human cellular signaling (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

References:

1. Lockyer PJ et al. (2001). Curr. Biol. 11: 981–986. [PubMed ID: 11525734] – CAPRI regulates Ca^2+-dependent inactivation of the Ras-MAPK pathway (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
2. Zhang J et al. (2005). Nat. Immunol. 6(9): 911–919. [Published Sep 2005] – A critical role for CAPRI in FcγR-mediated phagocytosis (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
3. Kupzig S et al. (2006). J. Biol. Chem. 281(15): 9891–9900. [Published Apr 2006] – GAP1 family members are bifunctional Ras/Rap GAPs (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
4. Dai Y et al. (2011). J. Biol. Chem. 286(22): 19905–19916. [Published Jun 2011] – Ca^2+-dependent monomer-dimer switch in CAPRI controls Ras vs Rap GAP activity (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
5. Xu X et al. (2021). Proc. Natl. Acad. Sci. U.S.A. 118(43): e2002162118. [Published Oct 2021] – CAPRI enables neutrophils to chemotax through high-range gradients (Ras adaptation) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
6. Chen J et al. (2021). Bioengineered 12(2): 10723–10733. [Published Dec 2021] – RASA4 inhibits HIF-1α pathway to suppress cervical cancer cell proliferation (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
7. Benoit A et al. (2022). Sci. Rep. 12: 779. [Published Jan 2022] – Mutations in Ras pathway regulators in relapsed DLBCL (includes RASA4 D111N) (pmc.ncbi.nlm.nih.gov).
8. Cherra SJ III & Lamb R (2024). Front. Mol. Neurosci. 17: 1352731. [Published Feb 2024] – Review: Intersections between Ras and Rap signaling (notes calcium-dependent RASA4 Ras/Rap GAP activity) (pmc.ncbi.nlm.nih.gov).

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49. AnnotationURLCitation(end_index=17860, start_index=17706, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=that%20the%20Ca%5E%7B2%2B%7D,defective%20activation%20of%20Cdc42%20and')
50. AnnotationURLCitation(end_index=17980, start_index=17861, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=Rac1,and%20Cdc42%20and%20Rac1%20and')
51. AnnotationURLCitation(end_index=18368, start_index=18214, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=that%20the%20Ca%5E%7B2%2B%7D,defective%20activation%20of%20Cdc42%20and')
52. AnnotationURLCitation(end_index=18504, start_index=18369, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=purified%20form,Fc%CE%B3R%20to%20Cdc42%20and%20Rac1')
53. AnnotationURLCitation(end_index=18882, start_index=18728, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=that%20the%20Ca%5E%7B2%2B%7D,defective%20activation%20of%20Cdc42%20and')
54. AnnotationURLCitation(end_index=19018, start_index=18883, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=purified%20form,Fc%CE%B3R%20to%20Cdc42%20and%20Rac1')
55. AnnotationURLCitation(end_index=19870, start_index=19698, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=Neutrophils%20sense%20and%20migrate%20through,chemoattractants%2C%20as%20a%20result%20of')
56. AnnotationURLCitation(end_index=20517, start_index=20361, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=gradients%20through%20adaptation,data%20reveal%20that%20CAPRI%20controls')
57. AnnotationURLCitation(end_index=20759, start_index=20628, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=calcium,data%20reveal%20that%20CAPRI%20controls')
58. AnnotationURLCitation(end_index=20932, start_index=20760, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=locally%20controls%20the%20GPCR,actin%20polymerization%3B%20and%20subsequent%20defective')
59. AnnotationURLCitation(end_index=21361, start_index=21231, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=GSK,findings%20show%20that%20CAPRI%20functions')
60. AnnotationURLCitation(end_index=21750, start_index=21583, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=Neutrophils%20provide%20first,mechanism%20largely%20remains%20elusive%2C%20although')
61. AnnotationURLCitation(end_index=21901, start_index=21751, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=recruit%20Ras%20inhibitor%2C%20CAPRI%2C%20to,mediated%20adaptation')
62. AnnotationURLCitation(end_index=22257, start_index=22090, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=Neutrophils%20provide%20first,mechanism%20largely%20remains%20elusive%2C%20although')
63. AnnotationURLCitation(end_index=22408, start_index=22258, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=recruit%20Ras%20inhibitor%2C%20CAPRI%2C%20to,mediated%20adaptation')
64. AnnotationURLCitation(end_index=23008, start_index=22855, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=Bacterial%20clearance%20in%20CAPRI,the%20expression%20of%20CAPRI%20in')
65. AnnotationURLCitation(end_index=23254, start_index=23106, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=macrophages%2C%20we%20examined%20the%20innate,5%7D%20CFU%2Fmouse')
66. AnnotationURLCitation(end_index=23549, start_index=23401, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=macrophages%2C%20we%20examined%20the%20innate,5%7D%20CFU%2Fmouse')
67. AnnotationURLCitation(end_index=23797, start_index=23668, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=whereas%20none%20of%20the%20controls,acquired')
68. AnnotationURLCitation(end_index=24120, start_index=23966, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=that%20the%20Ca%5E%7B2%2B%7D,defective%20activation%20of%20Cdc42%20and')
69. AnnotationURLCitation(end_index=24427, start_index=24273, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=that%20the%20Ca%5E%7B2%2B%7D,defective%20activation%20of%20Cdc42%20and')
70. AnnotationURLCitation(end_index=24563, start_index=24428, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=purified%20form,Fc%CE%B3R%20to%20Cdc42%20and%20Rac1')
71. AnnotationURLCitation(end_index=26657, start_index=26507, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=Luciferase%20reporter%20assays%20and%20LW6,Based%20on%20the%20TCGA')
72. AnnotationURLCitation(end_index=26937, start_index=26778, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=RAS%20p21%20protein%20activator%204,to%20determine%20the%20prognostic%20and')
73. AnnotationURLCitation(end_index=27266, start_index=27118, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=the%20CRISPR%2FCas9%20technology,was%20noted%20in%20the%20colony')
74. AnnotationURLCitation(end_index=27415, start_index=27267, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=the%20CRISPR%2FCas9%20technology,was%20noted%20in%20the%20colony')
75. AnnotationURLCitation(end_index=27714, start_index=27537, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=RASA4%20may%20negatively%20regulate%20HIF1%CE%B1,cells%20by%20inactivating%20the%20HIF1%CE%B1')
76. AnnotationURLCitation(end_index=27856, start_index=27715, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=Figure%205,Consistently%2C%20Western%20blotting%20results')
77. AnnotationURLCitation(end_index=28306, start_index=28168, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=that%20both%20RASA4%20and%20HIF1%CE%B1,As%20shown%20in')
78. AnnotationURLCitation(end_index=28473, start_index=28307, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=match%20at%20L366%20RASA4%20inactivated,revealed%20that%20survivin%20protein%20was')
79. AnnotationURLCitation(end_index=28759, start_index=28615, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=was%20also%20recorded%20in%20vivo,Our%20findings%20indicated')
80. AnnotationURLCitation(end_index=28913, start_index=28760, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=RASA4%20regulates%20proliferation%20through%20the,Furthermore%2C%20we')
81. AnnotationURLCitation(end_index=29539, start_index=29376, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=match%20at%20L44%20inactivating%20the,for%20targeted%20therapy%20against%20CESC')
82. AnnotationURLCitation(end_index=29811, start_index=29660, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=match%20at%20L251%20value%20of,of%20RASA4%20in%20CESC%20progression')
83. AnnotationURLCitation(end_index=30266, start_index=30152, title='Mutated RAS-associating proteins and ERK activation in relapse/refractory diffuse large B cell lymphoma - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8764096/#:~:text=SOS1%20%20,Possibly%20damaging')
84. AnnotationURLCitation(end_index=30368, start_index=30267, title='Mutated RAS-associating proteins and ERK activation in relapse/refractory diffuse large B cell lymphoma - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8764096/#:~:text=RASA2%20%20,N%2FA')
85. AnnotationURLCitation(end_index=30716, start_index=30567, title='Mutated RAS-associating proteins and ERK activation in relapse/refractory diffuse large B cell lymphoma - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8764096/#:~:text=hypothesized%20that%20RAS,In%20silico%20modeling%20indicated%20an')
86. AnnotationURLCitation(end_index=30842, start_index=30717, title='Mutated RAS-associating proteins and ERK activation in relapse/refractory diffuse large B cell lymphoma - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8764096/#:~:text=proteins,to%20a%20growth%20advantage%20in')
87. AnnotationURLCitation(end_index=32906, start_index=32723, title='RASA4 RAS p21 protein activator 4 [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene?Cmd=DetailsSearch&Db=gene&Term=10156#:~:text=that%20suppresses%20the%20Ras%2Fmitogen,provided%20by%20RefSeq%2C%20Jul%202008')
88. AnnotationURLCitation(end_index=33183, start_index=33016, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=Neutrophils%20provide%20first,mechanism%20largely%20remains%20elusive%2C%20although')
89. AnnotationURLCitation(end_index=33402, start_index=33248, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=that%20the%20Ca%5E%7B2%2B%7D,defective%20activation%20of%20Cdc42%20and')
90. AnnotationURLCitation(end_index=33618, start_index=33459, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=RAS%20p21%20protein%20activator%204,to%20determine%20the%20prognostic%20and')
91. AnnotationURLCitation(end_index=33928, start_index=33759, title='Ca2+-dependent Monomer and Dimer Formation Switches CAPRI Protein between Ras GTPase-activating Protein (GAP) and RapGAP Activities - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3103366/#:~:text=CAPRI%20increases%20upon%20cell%20stimulation,Ras%20and%20Rap1%20signaling%20pathways')
92. AnnotationURLCitation(end_index=34089, start_index=33929, title='Interactions between Ras and Rap signaling pathways during neurodevelopment in health and disease - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC10920261/#:~:text=%28Kupzig%20et%20al,translational%20modifications%20or%20second%20messenger')
93. AnnotationURLCitation(end_index=34569, start_index=34402, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=Neutrophils%20provide%20first,mechanism%20largely%20remains%20elusive%2C%20although')
94. AnnotationURLCitation(end_index=34720, start_index=34570, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=recruit%20Ras%20inhibitor%2C%20CAPRI%2C%20to,mediated%20adaptation')
95. AnnotationURLCitation(end_index=34989, start_index=34870, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=Rac1,and%20Cdc42%20and%20Rac1%20and')
96. AnnotationURLCitation(end_index=35415, start_index=35259, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=results%20indicate%20the%20tumor,Therefore%2C%20our%20results%20strongly')
97. AnnotationURLCitation(end_index=35593, start_index=35416, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=RASA4%20may%20negatively%20regulate%20HIF1%CE%B1,cells%20by%20inactivating%20the%20HIF1%CE%B1')
98. AnnotationURLCitation(end_index=36745, start_index=36574, title='Ca2+-dependent Monomer and Dimer Formation Switches CAPRI Protein between Ras GTPase-activating Protein (GAP) and RapGAP Activities - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3103366/#:~:text=RapGAP%20activity%20than%20RasGAP%20activity%2C,Ras%20and%20Rap1%20signaling%20pathways')
99. AnnotationURLCitation(end_index=36902, start_index=36746, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=gradients%20through%20adaptation,data%20reveal%20that%20CAPRI%20controls')
100. AnnotationURLCitation(end_index=37222, start_index=37081, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=CAPRI%20is%20a%20member%20of,cells%20but%20not%20in%20its')
101. AnnotationURLCitation(end_index=37356, start_index=37223, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=as%20a%20Ras,a%20critical%20role%20in%20Fc%CE%B3R')
102. AnnotationURLCitation(end_index=37660, start_index=37506, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=that%20the%20Ca%5E%7B2%2B%7D,defective%20activation%20of%20Cdc42%20and')
103. AnnotationURLCitation(end_index=37796, start_index=37661, title='A critical role for the Calcium-promoted Ras Inactivator in Fcγ receptor-mediated phagocytosis - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1464573/#:~:text=purified%20form,Fc%CE%B3R%20to%20Cdc42%20and%20Rac1')
104. AnnotationURLCitation(end_index=38077, start_index=37947, title='GAP1 Family Members Constitute Bifunctional Ras and Rap GTPase-activating Proteins - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1904491/#:~:text=GAP1,For%20this%20dual%20activity%2C%20regions')
105. AnnotationURLCitation(end_index=38213, start_index=38078, title='GAP1 Family Members Constitute Bifunctional Ras and Rap GTPase-activating Proteins - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC1904491/#:~:text=domain,association%20with%20the%20plasma%20membrane')
106. AnnotationURLCitation(end_index=38561, start_index=38392, title='Ca2+-dependent Monomer and Dimer Formation Switches CAPRI Protein between Ras GTPase-activating Protein (GAP) and RapGAP Activities - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3103366/#:~:text=CAPRI%20increases%20upon%20cell%20stimulation,Ras%20and%20Rap1%20signaling%20pathways')
107. AnnotationURLCitation(end_index=38722, start_index=38562, title='Interactions between Ras and Rap signaling pathways during neurodevelopment in health and disease - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC10920261/#:~:text=%28Kupzig%20et%20al,translational%20modifications%20or%20second%20messenger')
108. AnnotationURLCitation(end_index=39092, start_index=38920, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=Neutrophils%20sense%20and%20migrate%20through,chemoattractants%2C%20as%20a%20result%20of')
109. AnnotationURLCitation(end_index=39243, start_index=39093, title='Ras inhibitor CAPRI enables neutrophil-like cells to chemotax through a higher-concentration range of gradients - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8639426/#:~:text=recruit%20Ras%20inhibitor%2C%20CAPRI%2C%20to,mediated%20adaptation')
110. AnnotationURLCitation(end_index=39595, start_index=39418, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=RASA4%20may%20negatively%20regulate%20HIF1%CE%B1,cells%20by%20inactivating%20the%20HIF1%CE%B1')
111. AnnotationURLCitation(end_index=39737, start_index=39596, title='RASA4 inhibits the HIFα signaling pathway to suppress proliferation of cervical cancer cells - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8809920/#:~:text=Figure%205,Consistently%2C%20Western%20blotting%20results')
112. AnnotationURLCitation(end_index=40013, start_index=39899, title='Mutated RAS-associating proteins and ERK activation in relapse/refractory diffuse large B cell lymphoma - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8764096/#:~:text=SOS1%20%20,Possibly%20damaging')
113. AnnotationURLCitation(end_index=40388, start_index=40224, title='Interactions between Ras and Rap signaling pathways during neurodevelopment in health and disease - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC10920261/#:~:text=In%20addition%20to%20SynGAP%2C%20most,study%20suggests%20that%20modulation%20of')