GPCRs that Rhoar the Guanine nucleotide exchange factors

Figures & data

Figure 1. Schematic representation of some of the GEFs discussed in this review showing intra-domain interaction rendering them in autoinhibitory states. (a) GTPase switching mechanism between ON and OFF state. Canonical GEFs such as (b) p115-RhoGEF and (c) PDZ-RhoGEF are proposed to exhibit autoinhibition due to intra-domain interaction involving their RH domain. Similarly, autoinhibition of (d) AKAP-Lbc and (e) p114-RhoGEF is due to intra-domain interaction involving its non-RH, C terminal region. (f) In p63RhoGEF and (g) Vav GEF, the PH domain is suggested to be responsible for its autoinhibition. (h) In P-Rex GEF, the C terminal domains like IPI4, PDZ1, and PDZ2 are responsible for intra-domain interactions. (i) In TIAM1, the initial 50 amino acid domain (N50), N terminal PH domain (PHn) and the coiled coil extension (CC-Ex) domain of TIAM1 interact with each other to render the GEF to its autoinhibitory state. (j) In non-canonical GEFs like DOCK proteins interaction between N terminal SH3 domain and C terminal domains and/or DHR2 domain is responsible for their autoinhibition.

Table 1. Overview of canonical and non-canonical GEFs discussed in this review, their downstream Rho GTPases, tissue-specific expression and activating GPCRs

Guanine nucleotide exchange factors (GEFs)	GTPase specificity	Tissue Specificity		Activating GPCR	References
		High expression	Moderate/low expression
GPCR mediated heterotrimeric G protein subunit interaction with RH domains of GEFs
LARG	RhoA	Peripheral blood leukocytes, spleen, prostate, testis, ovary, small intestine and colon	Thymus	mAChR, Mas related receptor, G2A, S1P2, PAR and LPA	40,44,45,46,47
PDZ-RhoGEF	RhoA and Cdc42 (through non-RH domain)	Brain, testis, heart, ovary, central nervous system (CNS) and placenta	Kidney, pancreas, spleen, prostate, colon, skeletal muscle, lung and liver	GRPR, LPA1R, LPA2R and CXCR4	46,48,49,52,53,56,53,54
p115-RhoGEF	RhoA	Haematopoietic cells		LPAR, CXCR4, S1PR and β2AR	50,55,56,56
Activation of GEFs involving interaction between heterotrimeric G protein subunits and non-RH domains of GEFs
AKAP-Lbc	RhoA	Cardiac tissues		AT1R and LPAR	54,55,57
p63RhoGEF	RhoA, RhoB, and RhoC	Heart and brain		M3R and H1R	64,65,67
p114-RhoGEF	RhoA and Rac1	Kidney and pancreas	Liver, skeletal muscle and testis	M3R and LPAR	59–61,58
P-Rex1	Rac1/2/3, Cdc42 and RhoG	Neutrophils, macrophages, platelets, endothelial cells, neurons and brain tissues	Bone marrow, thymus, spleen, lymph nodes and lungs	LPA1R, CXCR4 and fMLPR	74,87,59,60,61
Activation of GEFs through phosphorylation, protein-lipid and protein-protein interactions mediated by GPCRs
αPIX	Rac1 and Cdc42	Heart, skeletal muscle and immune cells		fMLPR	13,82,62,63
βPIX	Cdc42 and Rac1	Ubiquitous		LPAR	13,76,82,64
Vav1	Rac1/2, RhoG, and Cdc42	Haematopoietic cells	Pancreas, the tooth enamel, and the trophoblast layer	CXCR4 and fMLPR	42,87,89,65,66
Vav2	RhoA, RhoG, Rac1, and Cdc42	Broadly expressed		CXCR4	42,88,89
Vav3	RhoA, RhoG, and Rac1	Broadly expressed, highest in brain and haematopoietic cells		fMLPR	87,89,67
TIAM1	Rac1	Brain, testis and epidermis		LPA1R and S1P1R	12,99
TIAM2	Rac1	Only restricted to neuronal cells		?	99,100,68
Activation of non-canonical GEFs through GPCR by phosphorylation, protein-lipid and protein-protein interactions
DOCK180/DOCK1	Rac1	Broadly expressed		CXCR4 and LPA	76,69,106,134
DOCK2	Rac1/2	Haematopoietic cells		fMLPR	110,70,71

Figure 2. Schematic representation of signalling pathways which leads to activation of GEFs through interaction between heterotrimeric G protein subunits and RH domains of GEFs. (a) GPCRs, Mas receptor and M1-mAChR activate the GEF, LARG, through Gα_q binding to its RH domain. (b) Activation of LARG through GPCRs, LPAR & S1P₂R occur through direct interaction with Gα_12/13 whereas, GPCRs, G2A & PAR activate LARG through Gα₁₃ subunit. (c) PDZ-RhoGEF mediated RhoA activation is established by GRPR, LPAR, and CXCR4 triggered Gα₁₃ heterotrimeric subunit binding to the RH domain of PDZ-RhoGEF. (d) GPCRs, LPAR, CXCR4, and S1P₂R activate p115-RhoGEF through interaction with Gα₁₃ heterotrimeric subunit. (e) Upon activation of β₂AR, p115-RhoGEF is triggered by β-arrestin activation through Gα_iβγ signalling.

Figure 3. Schematic representation of signalling pathways which leads to activation of GEFs through interaction between heterotrimeric G protein subunits and non-RH domains of GEFs. (a) Stimulation of prostaglandin receptor activates Gα_s, which in turn binds to PDZ-RhoGEF through DH, PH domains, and their linker region of GEF, thus making it active towards Cdc42. (b) AKAP-Lbc dependent activation of RhoA, by LPAR and AT₁R, happens through the interaction of Gα₁₂ with the C terminus of AKAP-Lbc. (c) and (d) Activation of Gβγ (through M₃R and LPAR) and Gα₁₂ triggers p114-RhoGEF, where Gβγ interacts with the C terminus of DH-PH domain of p114-RhoGEF. (e) p63RhoGEF mediated triggering of RhoA upon activation of M₃R and H₁R receptors. This involves the interaction of Gα_q/11 towards the PH domain of p63RhoGEF. (f) Stimulated GPCRs, LPAR, and CXCR4 activate P-Rex1 through the interaction of the Gβγ subunit with the IP4P domain and the tandem PDZ domains of P-Rex1. This may further lead to the activation of ELMO/DOCK2 through P-Rex activated RhoG. (g) Activation of ET₁R triggers βPIX mediated Cdc42 through the interaction of Gα_q with βPIX.

Figure 4. Activation of GEFs through Phosphorylation, protein-protein, and lipid-mediated interactions involving downstream of GPCR signalling. (a) Upon LPAR stimulation, activation of βPIX takes place through, Gα_i mediated Src kinase activation, which further phosphorylates βPIX. (b) Activation of fMLPR triggers Cdc42 through Gβγ-PAK1-αPIX and Rac1 through Gβγ-PAK1-αPIX-GIT2 sequential complex formation, where activation of αPIX takes place through PAK1 binding. Cdc42 and Rac1 are involved in feedforward mechanism where αPIX and Rac1 enhance PAK1 activity. Here activation of Cdc42 enhances GEF activity of αPIX towards Rac1. (c) CXCR4 activates Vav1 and Vav2 through JAK signalling. (d) Activation of Rac through fMLPR stimulation involves concerted signalling between Vav1/3 and P-Rex GEFs. (e) LPAR triggers Rac1 through PKC and CamKII mediated phosphorylation of TIAM1. (f) Stimulation of GPCRs, LPAR and S1P₁R, activate TIAM through Gα_iβγ/PI3K/TIAM1/Rac1 pathway, where TIAM1 interacts with a PI3K lipid product, PIP3. (g) Protein–protein interaction mediated triggering of RhoA when activated Frizzled7 recruits Dishevelled (Dvl) and Daam-1 proteins which activate the Rho WGEF.

Figure 5. Schematic representation of activation of GEFs upon direct interaction with GPCR domains. (a) GEFs, LARG, and PDZ-RhoGEF activate RhoA signalling upon interacting with the C terminus of LPAR through their PDZ domains. (b) Similarly, Vav2 interacts with the C terminus of PTHR through its DH and PH domains.

Figure 6. Schematic representation of activation of non-canonical GEFs by GPCRs. (a) CXCR4 activates ELMO1/DOCK180 through the interaction of ELMO1 with Gβγ and Gα_i subunits. (b) Stimulation of LPAR leads to Rac activation through Gα_i2-Src-p130Cas/ELMO/DOCK180/Rac1 pathway during which Src is proposed to phosphorylate DOCK180 at Tyr1811. (c) ELMO/DOCK2 mediated Rac activation takes place through binding of DHR-1 domain of DOCK2 to lipid, PIP3 upon fMLPR activation. (d) ELMO1 and ELMO2 are shown to interact with the C terminal of the BAI1 receptor through their N-terminal fragments to trigger the DOCK/Rac pathway.

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