Table 1.
Post translational modifications (PTMs) and functional consequences.
| Molecular Partners | Full Name | Function | Effect on Rac1 and Biological Impact | References Linked with CRC * |
|---|---|---|---|---|
| RAC1 | Ras-related C3 botulinum toxin substrate 1 | small GTP-binding protein, Small GTPase | Rac1 oligomerization via C-terminal domain (basic amino acid residues 183–188). Increased intrinsic GTPase activity, increased PAK1 activation [11]. | [6,12] |
| RAC1b | Ras-related C3 botulinum toxin substrate 1, transcript variant Rac1b | small GTP-binding protein, Small GTPase | In frame insertion of an extra 19-amino acid sequence. Preferentially in a GTP-bound active form. Overexpressed in CRC. Not sufficient to initiate colorectal carcinogenesis in transgenic mice, but cooperates with Wnt signaling, and promotes colon carcinogenesis upon chronic inflammation. | [13,14,15] |
| Post Translational Modifications (PTMs); Phosphorylation | ||||
| AKT1, PKB | AKT Serine/Threonine Kinase 1 | Serine/Threonine Kinase | Rac1 inhibition: Akt phosphorylates Rac1 at Ser 71 and inhibits GTP-binding activity [16]. | [17,18] |
| MAPK3, ERK1, P44 | Mitogen-Activated Protein Kinase 3 | Member of the MAP kinase family, Serine/Threonine protein kinase | Phosphorylation of RAC1 at Thr108: decreased Rac1 activity, partially due to inhibiting its interaction with phospholipase C-γ1 (PLC-γ1) [19]. | [18,20,21] |
| PTK2, FAK1 | Protein Tyrosine Kinase 2 | Cytoplasmic protein Tyosine kinase concentrated at focal adhesions | Phosphorylation of Rac1 at Tyr64 decreases GTP binding and Rac1 activity. Phosphorylation targets Rac1 to focal adhesions. Decreased association with βPIX, and increased binding with RHOGDI. Decreased cell spreading [22]. | [23] |
| PRKCG | Protein Kinase C Gamma | Protein Serine/Threonine kinase activated by calcium and diacylglycerol | Phosphorylation of Rac1 at Ser71 promotes invasion of A431 cells [24]. | [25] |
| PRKCZ | Protein Kinase C Zeta | Member of the PKC family of Serine/Threonine kinase. Independent of calcium and diacylglycerol but not of phosphatidylserine | Phosphorylation of RAC1 at Ser71 promotes proliferation, and migration of LoVo colon cancer cells via PAK1/β-catenin pathway. | [26] |
| ROCK1 | Rho Associated Coiled-Coil Containing Protein Kinase 1 | Protein Serine/Threonine kinase, activated when bound to GTP-Rho | Phosphorylation of Rac1b at Ser71 facilitates its interaction with cytochrome c. Increased mitochondrial ROS production [27]. | [28] |
| SRC | SRC Proto-Oncogene, Non-Receptor Tyrosine Kinase | Non receptor protein Tyrosine kinase | Phosphorylation of Rac1 at Tyr64 decreases GTP binding and Rac1 activity. Phosphorylation targets Rac1 to focal adhesions. Decreased association with βPIX, and increased binding with RhoGDI. Decreased cell spreading [22]. | [23,29] |
| SUMOylation/deSUMOylation | ||||
| PIAS3 | Protein Inhibitor of Activated STAT 3 | SUMO-E3 ligase | SUMOylation of Rac1 polybasic region (amino-acid residues 179–188) in the cytoplasm. Increased Rac1 activity and optimal cell migration in response to HGF signaling. PIAS3-mediated feedback loops controls cell proliferation and function as robust driving forces for Colitis Associated Cancer progression [30]. | [31] |
| TP53, P53 | Tumor Protein P53 | Tumor suppressor. Protein containing transcriptional activation, DNA binding, and oligomerization domains | Mutant TP53 competes with SENP1 for Rac1 interaction. Protects SUMOylated Rac1 from SENP1 peptidase. Increased Rac1 activity. In CRC, Rac1 SUMOylation/activation correlates with mutant TP53 status. | [32] |
| SENP1 | SUMO Specific Peptidase 1 | Cysteine protease, deconjugates sumoylated proteins. | Mutant TP53 competes with SENP1 for Rac1 interaction. Decreases Rac1 SUMOylation and activity [32]. | |
| Ubiquitination | ||||
| HACE1 | HECT Domain and Ankyrin Repeat Containing E3 Ubiquitin Protein Ligase 1 | E3 ubiquitin-protein ligase. Tumor suppressor. Mutated in cancers | Ubiquitinates and targets Rac1 (preferentially GTP-bound form) to proteasomal degradation. The Rac1 downstream kinase PAK phosphorylates and impairs HACE1 -induced Rac1 ubiquitination. Controls cell proliferation, cell migration, ROS production [33,34,35,36,37]. | [38,39] |
| FBXL19 | F-Box and Leucine Rich Repeat Protein 19 | E3 ubiquitin ligases (member of the Skp1-Cullin-F-box family) | Ubiquitination of Lys 166 of Rac1 (preferentially phosphorylated at Ser71). Rac1 degradation by proteasome [40]. | |
| BIRC2, c-IAP1 | Baculoviral IAP Repeat Containing 2 | E3 ubiquitin-protein ligase. Member of family of apoptotic suppressor proteins | Polyubiquitination of activated Rac1 at Lys147 and proteasomal degradation. Reverses mesenchymal morphology and cell migration [41,42]. | [43] |
| XIAP | X-Linked Inhibitor of Apoptosis | E3 ubiquitin-protein ligase. Member of family of apoptotic suppressor proteins | Polyubiquitination of activated Rac1 at Lys147 and proteasomal degradation. Reverses mesenchymal morphology and cell migration [41,42]. | [43] |
| Bacterial Toxins-Induced PTMs | ||||
| Toxin | Source | Rac1 PTM | Effect on Rac1 and Biological Impact | |
| TcdA, TcdB (toxin A, B) | Clostridium difficile | Mono-glycosylation of Thr35 of Rac1 (switch I). | Rac1 inactivation, actin depolymerisation, loss of cell-cell contacts and apoptosis [44] | |
| VopS | Vibrio parahaemolyticus | AMPylation of Thr35 of Rac1 (switch I). | Rac1 inactivation [45]. | |
| YopT | Yersinia enterocolitica | Cysteine protease; proteolysis of the Rac1 carboxy-terminal upstream CAAX box | Rac1 inactivation. Impaired membrane interaction [46]. | |
| VopC; DNT (dermonecrotic toxin); CNF1 | Vibrio parahaemolyticus; Bordetella dermonecrotizing; Escherichia coli | Deamidation Gln61 of Rac1 (switch II) Gln -> Glu | Constitutive Rac1 activation (decreased Rac1 GTPase activity) [47]. | |
| DNT (dermonecrotic toxin) | Bordetella dermonecrotizing | Polyamination of Gln61 (evidenced in vitro, not confirmed in vivo) | Constitutive Rac1 activation (decreased Rac1 GTPase activity). Note that DNT also deaminates Gln61 of Rac1 [48] | |
| ExoS (Exoenzyme S) | Pseudomonas aeruginosa | ADP-ribosylation of Arg 66 and Arg 68; cell line dependent (HT29 colon cancer cells sensitive; J774A.1 macrophages unsensitive) | Rac1 activation. Note that exoS ADP-ribosyltransferase activity is C-term, an antagonistic GTPase-activating domain is in the N-term of ExoS [49]. | |
* Highlighted references correspond to the involvement of the corresponding proteins (positively Red, negatively Green) to colorectal carcinogenesis, CRC aggressiveness, response to chemotherapy, or patient overall survival. Some discrepancies between their biological impact on Rac1 and their link with CRC might be related to the mobilization of distinct signaling pathways.