GTPase splice variants RAC1 and RAC1B display isoform-specific differences in localization, prenylation, and interaction with the chaperone protein SmgGDS
2023; Elsevier BV; Volume: 299; Issue: 6 Linguagem: Inglês
10.1016/j.jbc.2023.104698
ISSN1083-351X
AutoresOlivia J. Koehn, Ellen Lorimer, Bethany Unger, Ra’Mal M. Harris, Akansha Das, Kiall F. Suazo, Shelby A. Auger, Mark D. Distefano, Jeremy W. Prokop, Carol L. Williams,
Tópico(s)Viral Infectious Diseases and Gene Expression in Insects
ResumoIdentifying events that regulate the prenylation and localization of small GTPases will help define new strategies for therapeutic targeting of these proteins in disorders such as cancer, cardiovascular disease, and neurological deficits. Splice variants of the chaperone protein SmgGDS (encoded by RAP1GDS1) are known to regulate prenylation and trafficking of small GTPases. The SmgGDS-607 splice variant regulates prenylation by binding preprenylated small GTPases but the effects of SmgGDS binding to the small GTPase RAC1 versus the splice variant RAC1B are not well defined. Here we report unexpected differences in the prenylation and localization of RAC1 and RAC1B and their binding to SmgGDS. Compared to RAC1, RAC1B more stably associates with SmgGDS-607, is less prenylated, and accumulates more in the nucleus. We show that the small GTPase DIRAS1 inhibits binding of RAC1 and RAC1B to SmgGDS and reduces their prenylation. These results suggest that prenylation of RAC1 and RAC1B is facilitated by binding to SmgGDS-607 but the greater retention of RAC1B by SmgGDS-607 slows RAC1B prenylation. We show that inhibiting RAC1 prenylation by mutating the CAAX motif promotes RAC1 nuclear accumulation, suggesting that differences in prenylation contribute to the different nuclear localization of RAC1 versus RAC1B. Finally, we demonstrate RAC1 and RAC1B that cannot be prenylated bind GTP in cells, indicating that prenylation is not a prerequisite for activation. We report differential expression of RAC1 and RAC1B transcripts in tissues, consistent with these two splice variants having unique functions that might arise in part from their differences in prenylation and localization. Identifying events that regulate the prenylation and localization of small GTPases will help define new strategies for therapeutic targeting of these proteins in disorders such as cancer, cardiovascular disease, and neurological deficits. Splice variants of the chaperone protein SmgGDS (encoded by RAP1GDS1) are known to regulate prenylation and trafficking of small GTPases. The SmgGDS-607 splice variant regulates prenylation by binding preprenylated small GTPases but the effects of SmgGDS binding to the small GTPase RAC1 versus the splice variant RAC1B are not well defined. Here we report unexpected differences in the prenylation and localization of RAC1 and RAC1B and their binding to SmgGDS. Compared to RAC1, RAC1B more stably associates with SmgGDS-607, is less prenylated, and accumulates more in the nucleus. We show that the small GTPase DIRAS1 inhibits binding of RAC1 and RAC1B to SmgGDS and reduces their prenylation. These results suggest that prenylation of RAC1 and RAC1B is facilitated by binding to SmgGDS-607 but the greater retention of RAC1B by SmgGDS-607 slows RAC1B prenylation. We show that inhibiting RAC1 prenylation by mutating the CAAX motif promotes RAC1 nuclear accumulation, suggesting that differences in prenylation contribute to the different nuclear localization of RAC1 versus RAC1B. Finally, we demonstrate RAC1 and RAC1B that cannot be prenylated bind GTP in cells, indicating that prenylation is not a prerequisite for activation. We report differential expression of RAC1 and RAC1B transcripts in tissues, consistent with these two splice variants having unique functions that might arise in part from their differences in prenylation and localization. Small GTPases are critical signaling proteins that are regulated by guanine nucleotide binding state, as well as by prenylation and subcellular localization. The small GTPase RAC1 is a member of the Rho family of small GTPases and participates in signaling pathways that regulate cytoskeletal organization, cell proliferation and survival, and gene transcription (1Bosco E.E. Mulloy J.C. Zheng Y. Rac1 GTPase: a "rac" of all trades.Cell. Mol. Life Sci. 2009; 66: 370-374Crossref PubMed Scopus (249) Google Scholar, 2Gastonguay A. Berg T. Hauser A.D. Schuld N. Lorimer E. Williams C.L. The role of Rac1 in the regulation of NF-kB activity, cell proliferation, and cell migration in non-small cell lung carcinoma.Cancer Biol. Ther. 2012; 13: 647-656Crossref PubMed Google Scholar). The human RAC1 gene encodes two major splice variants that generate proteins referred to as RAC1 and RAC1B (Table 1 and Fig. 1A). RAC1B differs from RAC1 only by the presence of a 19-amino acid insertion encoded by exon 3b, immediately following the Switch-II region (3Fiegen D. Haeusler L.-C. Blumenstein L. Herbrand U. Dvorsky R. Vetter I.R. et al.Alternative splicing of Rac1 generates Rac1b, a self-activating GTPase.J. Biol. Chem. 2004; 279: 4743-4749Abstract Full Text Full Text PDF PubMed Scopus (129) Google Scholar, 4Singh A. Karnoub A.E. Palmby T.R. Lengyel E. Sondek J. Der C.J. Rac1b, a tumor associated, constitutively active Rac1 splice variant, promotes cellular transformation.Oncogene. 2004; 23: 9369-9380Crossref PubMed Scopus (150) Google Scholar, 5Gonçalves V. Matos P. Jordan P. Antagonistic SR proteins regulate alternative splicing of tumor-related Rac1b downstream of the PI3-kinase and Wnt pathways.Hum. Mol. Genet. 2009; 18: 3696-3707Crossref PubMed Scopus (90) Google Scholar). Compared to RAC1, RAC1B has distinct downstream signaling and altered activity, which are likely to have implications for normal cellular signaling and pathophysiology.Table 1Description of proteins encoded by transcripts from the RAC1 and RAP1GDS1 genesCommonprotein nameUniProtidentifierTranscriptidentifiersProtein sizeStructural elementsNotesRAC1 gene products RAC1 (9Schnelzer A. Prechtel D. Knaus U. Dehne K. Gerhard M. Graeff H. et al.Rac1 in human breast cancer: overexpression, mutation analysis, and characterization of a new isoform, Rac1b.Oncogene. 2000; 19: 3013-3020Crossref PubMed Scopus (343) Google Scholar, 15Matos P. Skaug J. Marques B. Beck S. Veríssimo F. Gespach C. et al.Small GTPase Rac1: structure, localization, and expression of the human gene.Biochem. Biophys. Res. Commun. 2000; 277: 741-751Crossref PubMed Scopus (59) Google Scholar, 16Jordan P. Brazão R. Boavida M.G. Gespach C. Chastre E. Cloning of a novel human Rac1b splice variant with increased expression in colorectal tumors.Oncogene. 1999; 18: 6835-6839Crossref PubMed Scopus (207) Google Scholar, 58Suazo K.F. Hurben A.K. Liu K. Xu F. Thao P. Sudheer C. et al.Metabolic labeling of prenylated proteins using alkyne-modified isoprenoid analogues.Curr. Protoc. Chem. Biol. 2018; 10: e46Crossref PubMed Scopus (11) Google Scholar)P63000-1RAC1-201ENST00000348035.9192 amino acidsFigure 1AThis isoform is the canonical sequence RAC1B (9Schnelzer A. Prechtel D. Knaus U. Dehne K. Gerhard M. Graeff H. et al.Rac1 in human breast cancer: overexpression, mutation analysis, and characterization of a new isoform, Rac1b.Oncogene. 2000; 19: 3013-3020Crossref PubMed Scopus (343) Google Scholar, 15Matos P. Skaug J. Marques B. Beck S. Veríssimo F. Gespach C. et al.Small GTPase Rac1: structure, localization, and expression of the human gene.Biochem. Biophys. Res. Commun. 2000; 277: 741-751Crossref PubMed Scopus (59) Google Scholar, 16Jordan P. Brazão R. Boavida M.G. Gespach C. Chastre E. Cloning of a novel human Rac1b splice variant with increased expression in colorectal tumors.Oncogene. 1999; 18: 6835-6839Crossref PubMed Scopus (207) Google Scholar)P63000-2RAC1-202ENST00000356142.4211 amino acidsFigure 1AIdentical to P63000-1 but has a 19 amino acid insertRAP1GDS1 gene products SmgGDS-607 (27Berg T.J. Gastonguay A.J. Lorimer E.L. Kuhnmuench J.R. Li R. Fields A.P. et al.Splice variants of SmgGDS control small GTPase prenylation and membrane localization.J. Biol. Chem. 2010; 285: 35255-35266Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar, 28Schuld N.J. Vervacke J.S. Lorimer E.L. Simon N.C. Hauser A.D. Barbieri J.T. et al.The chaperone protein SmgGDS interacts with small GTPases entering the prenylation pathway by recognizing the last amino acid in the CAAX motif.J. Biol. Chem. 2014; 289: 6862-6876Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar, 29Ntantie E. Gonyo P. Lorimer E.L. Hauser A.D. Schuld N. McAllister D. et al.An adenosine-mediated signaling pathway suppresses prenylation of the GTPase Rap1B and promotes cell scattering.Sci. Signal. 2013; 6: ra39Crossref PubMed Scopus (75) Google Scholar, 30Brandt A.C. McNally L. Lorimer E.L. Unger B. Koehn O.J. Suazo K.F. et al.Splice switching an oncogenic ratio of SmgGDS isoforms as a strategy to diminish malignancy.Proc. Natl. Acad. Sci. 2020; 117: 3627-3636Crossref PubMed Scopus (15) Google Scholar, 31Brandt A.C. Koehn O.J. Williams C.L. SmgGDS: an emerging master regulator of prenylation and trafficking by small GTPases in the ras and Rho families.Front. Mol. Biosci. 2021; 8685135Crossref PubMed Scopus (9) Google Scholar, 33García-Torres D. Fierke C.A. The chaperone SmgGDS-607 has a dual role, both activating and inhibiting farnesylation of small GTPases.J. Biol. Chem. 2019; 294: 11793-11804Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar, 35Jennings B.C. Lawton A.J. Rizk Z. Fierke C.A. SmgGDS-607 regulation of RhoA GTPase prenylation is nucleotide dependent.Biochemistry. 2018; 57: 4289-4298Crossref PubMed Scopus (8) Google Scholar)P52306-1RAP1GDS1-205ENST00000408927.8607 amino acidsFigure 1DThis isoform is the canonical sequence: It has 13 ARM domains labeled A - M SmgGDS-558 (27Berg T.J. Gastonguay A.J. Lorimer E.L. Kuhnmuench J.R. Li R. Fields A.P. et al.Splice variants of SmgGDS control small GTPase prenylation and membrane localization.J. Biol. Chem. 2010; 285: 35255-35266Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar, 28Schuld N.J. Vervacke J.S. Lorimer E.L. Simon N.C. Hauser A.D. Barbieri J.T. et al.The chaperone protein SmgGDS interacts with small GTPases entering the prenylation pathway by recognizing the last amino acid in the CAAX motif.J. Biol. Chem. 2014; 289: 6862-6876Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar, 29Ntantie E. Gonyo P. Lorimer E.L. Hauser A.D. Schuld N. McAllister D. et al.An adenosine-mediated signaling pathway suppresses prenylation of the GTPase Rap1B and promotes cell scattering.Sci. Signal. 2013; 6: ra39Crossref PubMed Scopus (75) Google Scholar, 30Brandt A.C. McNally L. Lorimer E.L. Unger B. Koehn O.J. Suazo K.F. et al.Splice switching an oncogenic ratio of SmgGDS isoforms as a strategy to diminish malignancy.Proc. Natl. Acad. Sci. 2020; 117: 3627-3636Crossref PubMed Scopus (15) Google Scholar, 31Brandt A.C. Koehn O.J. Williams C.L. SmgGDS: an emerging master regulator of prenylation and trafficking by small GTPases in the ras and Rho families.Front. Mol. Biosci. 2021; 8685135Crossref PubMed Scopus (9) Google Scholar, 33García-Torres D. Fierke C.A. The chaperone SmgGDS-607 has a dual role, both activating and inhibiting farnesylation of small GTPases.J. Biol. Chem. 2019; 294: 11793-11804Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar, 35Jennings B.C. Lawton A.J. Rizk Z. Fierke C.A. SmgGDS-607 regulation of RhoA GTPase prenylation is nucleotide dependent.Biochemistry. 2018; 57: 4289-4298Crossref PubMed Scopus (8) Google Scholar)P52306-2RAP1GDS1-204ENST00000408900.7558 amino acidsFigure 1DIdentical to P52306-1 but lacks ARM C SmgGDS-559 (32Hamel B. Monaghan-Benson E. Rojas R.J. Temple B.R.S. Marston D.J. Burridge K. et al.SmgGDS is a guanine nucleotide exchange factor that specifically activates RhoA and RhoC.J. Biol. Chem. 2011; 286: 12141-12148Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar)P52306-3RAP1GDS1-203ENST00000380158.8559 amino acidsFigure 1DIdentical to P52306-2, but has an additional alanine at residue 2 SmgGDS-608var (suggested)P52306-4RAP1GDS1-206ENST00000453712.6607 amino acidsFigure 1DIdentical to P52306-5 but lacks the alanine found at residue 434 in P52306-5 SmgGDS-608 (32Hamel B. Monaghan-Benson E. Rojas R.J. Temple B.R.S. Marston D.J. Burridge K. et al.SmgGDS is a guanine nucleotide exchange factor that specifically activates RhoA and RhoC.J. Biol. Chem. 2011; 286: 12141-12148Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar)P52306-5RAP1GDS1-202ENST00000339360.9608 amino acidsFigure 1DIdentical to P52306-1 but has an additional alanine at residue 2 SmgGDS-516 (suggested)P52306-6RAP1GDS1-201ENST00000264572.11516 amino acidsFigure 1DHas unique ARM domain organization SmgGDS-253 (suggested)H0Y8M2RAP1GDS1-214ENST0000050901.5253 amino acidsFigure 1DHas unique ARM domain organization SmgGDS-158A (suggested)D6REZ0RAP1GDS1-216ENST00000511212.5158 amino acidsFigure 1DHas unique ARM domain organization SmgGDS-158B (suggested)D6RB97RAP1GDS1-219ENST00000514122.5158 amino acidsFigure 1DHas unique ARM domain organization SmgGDS-147 (suggested)D6RHH8RAP1GDS1-211ENST00000508213.5147 amino acidsFigure 1DHas unique ARM domain organization SmgGDS-122 (suggested)D6RHZ7RAP1GDS1-213ENST00000509011.5122 amino acidsFigure 1DHas unique ARM domain organization SmgGDS-101 (suggested)U3KQJ4RAP1GDS1-220ENST00000514139.2101 amino acidsFigure 1DHas unique ARM domain organization Open table in a new tab Both RAC1 and RAC1B are able to activate the NF-kB pathway (2Gastonguay A. Berg T. Hauser A.D. Schuld N. Lorimer E. Williams C.L. The role of Rac1 in the regulation of NF-kB activity, cell proliferation, and cell migration in non-small cell lung carcinoma.Cancer Biol. Ther. 2012; 13: 647-656Crossref PubMed Google Scholar, 6Matos P. Collard J.G. Jordan P. Tumor-related alternatively spliced Rac1b is not regulated by Rho-GDP dissociation inhibitors and exhibits selective downstream signaling.J. Biol. Chem. 2003; 278: 50442-50448Abstract Full Text Full Text PDF PubMed Scopus (110) Google Scholar) and facilitate the production of reactive oxygen species (7Lee K. Chen Q.K. Lui C. Cichon M.A. Radisky D.C. Nelson C.M. Matrix compliance regulates Rac1b localization, NADPH oxidase assembly, and epithelial–mesenchymal transition.Mol. Biol. Cell. 2012; 23: 4097-4108Crossref PubMed Scopus (93) Google Scholar, 8Melzer C. Hass R. Lehnert H. Ungefroren H. RAC1B: a Rho GTPase with versatile functions in malignant transformation and tumor progression.Cells. 2019; 8: 21Crossref PubMed Scopus (36) Google Scholar). However, it has been reported that unlike RAC1, RAC1B is not able to bind to RHOGDI, cannot induce lamellipodia formation, and does not signal to p21 activated kinase (PAK) or Jun N-terminal kinase (6Matos P. Collard J.G. Jordan P. Tumor-related alternatively spliced Rac1b is not regulated by Rho-GDP dissociation inhibitors and exhibits selective downstream signaling.J. Biol. Chem. 2003; 278: 50442-50448Abstract Full Text Full Text PDF PubMed Scopus (110) Google Scholar). The unique exon present in RAC1B also causes it to have accelerated GDP/GTP exchange, which results in predominantly GTP-bound and active RAC1B within cells (3Fiegen D. Haeusler L.-C. Blumenstein L. Herbrand U. Dvorsky R. Vetter I.R. et al.Alternative splicing of Rac1 generates Rac1b, a self-activating GTPase.J. Biol. Chem. 2004; 279: 4743-4749Abstract Full Text Full Text PDF PubMed Scopus (129) Google Scholar, 9Schnelzer A. Prechtel D. Knaus U. Dehne K. Gerhard M. Graeff H. et al.Rac1 in human breast cancer: overexpression, mutation analysis, and characterization of a new isoform, Rac1b.Oncogene. 2000; 19: 3013-3020Crossref PubMed Scopus (343) Google Scholar). RAC1 and RAC1B are highly expressed in certain types of tumors and can promote cancer progression (4Singh A. Karnoub A.E. Palmby T.R. Lengyel E. Sondek J. Der C.J. Rac1b, a tumor associated, constitutively active Rac1 splice variant, promotes cellular transformation.Oncogene. 2004; 23: 9369-9380Crossref PubMed Scopus (150) Google Scholar, 9Schnelzer A. Prechtel D. Knaus U. Dehne K. Gerhard M. Graeff H. et al.Rac1 in human breast cancer: overexpression, mutation analysis, and characterization of a new isoform, Rac1b.Oncogene. 2000; 19: 3013-3020Crossref PubMed Scopus (343) Google Scholar, 10Zhou Y. Liao Q. Han Y. Chen J. Liu Z. Ling H. et al.Rac1 overexpression is correlated with epithelial mesenchymal transition and predicts poor prognosis in non-small cell lung cancer.J. Cancer. 2016; 7: 2100-2109Crossref PubMed Scopus (51) Google Scholar, 11Zhou C. Licciulli S. Avila J.L. Cho M. Troutman S. Jiang P. et al.The Rac1 splice form Rac1b promotes K-ras-induced lung tumorigenesis.Oncogene. 2013; 32: 903-909Crossref PubMed Scopus (86) Google Scholar, 12Stallings-Mann M.L. Waldmann J. Zhang Y. Miller E. Gauthier M.L. Visscher D.W. et al.Matrix metalloproteinase induction of Rac1b, a key effector of lung cancer progression.Sci. Transl. Med. 2012; 4: 142ra95Crossref PubMed Scopus (88) Google Scholar, 13Wertheimer E. Gutierrez-Uzquiza A. Rosemblit C. Lopez-Haber C. Sosa M.S. Kazanietz M.G. Rac signaling in breast cancer: a tale of GEFs and GAPs.Cell. Signal. 2012; 24: 353-362Crossref PubMed Scopus (149) Google Scholar, 14Zou T. Mao X. Yin J. Li X. Chen J. Zhu T. et al.Emerging roles of RAC1 in treating lung cancer patients.Clin. Genet. 2017; 91: 520-528Crossref PubMed Scopus (32) Google Scholar). RAC1B expression was identified predominantly in breast (4Singh A. Karnoub A.E. Palmby T.R. Lengyel E. Sondek J. Der C.J. Rac1b, a tumor associated, constitutively active Rac1 splice variant, promotes cellular transformation.Oncogene. 2004; 23: 9369-9380Crossref PubMed Scopus (150) Google Scholar, 9Schnelzer A. Prechtel D. Knaus U. Dehne K. Gerhard M. Graeff H. et al.Rac1 in human breast cancer: overexpression, mutation analysis, and characterization of a new isoform, Rac1b.Oncogene. 2000; 19: 3013-3020Crossref PubMed Scopus (343) Google Scholar, 13Wertheimer E. Gutierrez-Uzquiza A. Rosemblit C. Lopez-Haber C. Sosa M.S. Kazanietz M.G. Rac signaling in breast cancer: a tale of GEFs and GAPs.Cell. Signal. 2012; 24: 353-362Crossref PubMed Scopus (149) Google Scholar, 15Matos P. Skaug J. Marques B. Beck S. Veríssimo F. Gespach C. et al.Small GTPase Rac1: structure, localization, and expression of the human gene.Biochem. Biophys. Res. Commun. 2000; 277: 741-751Crossref PubMed Scopus (59) Google Scholar), lung (10Zhou Y. Liao Q. Han Y. Chen J. Liu Z. Ling H. et al.Rac1 overexpression is correlated with epithelial mesenchymal transition and predicts poor prognosis in non-small cell lung cancer.J. Cancer. 2016; 7: 2100-2109Crossref PubMed Scopus (51) Google Scholar, 11Zhou C. Licciulli S. Avila J.L. Cho M. Troutman S. Jiang P. et al.The Rac1 splice form Rac1b promotes K-ras-induced lung tumorigenesis.Oncogene. 2013; 32: 903-909Crossref PubMed Scopus (86) Google Scholar, 12Stallings-Mann M.L. Waldmann J. Zhang Y. Miller E. Gauthier M.L. Visscher D.W. et al.Matrix metalloproteinase induction of Rac1b, a key effector of lung cancer progression.Sci. Transl. Med. 2012; 4: 142ra95Crossref PubMed Scopus (88) Google Scholar, 14Zou T. Mao X. Yin J. Li X. Chen J. Zhu T. et al.Emerging roles of RAC1 in treating lung cancer patients.Clin. Genet. 2017; 91: 520-528Crossref PubMed Scopus (32) Google Scholar), and colorectal (4Singh A. Karnoub A.E. Palmby T.R. Lengyel E. Sondek J. Der C.J. Rac1b, a tumor associated, constitutively active Rac1 splice variant, promotes cellular transformation.Oncogene. 2004; 23: 9369-9380Crossref PubMed Scopus (150) Google Scholar, 15Matos P. Skaug J. Marques B. Beck S. Veríssimo F. Gespach C. et al.Small GTPase Rac1: structure, localization, and expression of the human gene.Biochem. Biophys. Res. Commun. 2000; 277: 741-751Crossref PubMed Scopus (59) Google Scholar, 16Jordan P. Brazão R. Boavida M.G. Gespach C. Chastre E. Cloning of a novel human Rac1b splice variant with increased expression in colorectal tumors.Oncogene. 1999; 18: 6835-6839Crossref PubMed Scopus (207) Google Scholar) cancers. Both splice variants were found to promote KRAS-induced lung cancer (11Zhou C. Licciulli S. Avila J.L. Cho M. Troutman S. Jiang P. et al.The Rac1 splice form Rac1b promotes K-ras-induced lung tumorigenesis.Oncogene. 2013; 32: 903-909Crossref PubMed Scopus (86) Google Scholar, 17Kissil J.L. Walmsley M.J. Hanlon L. Haigis K.M. Bender Kim C.F. Sweet-Cordero A. et al.Requirement for Rac1 in a K-ras–induced lung cancer in the mouse.Cancer Res. 2007; 67: 8089-8094Crossref PubMed Scopus (148) Google Scholar, 18Marei H. Malliri A. Rac1 in human diseases: the therapeutic potential of targeting Rac1 signaling regulatory mechanisms.Small GTPases. 2017; 8: 139-163Crossref PubMed Scopus (86) Google Scholar) and the epithelial-mesenchymal transition (12Stallings-Mann M.L. Waldmann J. Zhang Y. Miller E. Gauthier M.L. Visscher D.W. et al.Matrix metalloproteinase induction of Rac1b, a key effector of lung cancer progression.Sci. Transl. Med. 2012; 4: 142ra95Crossref PubMed Scopus (88) Google Scholar, 18Marei H. Malliri A. Rac1 in human diseases: the therapeutic potential of targeting Rac1 signaling regulatory mechanisms.Small GTPases. 2017; 8: 139-163Crossref PubMed Scopus (86) Google Scholar). Interestingly, studies of breast and pancreatic cancers revealed different effects of RAC1 and RAC1B on transforming growth factor (TGF)-β1-induced cell migration (8Melzer C. Hass R. Lehnert H. Ungefroren H. RAC1B: a Rho GTPase with versatile functions in malignant transformation and tumor progression.Cells. 2019; 8: 21Crossref PubMed Scopus (36) Google Scholar, 19Melzer C. Hass R. von der Ohe J. Lehnert H. Ungefroren H. The role of TGF-β and its crosstalk with RAC1/RAC1b signaling in breast and pancreas carcinoma.Cell Commun. Signal. 2017; 15: 19Crossref PubMed Scopus (47) Google Scholar, 20Ungefroren H. Sebens S. Giehl K. Helm O. Groth S. Faendrich F. et al.Rac1b negatively regulates TGF-β1-induced cell motility in pancreatic ductal epithelial cells by suppressing Smad signalling.Oncotarget. 2013; 5: 277-290Crossref Scopus (44) Google Scholar), where RAC1 promoted and RAC1B inhibited TGF-β1-induced cell motility. In the brain, Rho GTPases, including RAC1, are crucial for the formation, organization, and maturation of neuronal dendrites (21Perez S.E. Getova D.P. He B. Counts S.E. Geula C. Desire L. et al.Rac1b increases with progressive Tau pathology within cholinergic nucleus basalis neurons in Alzheimer's disease.Am. J. Pathol. 2012; 180: 526-540Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar, 22Samuel F. Hynds D.L. RHO GTPase signaling for axon extension: is prenylation important?.Mol. Neurobiol. 2010; 42: 133-142Crossref PubMed Scopus (42) Google Scholar). Misregulation of RAC1, causing alterations in neuronal cytoskeletal organization, has been implicated in neurodegenerative diseases, including Alzheimer's disease (21Perez S.E. Getova D.P. He B. Counts S.E. Geula C. Desire L. et al.Rac1b increases with progressive Tau pathology within cholinergic nucleus basalis neurons in Alzheimer's disease.Am. J. Pathol. 2012; 180: 526-540Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar). RAC1 splicing has been found to be altered in brains with Alzheimer's disease and RAC1B expression in some neuronal populations in Alzheimer's disease has been linked to increased neurofibrillary tangles and membrane dysfunction (21Perez S.E. Getova D.P. He B. Counts S.E. Geula C. Desire L. et al.Rac1b increases with progressive Tau pathology within cholinergic nucleus basalis neurons in Alzheimer's disease.Am. J. Pathol. 2012; 180: 526-540Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar). Misregulation of RAC1 has also been implicated in cardiovascular disease (18Marei H. Malliri A. Rac1 in human diseases: the therapeutic potential of targeting Rac1 signaling regulatory mechanisms.Small GTPases. 2017; 8: 139-163Crossref PubMed Scopus (86) Google Scholar), though potential roles of RAC1B in the cardiovascular system have not yet been defined. Most small GTPases, including RAC1 and RAC1B, are posttranslationally modified by prenylation to facilitate their association with membranes, where small GTPases participate in many signaling pathways. A farnesyl or geranylgeranyl isoprenoid group is added to the cysteine in the C-terminal CAAX motif by farnesyltransferase or geranylgeranyltransferase-I, respectively (23Winter-Vann A.M. Casey P.J. Post-prenylation-processing enzymes as new targets in oncogenesis.Nat. Rev. Cancer. 2005; 5: 405-412Crossref PubMed Scopus (283) Google Scholar, 24Wright L.P. Philips M.R. Thematic review series: lipid posttranslational modifications CAAX modification and membrane targeting of Ras.J. Lipid Res. 2006; 47: 883-891Abstract Full Text Full Text PDF PubMed Scopus (266) Google Scholar). The specific isoprenoid modification depends on the last amino acid in the CAAX motif and both RAC1 and RAC1B become geranylgeranylated (25Moores S.L. Schaber M.D. Mosser S.D. Rands E. O'Hara M.B. Garsky V.M. et al.Sequence dependence of protein isoprenylation.J. Biol. Chem. 1991; 266: 14603-14610Abstract Full Text PDF PubMed Google Scholar). Blocking the prenylation of small GTPases is an attractive therapeutic strategy but farnesyltransferase and GGTase-I inhibitors have proven to be limited in their clinical effectiveness (23Winter-Vann A.M. Casey P.J. Post-prenylation-processing enzymes as new targets in oncogenesis.Nat. Rev. Cancer. 2005; 5: 405-412Crossref PubMed Scopus (283) Google Scholar, 26Berndt N. Hamilton A.D. Sebti S.M. Targeting protein prenylation for cancer therapy.Nat. Rev. Cancer. 2011; 11: 775-791Crossref PubMed Scopus (462) Google Scholar), highlighting the need for further investigation of the mechanisms controlling the prenylation pathway. The chaperone protein SmgGDS has been shown to play a role in regulating the prenylation and trafficking of small GTPases in the Ras and Rho families (27Berg T.J. Gastonguay A.J. Lorimer E.L. Kuhnmuench J.R. Li R. Fields A.P. et al.Splice variants of SmgGDS control small GTPase prenylation and membrane localization.J. Biol. Chem. 2010; 285: 35255-35266Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar, 28Schuld N.J. Vervacke J.S. Lorimer E.L. Simon N.C. Hauser A.D. Barbieri J.T. et al.The chaperone protein SmgGDS interacts with small GTPases entering the prenylation pathway by recognizing the last amino acid in the CAAX motif.J. Biol. Chem. 2014; 289: 6862-6876Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar, 29Ntantie E. Gonyo P. Lorimer E.L. Hauser A.D. Schuld N. McAllister D. et al.An adenosine-mediated signaling pathway suppresses prenylation of the GTPase Rap1B and promotes cell scattering.Sci. Signal. 2013; 6: ra39Crossref PubMed Scopus (75) Google Scholar, 30Brandt A.C. McNally L. Lorimer E.L. Unger B. Koehn O.J. Suazo K.F. et al.Splice switching an oncogenic ratio of SmgGDS isoforms as a strategy to diminish malignancy.Proc. Natl. Acad. Sci. 2020; 117: 3627-3636Crossref PubMed Scopus (15) Google Scholar, 31Brandt A.C. Koehn O.J. Williams C.L. SmgGDS: an emerging master regulator of prenylation and trafficking by small GTPases in the ras and Rho families.Front. Mol. Biosci. 2021; 8685135Crossref PubMed Scopus (9) Google Scholar). SmgGDS binds to many different small GTPases that contain a C-terminal polybasic region (PBR) but it is reported to be a guanine nucleotide exchange factor (GEF) only for RHOA and RHOC (32Hamel B. Monaghan-Benson E. Rojas R.J. Temple B.R.S. Marston D.J. Burridge K. et al.SmgGDS is a guanine nucleotide exchange factor that specifically activates RhoA and RhoC.J. Biol. Chem. 2011; 286: 12141-12148Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar). SmgGDS is encoded by the RAP1GDS1 gene, which generates 21 different spliced transcripts, and twelve of these alternatively spliced transcripts are reported to generate proteins (Table 1). The best characterized isoforms are the longer SmgGDS-607 isoform and a shorter SmgGDS-558 isoform (27Berg T.J. Gastonguay A.J. Lorimer E.L. Kuhnmuench J.R. Li R. Fields A.P. et al.Splice variants of SmgGDS control small GTPase prenylation and membrane localization.J. Biol. Chem. 2010; 285: 35255-35266Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar). SmgGDS-558 binds only prenylated small GTPases and helps traffic them throughout the cell (27Berg T.J. Gastonguay A.J. Lorimer E.L. Kuhnmuench J.R. Li R. Fields A.P. et al.Splice variants of SmgGDS control small GTPase prenylation and membrane localization.J. Biol. Chem. 2010; 285: 35255-35266Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar). In contrast, SmgGDS-607 binds only preprenylated small GTPases and regulates their entry into the prenylation pathway (27Berg T.J. Gastonguay A.J. Lorimer E.L. Kuhnmuench J.R. Li R. Fields A.P. et al.Splice variants of SmgGDS control small GTPase prenylation and membrane localization.J. Biol. Chem. 2010; 285: 35255-35266Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar). SmgGDS-607 may promote prenylation by delivering preprenylated small GTPases to prenyltransferases (29Ntantie E. Gonyo P. Lorimer E.L. Hauser A.D. Schuld N. McAllister D. et al.An adenosine-mediated signaling pathway suppresses prenylation of the GTPase Rap1B and promotes cell scattering.Sci. Signal. 2013; 6: ra39Crossref PubMed Scopus (75) Google Scholar, 31Brandt A.C. Koehn O.J. Williams C.L. SmgGDS: an emerging master regulator of prenylation and trafficking by small GTPases in the ras and Rho families.Front. Mol. Biosci. 2021; 8685135Crossref PubMed Scopus (9) Google Scholar, 33García-Torres D. Fierke C.A. The chaperone SmgGDS-607 has a dual role, both activating and inhibiting farnesylation of small GTPases.J. Biol. Chem. 2019; 294: 11793-11804Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar, 34Wilson J.M. Prokop J.W. Lorimer E. Ntantie E. Williams C.L. Differences in the phosphorylation-dependent regulation of prenylation of Rap1A and Rap1B.J. Mol. Biol. 2016; 428: 4929-4945Crossref PubMed Scopus (20) Google Scholar) but SmgGDS-607 can also suppress prenylation by retaining preprenylated small GTPases and not rele
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