Cdc42
PDB rendering based on 1a4r
Identifiers
Symbol	Cdc42
Alt. symbols	G25K; CDC42Hs
Entrez	998
HUGO	1736
OMIM	116952
PDB	1a4r
UniProt	P60953
Other data
Locus	Chr. 1 p36.12

Cdc42 is a highly conserved 25kD small Ras-like GTPase, it is a member of the sub-family of Rho GTPases. It is functionally implicated in the establishment and maintenance of cell polarity and cell cycle progression. Specifically it was found that vesicle trafficking, cell migration and morphology are affected. ^[1] Fast cycling mutants which have acquired the ability to exchange GDP for GTP in absence of Guanine nucleotide exchange factors have been shown to be potent activators of malignant transformation. Activated Cdc42 binds specifically to the γ-COP subunit which is essential for transforming signals distinct from those elicited by Ras. ^{[2] [3]}

Contents 1 Introduction 2 Structure 3 Cellular location 4 Regulation 5 Effectors 6 Function 6.1 Cell polarity 6.2 Axon guidance 6.3 Stress response 7 Cdc42 in pathogenesis 8 References

[edit] Introduction

Human Cdc42 was first described in 1990 by Shinjo et al. ^[4] who cloned it from a human placenta library and found the protein sequence to be very similar to the Rho family of GTPases; the highest sequence homology was to the budding yeast Cdc42p. Yeast cdc42-1 mutants could in deed be complemented by human Cdc42 revealing its functional conservaton. The cycling between the GDP- and GTP-bound forms has been shown to be essential for transformation. ^[5] The Cdc42(F28L) mutant cycles constitutively between GDP and GTP and has been shown to induce transformation causing a significant loss of actin stress fibres, a phenotype reminiscent of the one observed in Ras transformed fibroblasts.

[edit] Structure

The human gene product Cdc42 is a 191 amino acid peptide. The C-terminal CAAX-domain's cystein residue is postranslationally modified to either a methylester or a geranyl-geranyl cystein, AAX is cleaved off. The crystal structure has been solved in high resolution bound to the GTP analogue GMPPNP and in complex with the GAP domain of p50rhoGAP by Rittinger et al. ^[6] and bound to the GTP analogue GMP-PCP (guanylyl β,γ-methylenediphosphonate) by Philipps et al. ^[7] . Like all GTPases of the RhoGTPase family Cdc42 comprises a conserved architecture of five α-helices and six β-strands, and as many of the known GTPases, Cdc42 has a switch I and a switch II region, which are important regions for GTP hydrolysis. The nucleotide binding occurs in the so-called G-box or G-domain, which consists of 5 parts that are distributed all over the protein:
G1: GXXXXGKS/T (also called the P-loop)
G2: T
G3: DXXG
G4: NKXD
G5: SAK.
Mutations in this G-domain can lock Cdc42 in either the GTP- or the GDP-bound conformation. Often the conformational change in switch I and II is the principal reason for the engagement of downstream effectors, in Cdc42 however the switch regions of the GTP- and the GDP-bound form are highly similar (RMSD ~ 0.75 Å), even though only the GTP-bound conformer engages downstream effectors. Effector proteins recognize the GTP-bound conformer specifically and induce conformational changes to enable signal propagation to occur. Cdc42 has, like all RhoGTPases, an C-terminal "Rho-insert region", which is thought to be important for the interaction with different downstream effectors.

[edit] Cellular location

Cdc42 is mostly located at the cytoplasmic side of the cellular membrane and anchored via a geranyl-geranyl-anchor, but it has also been reported to reside at the Golgi membrane. The location of activated Cdc42 is important for its cellular function, since it defines a polarization point by localising to the division area and growing tips such as the site of budding in S.cerevisiae and the location for the formation of some internal membrane structures (lamellopodia and filopodia) (see Functions).

[edit] Regulation

Like all Rho-family proteins, Cdc42 undergoes a GDP-dissociation/GTP-binding/GTP-hydrolysis cycle which enables it to act as a molecular switch. Cdc42 is found in an so called active GTP-bound and an "inactive" GDP-bound form. The GTPase Cdc42 has a low intrinsic GTP-hydrolysis activity. However this GTPase activity is controlled by GEFs (Guanine nucleotide exchange factors) and GAPs (GTPase activating proteins). GEFs exchange bound GDP with GTP, GAPs enhance the rate of hydrolysis of Cdc42-GTP. The main GEFs of Cdc42 are of the Dbl-family and the DOCK180 family, which are thought to report about the cellular localization of Cdc42-GTP. So far, more than 70 mammalian GEFs have been shown to interact with Cdc42.

There are have also been different GAPs identified that interact with Cdc42, amongst them are RICS (RhoGAP involved in the beta-catenin-N-cadherin and NMDA receptor signaling), SNX26 (from the PX domain containing GAP family) and Rho GTPase-activating protein 1. The activation of Cdc42 is spatiotemporally tightly restricted. Activated Cdc42 has been shown to be localized primarily at sites of membrane growth such as during bud formation in S. cerivisiae, cytokinesis (membrane growth at the indentation sites), axon growth and guidance and cell migration.

[edit] Effectors

Effectors of Cdc42-GTP and Rac1-GTP, like IQGAP1 and Par6, mediate cell-cell adhesion and cell polarity. Also actin polymerization at cell protrusions, stabilization and capture of microtubules, and also positioning of the cytoskeleton and organelles is mediated by Cdc42effector molecules. The Cdc42 and Rac effector protein kinases, PAKs, are important mediators of cytoskeletal organization. Cdc42-GTP binding to N-WASP initiates the activation of the ARP2/3 complex that is required for actin polymerization. Another effector protein of Cdc42 is MRCK, Myotonic dystrophy kinase-related CDC42-binding kinase, which regulates actomyosin contraction. ^[8]

[edit] Function

[edit] Cell polarity

Cdc42 GTPase regulates cell morphology especially via the polarization of the actin cytoskeleton in all eukaryotic cells. A local activation of this GTPase at the presumptive membrane growth site is required in order to selectively regulate the multiple signal transduction pathways that will execute growth only at this location.

Its activity is precisely controlled both temporally and spatially. Although the functional characteristics of cell polarity are extremely variable (depending on the cell type and the biological context), Cdc42 has a huge capacity to coordinate the control of multiple signal transduction pathways. Cdc42 in particular stands out as playing a central role in establishing cell polarity in all eukaryotic cells, irrespective of the biological context. ^[9]

[edit] Axon guidance

Cdc42 has been shown to be involved in the reorganization of the actin cytoskeleton and in the generation of cell extensions in non-neuronal cells, suggesting that it also may be involved in neuronal axon guidance. ^[10] Cortical neurons expressing dominant-negatively mutated Cdc42 showed a decreased number of dendrites, whereas neurons expressing constitutively active Cdc42 had more dendrites. ^[11] Immature neurons continuously extend and retract neurites until one such neurite receives dominant signals that promote its development into an axon. Cdc42 has been shown to promote the formation of filopodia at the neurite growth cone. Additionally, interference with Cdc42 and Rac1 activity blocks axon formation. During axon specification, extrinsic stimuli such as growth factors finally induce spatially restricted activation of Rap1B, a Ras-like GTPase. Cdc42 is downstream of Rap1B, which further activates the partitioning defective PAR complex and the Rac-specific GEF TIAM1. Subsequent activation of Rac promotes actin polymeriyation and microtubule stabilization. ^[12]

[edit] Stress response

Shear stress induces the translocation of Cdc42 and Rho from cytosol to membrane. Although both Cdc42 and Rho were involved in the shear stress–induced transcription factor AP-1,only Cdc42 was sufficient to activate AP-1/TRE. ^[13] Cdc42 is critical for correct reorientation of the MTOC after shear stress, and especially the correct spatial activation of Cdc42 rather than its activity per se. It controls the polarization of the microtubule organizing center. ^[14]

[edit] Cdc42 in pathogenesis

Cdc42 helps bacterial infection. Streptococcus pneumoniae (pneumococci) infection is dependent on the host cell's actin and microtubules, and Cdc42 has been shown to be time dependently activated during ingestion of the bacteria and to be essential for this process. ^[15] The Salmonella Pathogenicity Island 1 (SPI1) locus encodes a type III protein secretion system (TTSS) that translocates effector proteins into cells to promote bacterial invasion through actin rearrangements. SPI1 effectors interact directly with actin and also alter the cytoskeleton through activation of the regulatory proteins, Cdc42 and Rac, to produce membrane ruffles that engulf the bacteria.^[16] This activation of RhoGTPases is done by molecular mimicry: Bacterial SopE and SopE2 mimic eukaryotic G-nucleotide exchange factors and thereby activate RhoGTPase signaling pathways in infected host cells, whereas SptP inactivates RhoGTPases by mimicking the activity of eukaryotic GTPase-activating proteins. ^[17]

In breast cancer elevated levels of Cdc42 are often observed and it is thought that this increases the activity of Jun kinase, which is a common oncoprotein in mammary cancers. ^[18] Cdc42 also plays a role in renal and in pancreatic cancer. Additionally, decreased Cdc42 signaling leads to impaired neuronal development, as Cdc42 stimulates filopodia formation, neurite formation and dendritic branch dynamics, whereas dominant negative Cdc42 induces fewer and shorter dendrites. This leads to more pathfinding errors, decreased or aberrant connectivity in the neuronal networks and thereby to mental retardation ^[19].

[edit] References

1. ^ Etienne-Manneville S (2004 Mar 15). "Cdc42 — the centre of cell polarity". J Cell Sci. 117(Pt 8): 1291-300. PMID 15020669. 
2. ^ Qiu RG, Abo A, McCormick F, Symons M. (1997 Jun). "Cdc42 regulates anchorage-independent growth and is necessary for Ras transformation.". Mol Cell Biol. 17(6): 3449-58. PMID 9154844. 
3. ^ Wu WJ, Erickson JW, Lin R, Cerione RA (2000 Jun 15). "The gamma-subunit of the coatomer complex binds Cdc42 to mediate transformation.". Nature 405(6788): 800-4. PMID 10866202. 
4. ^ Shinjo K, Koland JG, Hart MJ, Narasimhan V, Johnson DI, Evans T, Cerione RA. (1990 Dec). "Molecular cloning of the gene for the human placental GTP-binding protein Gp (G25K): identification of this GTP-binding protein as the human homolog of the yeast cell-division-cycle protein CDC42.". Proc Natl Acad Sci U S A 87(24): 9853-7. PMID 2124704. 
5. ^ Tu SS, Wu WJ, Yang W, Nolbant P, Hahn K, Cerione RA. (2002 Oct 15). "Antiapoptotic Cdc42 mutants are potent activators of cellular transformation.". Biochemistry 41(41): 12350-8. PMID 12369824. 
6. ^ Katrin Rittinger, Philip A.Walker, John F. Eccleston, Kurshid Nurmahomed, Darerca Owen, Ernest Laue, Steven J. Gamblin, Stephen J. Smerdon (1997 Aug 14). "Crystal structure of a small G-protein in complex with the GTPase-activating protein rhoGAP". Nature 388: 693-697. 
7. ^ Matthew J. Phillips, Guillermo Calero, Britton Chan, Sekar Ramachandran, and Richard A. Cerione (2008 May 16). "Effector Proteins Exert an Important Influence on the Signaling-active State of the Small GTPase Cdc42". Journal of Biological Chemistry 283: 14153-14164. 
8. ^ Iden S, Collard JG. (2008 Nov). "Crosstalk between small GTPases and polarity proteins in cell polarization.". Nature Review. Molecular Cell Biology 9: 846-859. PMID 18946474. 
9. ^ Sandrine Etienne-Manneville (2004 March 15). "Cdc42 – the centre of polarity". Journal of Cell Science 117: 1291-1300. PMID 15020669. 
10. ^ Tapon N, Hall A (2002 Aug 3). "Rho, Rac and Cdc42 GTPases regulate the organization of the actin cytoskeleton.". Current Opinion in Cell Biology 9: 86-92. PMID 9013670. 
11. ^ Gallo G, Letourneau PC. (1998 Jan 29). "Axon guidance: GTPases help axons reach their targets.". Current Biology 8: 80-82. PMID 9443904. 
12. ^ Iden S, Collard JG. (2008 Nov). "Crosstalk between small GTPases and polarity proteins in cell polarization.". Nature Review. Molecular Cell Biology 9: 846-859. PMID 18946474. 
13. ^ Li S, Chen BP, Azuma N, Hu YL, Wu SZ, Sumpio BE, Shyy JY, Chien S. (1999 April 15). "Distinct roles for the small GTPases Cdc42 and Rho in endothelial responses to shear stress.". The Journal of Clinical Investigation 103: 1141-1150. PMID 10207166. 
14. ^ Tzima E. (2006 Feb 3). "Role of small GTPases in endothelial cytoskeletal dynamics and the shear stress response.". Circulation Research 98: 176-185. PMID 16456110. 
15. ^ Agarwal V, Hammerschmidt S (2009 May 27). "CDC42 and PI3-kinase-AKT pathway are essential for PspC mediated internalization of pneumococci by respiratory epithelial cells.". J Biol Chem Epub ahead of print. PMID 19473971. 
16. ^ Guiney DG, Lesnick M (2005 Mar). "Targeting of the actin cytoskeleton during infection by Salmonella strains.". Clin Immunol 114(3): 248-55. PMID 17981586. 
17. ^ Schlumberger MC, Hardt WD (2005). "Triggered phagocytosis by Salmonella: bacterial molecular mimicry of RhoGTPase activation/deactivation.". Curr Top Microbiol Immunol. 291: 29-42. PMID 15981458. 
18. ^ Tang Y, Olufemi L, Wang MT, Nie D. (2008 Jan 1). "Role of Rho GTPases in breast cancer.". Front Biosci. 13: 759-76. PMID 17981586. 
19. ^ Ramakers GJ (2002 Apr). "Rho proteins, mental retardation and the cellular basis of cognition.". Trends Neurosci. 25(4): 191-9. PMID 11998687.