Mutations on the Switch III region and the alpha3 helix of Galpha16 differentially affect receptor coupling and regulation of downstream effectors

Structural mechanism of G protein activation by G protein-coupled receptor

G protein-coupled receptors (GPCRs) are a family of membrane receptors that regulate physiology and pathology of various organs. Consequently, about 40% of drugs in the market targets GPCRs. Heterotrimeric G proteins are composed of α, β, and γ subunits, and act as the key downstream signaling molecules of GPCRs. The structural mechanism of G protein activation by GPCRs has been of a great interest, and a number of biochemical and biophysical studies have been performed since the late 80's. These studies investigated the interface between GPCR and G proteins and the structural mechanism of GPCR-induced G protein activation. Recently, arrestins are also reported to be important molecular switches in GPCR-mediated signal transduction, and the physiological output of arrestin-mediated signal transduction is different from that of G protein-mediated signal transduction. Understanding the structural mechanism of the activation of G proteins and arrestins would provide fundamental information for the downstream signaling-selective GPCR-targeting drug development. This review will discuss the structural mechanism of GPCR-induced G protein activation by comparing previous biochemical and biophysical studies.

Structural Aspects of GPCR-G Protein Coupling

G protein-coupled receptors (GPCRs) are membrane receptors; approximately 40% of drugs on the market target GPCRs. A precise understanding of the activation mechanism of GPCRs would facilitate the development of more effective and less toxic drugs. Heterotrimeric G proteins are important molecular switches in GPCR-mediated signal transduction. An agonist-activated receptor interacts with specific sites on G proteins and promotes the release of GDP from the Gα subunit. Because of the important biological role of the GPCR-G protein coupling, conformational changes in the G protein upon receptor coupling have been of great interest. One of the most important questions was the interface between the GPCR and G proteins and the structural mechanism of GPCR-induced G protein activation. A number of biochemical and biophysical studies have been performed since the late 80s to address these questions; there was a significant breakthrough in 2011 when the crystal structure of a GPCR-G protein complex was solved. This review discusses the structural aspects of GPCR-G protein coupling by comparing the results of previous biochemical and biophysical studies to the GPCR-G protein crystal structure.

Activation of Ras-dependent signaling pathways by G(14) -coupled receptors requires the adaptor protein TPR1

Many G(q) -coupled receptors mediate mitogenic signals by stimulating extracellular signal-regulated protein kinases (ERKs) that are typically regulated by the small GTPase Ras. Recent studies have revealed that members of the Gα(q) family may possess the ability to activate Ras/ERK by interacting with the adaptor protein tetratricopeptide repeat 1 (TPR1). Within the Gα(q) family, the highly promiscuous Gα(14) can relay signals from numerous receptors. Here, we examined if Gα(14) interacts with TPR1 to stimulate Ras signaling pathways. Expression of the constitutively active Gα(14) QL mutant in HEK293 cells led to the formation of GTP-bound Ras as well as increased phosphorylations of downstream signaling molecules including ERK and IκB kinase. Stimulation of endogenous G(14) -coupled somatostatin type 2 and α(2) -adrenergic receptors produced similar responses in human hepatocellular HepG2 carcinoma cells. Co-immunoprecipitation assays using HEK293 cells demonstrated a stronger association of TPR1 for Gα(14) QL than Gα(14) , suggesting that TPR1 preferentially binds to the GTP-bound form of Gα(14) . Activated Gα(14) also interacted with the Ras guanine nucleotide exchange factors SOS1 and SOS2. Expression of a dominant negative mutant of TPR1 or siRNA-mediated knockdown of TPR1 effectively abolished the ability of Gα(14) to induce Ras signaling in native HepG2 or transfected HEK293 cells. Although expression of the dominant negative mutant of TPR1 suppressed Gα(14) QL-induced phosphorylations of ERK and IκB kinase, it did not affect Gα(14) QL-induced stimulation of phospholipase Cβ or c-Jun N-terminal kinase. Our results suggest that TPR1 is required for Gα(14) to stimulate Ras-dependent signaling pathways, but not for the propagation of signals along Ras-independent pathways.

Gα16 interacts with tetratricopeptide repeat 1 (TPR1) through its β3 region to activate Ras independently of phospholipase Cβ signaling

Background

G protein-coupled receptors constitute the largest family of cell surface receptors in the mammalian genome. As the core of the G protein signal transduction machinery, the Gα subunits are required to interact with multiple partners. The GTP-bound active state of many Gα subunits can bind a multitude of effectors and regulatory proteins. Yet it remains unclear if the different proteins utilize distinct or common structural motifs on the Gα subunit for binding. Using Gα₁₆ as a model, we asked if its recently discovered adaptor protein tetratricopeptide repeat 1 (TPR1) binds to the same region as its canonical effector, phospholipase Cβ (PLCβ).

Results

We have examined the specificity of Gα₁₆/TPR1 association by testing a series of chimeras between Gα₁₆ and Gα_z. TPR1 co-immunoprecipitated with Gα₁₆ and more tightly with its constitutively active Gα₁₆QL, but not Gα_z. Progressive replacement of Gα₁₆ sequence with the corresponding residues of Gα_z eventually identified a stretch of six amino acids in the β3 region of Gα₁₆ which are responsible for TPR1 interaction and the subsequent Ras activation. Insertion of these six residues into Gα_z allowed productive TPR1-interaction. Since the β3 region only minimally contributes to interact with PLCβ, several chimeras exhibited differential abilities to stimulate PLCβ and Ras. The ability of the chimeras to activate downstream transcription factors such as signal transducer and activator of transcription 3 and nuclear factor κB appeared to be associated with PLCβ signaling.

Conclusions

Our results suggest that Gα₁₆ can signal through TPR1/Ras and PLCβ simultaneously and independently. The β3 region of Gα₁₆ is essential for interaction with TPR1 and the subsequent activation of Ras, but has relatively minor influence on the PLCβ interaction. Gα₁₆ may utilize different structural domains to bind TPR1 and PLCβ.

The RhoA-specific guanine nucleotide exchange factor p63RhoGEF binds to activated Galpha(16) and inhibits the canonical phospholipase Cbeta pathway

Heterotrimeric G proteins regulate diverse physiological processes by modulating the activities of intracellular effectors. Members of the Galpha(q) family link G protein-coupled receptor activation to phospholipase Cbeta (PLCbeta) activity and intracellular calcium signaling cascades. However, they differ markedly in biochemical properties as well as tissue distribution. Recent findings have shown that some of the cellular activities of Galpha(q) family members are independent of PLCbeta activation. A guanine nucleotide exchange factor, p63RhoGEF, has been shown to interact with Galpha(q) proteins and thus provides linkage to RhoA activation. However, it is not known if p63RhoGEF can associate with other Galpha(q) family members such as Galpha(16). In the present study, we employed co-immunoprecipitation studies in HEK293 cells to demonstrate that p63RhoGEF can form a stable complex with the constitutively active mutant of Galpha(16) (Galpha(16)QL). Interestingly, overexpression of p63RhoGEF inhibited Galpha(16)QL-induced IP(3) production in a concentration-dependent manner. The binding of PLCbeta(2) to Galpha(16)QL could be displaced by p63RhoGEF. Similarly, p63RhoGEF inhibited the binding of tetratricopeptide repeat 1 to Galpha(16)QL, leading to a suppression of Galpha(16)QL-induced Ras activation. In the presence of p63RhoGEF, Galpha(16)QL-induced STAT3 phosphorylation was significantly reduced and Galpha(16)QL-mediated SRE transcriptional activation was attenuated. Taken together, these results suggest that p63RhoGEF binds to activated Galpha(16) and inhibits its signaling pathways.