From the Department of Molecular and Integrative Physiology, The University of Michigan, Ann Arbor, Michigan 48109-0622

Entry Version: 

1.0, January 2, 2010


Sabbatini, Maria Eugenia. (2010). Rap1.
Pancreapedia: Exocrine Pancreas Knowledge Base, DOI: 10.3998/panc.2010.4
PDF icon Rap1291.55 KB

Gene symbols: Rap1a, Rap1b

1. General Function

Rap1 (Ras-proximate-1) is a small GTP-binding protein of Mr 21,000, which belongs to the Ras family. Rap1 was identified in 1989 as a suppressor structurally similar to Ras (ca. 50 % homology) that reversed the phenotype of NIH3T3 transformed by Kirsten sarcoma virus and was referred to as Krev-1 (27, 38), though nowadays several studies indicate that Rap1 can act in a Ras-independent manner (42). Rap1 is also referred to in early literature as smg p21, when it was identified as a small GTP binding protein of 21 KDa purified from the cytosol fraction of human platelets (26, 37). There are two isoforms of Rap1, Rap1a and Rap1b, which are 95 % identical at the amino acid sequence (51). Some biological actions of Rap1 have been associated with a specific isoform (32) and some cells, such as B cells and platelets, express primarily one isoform, Rap1b (10, 28). Both isoforms are geranylgeranylated at the carboxyl-terminal Cys residue, which allows Rap1 to attach to biological membranes. In addition, they have clustered polybasic amino acids, which interact with the polar head groups of the acidic phospholipids contributing to membrane association (25, 51).

Rap1 is ubiquitously expressed and well known for its role in cell proliferation, differentiation, polarity as well as integrin-mediated cell adhesion and cadherin-mediated cell junction formation (6). In a variety of cell types, Rap1 regulates cell spreading by mediating the functions of integrins, by binding to and, thereby localizing two Rac GEFs VAV1 and TIAM1 to the sites of cell spreading, by regulating cadherin-mediated cell-cell contacts, and by interacting with and regulating myosin II, which is a constituent of the cytoskeleton (19). Rap1 has been implicated in adhesion-dependent signals during leukocyte migration and extravasation. CD31, which is an important integrin adhesion amplifier, is able to activate Rap1, and thereby, stimulate T lymphocyte adhesion to intercellular adhesion molecule (ICAM) and vascular cell adhesion molecule (VCAM) (43). Rap1 regulates the ERK cascade in different manners depending on the cell type. In HEK293 and NIH 3T3 cells Rap1 inhibits Ras-induced ERK activation via sequestration of Raf-1 (46) while in Rat-1 cells Rap1 does not affect Ras-induced ERK activation (60). In other cell types, unlike Ras, Rap1 activates the ERK pathway through direct association with B-Raf (49, 53, 59).

In addition to Rap1's role in normal cells, Rap1 activation has been implicated in the reduction of pancreatic tumor cell proliferation. In pancreatic cancer Panc1 and MiaPaCa cell lines Rap1 has been activated by forskolin, a direct adenylate cyclase stimulator, and a reduction of cell growth has been observed in Panc1 cells independently on Ras/Raf/MEK/ERK pathway (35).

Rap1, like other small G proteins, cycles between an inactive GDP-bound and an active GTP-bound forms and this cycle is regulated by two groups of regulatory proteins: guanine nucleotide exchange factors (GEFs), which induce dissociation of GDP from Rap1 followed by binding of GTP, and GTPase-activating proteins (GAPs), which convert active GTP-bound to the inactive GDP-bound form. The activation of Rap1 by GEFs is induced by second messengers, including calcium, diacylglycerol (DAG), phospholipase Cγ (PLCγ) and cAMP (51, 59). The Rap1GEF activated by an increase in cAMP is the Exchange protein activating cAMP (Epac) family which is constituted by two isoforms, Epac1 and Epac2 (14, 24). An increase in calcium and DAG levels induce CalDAG-GEF activation which is another family of Rap1GEF constituted by four isoforms, CalDAG-GEF I, II, III and IV; only CalDAG-GEF I and III are able to activate Rap1 (56). All CalDAG-GEFs have a CD25 domain, which is necessary for the GEF activity, as well as calcium- and DAG-binding domains. C3G was the first RapGEF identified with a characteristic catalytic region consisting of a CDC25 homology domain, a Ras exchange motif (REM) and a proline-rich sequences which bind to the SH3 domain of the adaptor proteins Crk and Grb2 (40). Another GEF for Rap1 is PDZ-GEF, which contains Ras association, and Ras-GEF domains, as well as a carboxyl-terminal motif for binding to PDZ domains (31). DOCK4 is an atypical GEF, which has GEF activity for both Rap and Rac (55). The finding that Rap1 could be activated by stimulation of a number of membrane receptors broadened its possible roles in signal transduction.

Protein kinase A (PKA), which is activated by cAMP formation, participates in Rap1 phosphorylation in certain cell types, including neutrophils, platelets, fibroblasts, thyroids, and enteroendocrine cells (41, 52). Phosphorylation appears to regulate both Rap1 activation as well as Rap1 membrane association. In platelets where Rap1 is one of the most abundant phosphoproteins, PKA-catalyzed phosphorylation at Ser179 affects Rap1 membrane binding and makes Rap1 sensitive to the action of Rap1GEF to stimulate its GDP/GTP change reaction, but does not by itself affect GDP/GTP binding or intrinsic GTPase activity (18). Ser179 belongs to the hydrophobic acid polybasic region of Rap1, which is essential for Rap1 to bind to membrane. Since PKA phosphorylates Rap1 downstream of this polybasic region, PKA reduces Rap1 membrane-binding affinity. However, the role of PKA-dependent phosphorylation of Rap1 can not be generalized since in thyroid follicular cells the phosphorylation of Rap1 does not affect its perinuclear subcellular localization (44). In mouse pancreatic acini PKA does not seem to be required for Rap1 activation and translocation since the PKA inhibitor H-89 does not either block forskolin-induced Rap1 activation or affect Rap1 membrane binding (45). Recently, a mass spectroscopic study has shown that when PKA phosphorylates Rap1b on Ser179, it produces a conformational switch in active Rap1, which affects allosterically the effector domain (15).

Active Rap1 activates multiple downstream effectors including RapL, RalGDS, Afadin, Arap3, Nore1B, RIAM, Raf-1, and B-Raf (42). RapL is a major effector of Rap1 in immune cells (23). RIAM (Rap1-GTP-interacting adaptor molecule) links Rap1 to integrin activation and interacts with profilin and Ena/VASP proteins which regulate actin dynamics (30). In some cells, Rap1 is activated by C3G and stimulates ERK pathway via interaction with B-Raf (58).

Several actin modulating proteins have been directly regulated by Rap1 (40). Recently, RA-RhoGAP, an inhibitor of RhoA activation, has demonstrated to be a Rap1 effector (1).

Rap1 interacts with other small GTP-binding proteins including Rac, which is involved in the regulation of actin cytoskeleton dynamics (36). In certain cell types upon Rap1 activation, Rac1 is activated contributing to cell movement and spreading (2, 50). In MCF-7 human breast epithelial cells IQGAP, an effector of Rac1 (29), has mostly interacted with the active form of Rap1 and reduced Rap1 activation induced by cAMP, resulting in a decrease in cell adhesion (20). In PC12 cells, active Ras regulates Epac2 function and thereby Epac2-mediated activation of Rap1 (34). In addition to its role as Rap1GEF, Epac2 has been shown to exert physiological function in pancreatic β cells, where participates in cAMP-regulated insulin granule exocytosis (16, 22).

2. Specific function in the pancreas

In mouse pancreatic acini Rap1A and Rap1B have been identified by RT-PCR. Moreover, western-blotting demonstrates the presence of Rap1 protein but the antibody is not able to distinguish between both isoforms (45). Rap1 is activated following stimulation of acini with CCK, carbachol, and VIP as shown using a pull-down assay (45). Several second messengers are able to activate Rap1; calcium ionophore A-23187, phorbol ester, forskolin, 8-bromo-cAMP, and the Epac-selective cAMP analog 8-pCPT-2'-O-Me-cAMP all induce an increase in GTP-Rap1 levels. Using RT-PCR two Rap1GEFs have been found in mouse pancreatic acini: Epac1 and CalDAG-GEF III. The presence of other Rap1GEFs such as C3G and PDZ-GEF has not been evaluated. Epac1 protein is present in the zymogen granule area likely associated with Rap1 as shown using western-blotting and immunocytochemistry. Rap1 is present on zymogen granule membranes, as shown by mass spectrometry, immunohistochemistry and Western-blotting (8). Immnunohistochemistry localization in mouse and rat acini is shown in Fig. 1 and Fig. 2.

Figure 1

Figure 1. Immunostaining of purified isolated rat zymogen granules for Rap1. Zymogen granules were purified using Percoll gradient procedure and allowed to attach to a glass slide. Rap1 localization was demonstrated by confocal microscopy using rabbit anti-Rap1 antibody from Santa Cruz (red). Zymogen granule fluorescence image is paired with the corresponding Nomarski image (Reproduced from Reference 8).

With respect to its role in pancreatic exocrine function, Rap1 has been reported to be required for pancreatic amylase secretion (45). The overexpression of Rap1GAP, a protein involved in the inhibition of Rap1 activation, decreases pancreatic amylase secretion induced by carbachol and cholecystokinin (CCK) as well as stimulators of cAMP pathway (45). A proposed model for Rap1-mediated response on pancreatic amylase secretion is shown in Fig. 3. Rap1 activation is not required for CCK-induced calcium mobilization since the expression of Rap1GAP in pancreatic acini does not affect the response to CCK (45). The participation of Rap1 in exocrine secretion has also been shown in other tissues and cell types such as parotid glands and acrosomes. Rap1 is present in secretory granules of parotid glands (11, 21, 48). It translocates from the membrane to the cytosol upon stimulation with the β-adrenergic agonist isoproterenol, and this event occurs in parallel to an increase in amylase secretion (12). In the acrosome, unlike in pancreatic acini, Rap1, which is activated by Epac1, induces intracellular calcium mobilization to achieve exocytosis (7).

Although in some tissues, Rap1 translocation from the membrane to the cytosol is correlated to Rap1 activation, in mouse pancreatic acini Rap1 does not translocate upon stimulation with different secretagogues (45).

Figure 2

Figure 2. Confocal immunostaining of isolated mouse pancreatic acini for Rap1. Isolated mouse pancreatic acini were prepared and fixed with paraformaldehyde. Cryostat sections were mounted on SuperFrost Plus slides (Fischer). The sections were incubated with the following primary antibodies: polyclonal rabbit anti-Rap1 (Santa Cruz Biotechnology) (1:200) (red), polyclonal sheep anti-human salivary amylase (U.S. Biological) (1:100 to 1:200) (green). The sections were then incubated with secondary antibodies Cy3-conjugated donkey anti-rabbit IgG (1:200) or fluorescein-conjugated anti-sheep IgG (1;200) (Jackson Immunoresearch Laboratories, Inc). Prolong Gold with 4,6-diamidino-2-phenylindole (DAPI) (blue) was added to mounting medium to counterstain nuclei. Digitized images were collected with an Olympus Fluoview 500 confocal microscope. Rap1 is localized on zymogen granules in close proximity to amylase. The figure was kindly provided by Dr. Stephen Ernst, The University of Michigan.

While Rap1 has been involved in ERK activation in diverse cells as previously mention, in mouse pancreatic acini CCK-induced ERK phosphorylation (13) has not been affected by Rap1 activation since the expression of Rap1GAP does not modify the response to CCK (unpublished data).

Recently, a study has shown that Rap1, which is activated by PKCα, mediates integrin-induced aggregation in platelets (17). In pancreatic acini high concentrations CCK induces acini aggregation, an effect which seems to be mediated by PKC (unpublished data). Although CCK induces Rap1 activation in mouse pancreatic acini, the participation of Rap1 in aggregation is unlikely since the expression of Rap1GAP does not affect the induction of aggregation by CCK (unpublished data).

To date, the effectors involved in the pancreatic exocytotic response to Rap1, as well as the ability of Rap1 to interact with other small G proteins have not been studied.

3. Tools for study of Rap1

a. cDNA clones

cDNA clones for human wild-type, constitutively active and dominant negative Rap1A in pcDNA 3.1 are available from UMR cDNA Resource center, Missouri University of Science and Technology (www.cdna.org). A plasmid coding for GST-RalGDS-Rap1 binding domain (RBD) is available from this lab.

Figure 4

Figure 3. Activation of Rap1 after the stimulation by secretagogues CCK, carbachol and VIP in mouse pancreatic acini. Different second messengers including DAG, calcium and cAMP generated from the activation of CCK, muscarinic and VPAC receptors are able to induce Rap1 activation via either CalDAG-GEF or Epac1, and thereby regulate amylase secretion in mouse pancreatic acinar cells (From Reference 45).

b. Antibodies

Rabbit polyclonal antibody raised against a peptide mapping near the C-terminus of Rap1 of human origin from Santa Cruz Biotechnology (# sc-65) has been used to identify Rap1 by western-blotting and immunohistochemistry in pancreatic acini (8, 45). We have also successfully used a mouse monoclonal Rap1 antibody from BD Transduction Laboratories (# 610195). Specific antibodies against Rap1a and Rap1b have been generated by immunization of rabbits with peptides derived from the C-terminus of the Rap1a and Rap1b proteins (28), though they have not been used by us on pancreas. There are several antibodies raised against Rap1GEFs and Rap1GAP as well as Rap1 effectors available: rabbit polyclonal Rap1GAP antibody from Santa Cruz Biotechnology (# sc-28189), polyclonal C3G and PDZ-GEFs antibodies from Bethyl Laboratories. These two last antibodies have not been checked by us on pancreas. Polyclonal Epac antibodies are available which are discussed under that molecule.

c. Viruses

Sindbis virus coding for constitutively active Rap1B (V12) and dominant negative Rap1B (N17) have been described (3). An adenovirus expressing Rap1GAP has been prepared by Dr. Patrick Casey of Duke University (54) and is available from us with permission of Dr. Casey.

d. The following knock-out and transgenic mice have been developed:

  1. Conditional knockout of rap1a/rap1b in forebrain (39).
  2. transgenic mice expressing active Rap1A within the T cell lineage (47).
  3. Rap1a and Rap1b null mice (33, 57). Some Rap1a -/- embryos have died in utero (33). Rap1b -/- mice have shown an increase in embryonic lethality and evidence of prolonged bleeding time and hemorrhage in liver, brain, and abdominal cavity (9).

e. Assay of the active state of Rap1

Assay of the active state of Rap1 has been carried out by use of a pull-down assay using GST-RalGDS (45). We carry out this assay using GST-RalGDS prepared in our lab (see Methods). A commercial kit, StressXpress Rap1 activation kit, is now available from Stressgen Bioreagents (# EKS-455).

4. References

  1. Aivatiadou E, Ripolone M, Brunetti F, and Berruti G. cAMP-Epac2-mediated activation of Rap1 in developing male germ cells: RA-RhoGAP as a possible direct down-stream effector. Mol Reprod Dev 76: 407-416, 2009. PMID: 18937323
  2. Arthur WT, Quilliam LA, and Cooper JA. Rap1 promotes cell spreading by localizing Rac guanine nucleotide exchange factors. J Cell Biol 167: 111-122, 2004. PMID: 15479739
  3. Bertoni A, Tadokoro S, Eto K, Pampori N, Parise LV, White GC, and Shattil SJ. Relationships between Rap1b, affinity modulation of integrin alpha IIbbeta 3, and the actin cytoskeleton. J Biol Chem 277: 25715-25721, 2002. PMID: 119943014. Borland G, Smith BO, and Yarwood SJ. EPAC proteins transduce diverse cellular actions of cAMP. Br J Pharmacol 158: :70-86, 2009. PMID: 19210747
  4. Bos JL. Epac proteins: multi-purpose cAMP targets. Trends Biochem Sci 31: 680-686, 2006. PMID: 17084085
  5. Bos JL. Linking Rap to cell adhesion. Curr Opin Cell Biol 17: 123-128, 2005. PMID: 15780587
  6. Branham MT, Bustos MA, De Blas GA, Rehmann H, Zarelli VE, Trevino CL, Darszon A, Mayorga LS, and Tomes CN. Epac activates the small G proteins Rap1 and Rab3A to achieve exocytosis. J Biol Chem 284: 24825-39,2009. PMID: 19546222
  7. Chen X, Walker AK, Strahler JR, Simon ES, Tomanicek-Volk SL, Nelson BB, Hurley MC, Ernst SA, Williams JA, and Andrews PC. Organellar proteomics: analysis of pancreatic zymogen granule membranes. Mol Cell Proteomics 5: 306-312, 2006. PMID: 16278343
  8. Chrzanowska-Wodnicka M, Smyth SS, Schoenwaelder SM, Fischer TH, and White GC, 2nd. Rap1b is required for normal platelet function and hemostasis in mice. J Clin Invest 115: 680-687, 2005. PMID: 15696195
  9. Chu H, Awasthi A, White GC, 2nd, Chrzanowska-Wodnicka M, and Malarkannan S. Rap1b regulates B cell development, homing, and T cell-dependent humoral immunity. J Immunol 181: 3373-3383, 2008. PMID: 18714009
  10. D'Silva NJ, DiJulio DH, Belton CM, Jacobson KL, and Watson EL. Immunolocalization of rap1 in the rat parotid gland: detection on secretory granule membranes. J Histochem Cytochem 45: 965-973, 1997. PMID: 9212822
  11. D'Silva NJ, Jacobson KL, Ott SM, and Watson EL. Beta-adrenergic-induced cytosolic redistribution of Rap1 in rat parotid acini: role in secretion. Am J Physiol 274: C1667-1673, 1998. PMID: 9611133
  12. Dabrowski A, Groblewski GE, Schafer C, Guan KL, and Williams JA. Cholecystokinin and EGF activate a MAPK cascade by different mechanisms in rat pancreatic acinar cells. Am J Physiol 273: C1472-1479, 1997. PMID: 9374631
  13. de Rooij J, Zwartkruis FJ, Verheijen MH, Cool RH, Nijman SM, Wittinghofer A, and Bos JL. Epac is a Rap1 guanine-nucleotide-exchange factor directly activated by cyclic AMP. Nature 396: 474-477, 1998. PMID: 9853756
  14. Edreira MM, Li S, Hochbaum D, Wong S, Gorfe AA, Ribeiro-Neto F, Woods VL, and Altschuler DL. Phosphorylation-induced conformational changes in Rap1b: allosteric effects on switch domains and effector loop. J Biol Chem 284:27480-6, 2009. PMID: 19651783
  15. Fujimoto K, Shibasaki T, Yokoi N, Kashima Y, Matsumoto M, Sasaki T, Tajima N, Iwanaga T, and Seino S. Piccolo, a Ca2+ sensor in pancreatic beta-cells. Involvement of cAMP-GEFII.Rim2.Piccolo complex in cAMP-dependent exocytosis. J Biol Chem 277: 50497-50502, 2002. PMID: 12401793
  16. Han J, Lim CJ, Watanabe N, Soriani A, Ratnikov B, Calderwood DA, Puzon-McLaughlin W, Lafuente EM, Boussiotis VA, Shattil SJ, and Ginsberg MH. Reconstructing and deconstructing agonist-induced activation of integrin alphaIIbbeta3. Curr Biol 16: 1796-1806, 2006. PMID: 16979556
  17. Hata Y, Kaibuchi K, Kawamura S, Hiroyoshi M, Shirataki H, and Takai Y. Enhancement of the actions of smg p21 GDP/GTP exchange protein by the protein kinase A-catalyzed phosphorylation of smg p21. J Biol Chem 266: 6571-6577, 1991. PMID: 1901063
  18. Jeon TJ, Lee DJ, Merlot S, Weeks G, and Firtel RA. Rap1 controls cell adhesion and cell motility through the regulation of myosin II. J Cell Biol 176: 1021-1033, 2007. PMID: 17371831
  19. Jeong HW, Li Z, Brown MD, and Sacks DB. IQGAP1 binds Rap1 and modulates its activity. J Biol Chem 282: 20752-20762, 2007. PMID: 17517894
  20. Kameyama Y, Nagata K, Mizuno-Kamiya M, Yokota Y, Fujita A, and Nozawa Y. Localization of a low Mr GTP-binding protein, rap1 protein, in plasma membranes and secretory granule membranes of rat parotid gland. Life Sci 55: 213-219, 1994. PMID: 8007763
  21. Kashima Y, Miki T, Shibasaki T, Ozaki N, Miyazaki M, Yano H, and Seino S. Critical role of cAMP-GEFII--Rim2 complex in incretin-potentiated insulin secretion. J Biol Chem 276: 46046-46053, 2001.
  22. Katagiri K, Maeda A, Shimonaka M, and Kinashi T. RAPL, a Rap1-binding molecule that mediates Rap1-induced adhesion through spatial regulation of LFA-1. Nat Immunol 4: 741-748, 2003. PMID: 12845325
  23. Kawasaki H, Springett GM, Mochizuki N, Toki S, Nakaya M, Matsuda M, Housman DE, and Graybiel AM. A family of cAMP-binding proteins that directly activate Rap1. Science 282: 2275-2279, 1998. PMID: 9856955
  24. Kawata M, Farnsworth CC, Yoshida Y, Gelb MH, Glomset JA, and Takai Y. Posttranslationally processed structure of the human platelet protein smg p21B: evidence for geranylgeranylation and carboxyl methylation of the C-terminal cysteine. Proc Natl Acad Sci U S A 87: 8960-8964, 1990. PMID: 2123345
  25. Kawata M, Matsui Y, Kondo J, Hishida T, Teranishi Y, and Takai Y. A novel small molecular weight GTP-binding protein with the same putative effector domain as the ras proteins in bovine brain membranes. Purification, determination of primary structure, and characterization. J Biol Chem 263: 18965-18971, 1988. PMID: 3143720
  26. Kitayama H, Sugimoto Y, Matsuzaki T, Ikawa Y, and Noda M. A ras-related gene with transformation suppressor activity. Cell 56: 77-84, 1989. PMID: 2642744
  27. Klinz FJ, Seifert R, Schwaner I, Gausepohl H, Frank R, and Schultz G. Generation of specific antibodies against the rap1A, rap1B and rap2 small GTP-binding proteins. Analysis of rap and ras proteins in membranes from mammalian cells. Eur J Biochem 207: 207-213, 1992. PMID: 1628649
  28. Kuroda S, Fukata M, Kobayashi K, Nakafuku M, Nomura N, Iwamatsu A, and Kaibuchi K. Identification of IQGAP as a putative target for the small GTPases, Cdc42 and Rac1. J Biol Chem 271: 23363-23367, 1996. PMID: 8798539
  29. Lafuente EM, van Puijenbroek AA, Krause M, Carman CV, Freeman GJ, Berezovskaya A, Constantine E, Springer TA, Gertler FB, and Boussiotis VA. RIAM, an Ena/VASP and Profilin ligand, interacts with Rap1-GTP and mediates Rap1-induced adhesion. Dev Cell 7: 585-595, 2004. PMID: 15469846
  30. Lee JH, Cho KS, Lee J, Kim D, Lee SB, Yoo J, Cha GH, and Chung J. Drosophila PDZ-GEF, a guanine nucleotide exchange factor for Rap1 GTPase, reveals a novel upstream regulatory mechanism in the mitogen-activated protein kinase signaling pathway. Mol Cell Biol 22: 7658-7666, 2002. PMID: 12370312
  31. Li X, Stankovic M, Lee BP, Aurrand-Lions M, Hahn CN, Lu Y, Imhof BA, Vadas MA, and Gamble JR. JAM-C induces endothelial cell permeability through its association and regulation of {beta}3 integrins. Arterioscler Thromb Vasc Biol 29: 1200-1206, 2009. PMID: 19461049
  32. Li Y, Yan J, De P, Chang HC, Yamauchi A, Christopherson KW, 2nd, Paranavitana NC, Peng X, Kim C, Munugalavadla V, Kapur R, Chen H, Shou W, Stone JC, Kaplan MH, Dinauer MC, Durden DL, and Quilliam LA. Rap1a null mice have altered myeloid cell functions suggesting distinct roles for the closely related Rap1a and 1b proteins. J Immunol 179: 8322-8331, 2007. PMID: 18056377
  33. Liu C, Takahashi M, Li Y, Song S, Dillon TJ, Shinde U, and Stork PJ. Ras is required for the cyclic AMP-dependent activation of Rap1 via Epac2. Mol Cell Biol 28: 7109-7125, 2008. PMID: 18824540
  34. Lorenz R, Aleksic T, Wagner M, Adler G, and Weber CK. The cAMP/Epac1/Rap1 pathway in pancreatic carcinoma. Pancreas 37: 102-103, 2008. PMID: 18580452
  35. Maillet M, Robert SJ, Cacquevel M, Gastineau M, Vivien D, Bertoglio J, Zugaza JL, Fischmeister R, and Lezoualc'h F. Crosstalk between Rap1 and Rac regulates secretion of sAPPalpha. Nat Cell Biol 5: 633-639, 2003. PMID: 12819788
  36. Nagata K, Itoh H, Katada T, Takenaka K, Ui M, Kaziro Y, and Nozawa Y. Purification, identification, and characterization of two GTP-binding proteins with molecular weights of 25,000 and 21,000 in human platelet cytosol. One is the rap1/smg21/Krev-1 protein and the other is a novel GTP-binding protein. J Biol Chem 264: 17000-17005, 1989. PMID: 2507536
  37. Noda M, Kitayama H, Kanazawa S, Murata S, Matsuzaki T, and Ikawa Y. Transformation suppressor genes. Princess Takamatsu Symp 20: 233-239, 1989. PMID: 2518686
  38. Pan BX, Vautier F, Ito W, Bolshakov VY, and Morozov A. Enhanced cortico-amygdala efficacy and suppressed fear in absence of Rap1. J Neurosci 28: 2089-2098, 2008. PMID: 18305243
  39. Pannekoek WJ, Kooistra MR, Zwartkruis FJ, and Bos JL. Cell-cell junction formation: the role of Rap1 and Rap1 guanine nucleotide exchange factors. Biochim Biophys Acta 1788: 790-796, 2009. PMID: 19159611
  40. Quilliam LA, Mueller H, Bohl BP, Prossnitz V, Sklar LA, Der CJ, and Bokoch GM. Rap1A is a substrate for cyclic AMP-dependent protein kinase in human neutrophils. J Immunol 147: 1628-1635, 1991. PMID: 1908879
  41. Raaijmakers JH and Bos JL. Specificity in Ras and Rap signaling. J Biol Chem 284: 10995-10999, 2009. PMID: 19091745
  42. Reedquist KA, Ross E, Koop EA, Wolthuis RM, Zwartkruis FJ, van Kooyk Y, Salmon M, Buckley CD, and Bos JL. The small GTPase, Rap1, mediates CD31-induced integrin adhesion. J Cell Biol 148: 1151-1158, 2000. PMID: 10725328
  43. Ribeiro-Neto F, Urbani J, Lemee N, Lou L, and Altschuler DL. On the mitogenic properties of Rap1b: cAMP-induced G(1)/S entry requires activated and phosphorylated Rap1b. Proc Natl Acad Sci U S A 99: 5418-5423, 2002. PMID: 11959997
  44. Sabbatini ME, Chen X, Ernst SA, and Williams JA. Rap1 activation plays a regulatory role in pancreatic amylase secretion. J Biol Chem 283: 23884-23894, 2008. PMID: 19088252
  45. Schmitt JM and Stork PJ. Cyclic AMP-mediated inhibition of cell growth requires the small G protein Rap1. Mol Cell Biol 21: 3671-3683, 2001. PMID: 11340161
  46. Sebzda E, Bracke M, Tugal T, Hogg N, and Cantrell DA. Rap1A positively regulates T cells via integrin activation rather than inhibiting lymphocyte signaling. Nat Immunol 3: 251-258, 2002. PMID: 11836528
  47. Shimomura H, Imai A, and Nashida T. Evidence for the involvement of cAMP-GEF (Epac) pathway in amylase release from the rat parotid gland. Arch Biochem Biophys 431: 124-128, 2004. PMID: 15464734
  48. Stork PJ. Does Rap1 deserve a bad Rap? Trends Biochem Sci 28: 267-275, 2003. PMID: 12765839
  49. Takahashi M, Rikitake Y, Nagamatsu Y, Hara T, Ikeda W, Hirata K, and Takai Y. Sequential activation of Rap1 and Rac1 small G proteins by PDGF locally at leading edges of NIH3T3 cells. Genes Cells 13: 549-569, 2008. PMID: 18422604
  50. Takai Y, Sasaki T, and Matozaki T. Small GTP-binding proteins. Physiol Rev 81: 153-208, 2001. PMID: 11152757
  51. Tsygankova OM, Saavedra A, Rebhun JF, Quilliam LA, and Meinkoth JL. Coordinated regulation of Rap1 and thyroid differentiation by cyclic AMP and protein kinase A. Mol Cell Biol 21: 1921-1929, 2001. PMID: 11238928
  52. Wang Z, Dillon TJ, Pokala V, Mishra S, Labudda K, Hunter B, and Stork PJ. Rap1-mediated activation of extracellular signal-regulated kinases by cyclic AMP is dependent on the mode of Rap1 activation. Mol Cell Biol 26: 2130-2145, 2006. PMID: 16507992
  53. Wittchen ES, Worthylake RA, Kelly P, Casey PJ, Quilliam LA, and Burridge K. Rap1 GTPase inhibits leukocyte transmigration by promoting endothelial barrier function. J Biol Chem 280: 11675-11682, 2005. PMID: 15661741
  54. Yajnik V, Paulding C, Sordella R, McClatchey AI, Saito M, Wahrer DC, Reynolds P, Bell DW, Lake R, van den Heuvel S, Settleman J, and Haber DA. DOCK4, a GTPase activator, is disrupted during tumorigenesis. Cell 112: 673-684, 2003. PMID: 12628187
  55. Yamashita S, Mochizuki N, Ohba Y, Tobiume M, Okada Y, Sawa H, Nagashima K, and Matsuda M. CalDAG-GEFIII activation of Ras, R-ras, and Rap1. J Biol Chem 275: 25488-25493, 2000. PMID: 10835426
  56. Yan J, Li F, Ingram DA, and Quilliam LA. Rap1a is a key regulator of fibroblast growth factor 2-induced angiogenesis and together with Rap1b controls human endothelial cell functions. Mol Cell Biol 28: 5803-5810, 2008. PMID: 18625726
  57. York RD, Yao H, Dillon T, Ellig CL, Eckert SP, McCleskey EW, and Stork PJ. Rap1 mediates sustained MAP kinase activation induced by nerve growth factor. Nature 392: 622-626, 1998. PMID: 9560161
  58. Zwartkruis FJ and Bos JL. Ras and Rap1: two highly related small GTPases with distinct function. Exp Cell Res 253: 157-165, 1999. PMID: 10579920
  59. Zwartkruis FJ, Wolthuis RM, Nabben NM, Franke B, and Bos JL. Extracellular signal-regulated activation of Rap1 fails to interfere in Ras effector signalling. EMBO J 17: 5905-5912, 1998. PMID: 9774335