Conformational switch that induces GDP release from Gi

Balancing G protein selectivity and efficacy in the adenosine A2A receptor

The adenosine A2A receptor (A2AR) engages several G proteins, notably Go and its cognate Gs protein. This coupling promiscuity is facilitated by a dynamic ensemble, revealed by 19F nuclear magnetic resonance imaging of A2AR and G protein. Two transmembrane helix 6 (TM6) activation states, formerly associated with partial and full agonism, accommodate the differing volumes of Gs and Go. While nucleotide depletion biases TM7 toward a fully active state in A2AR-Gs, A2AR-Go is characterized by a dynamic inactive/intermediate fraction. Molecular dynamics simulations reveal that the NPxxY motif, a highly conserved switch, establishes a unique configuration in the A2AR-Go complex, failing to stabilize the helix-8 interface with Gs, and adoption of the active state. The resulting TM7 dynamics hamper G protein coupling, suggesting kinetic gating may be responsible for reduced efficacy in the noncognate G protein complex. Thus, dual TM6 activation states enable greater diversity of coupling partners while TM7 dynamics dictate coupling efficacy.

Enhancing Rab7 Activity by Inhibiting TBC1D5 Expression Improves Mitophagy in Alzheimer's Disease Models

Background

Mitochondrial dysfunction exists in Alzheimer's disease (AD) brain, and damaged mitochondria need to be removed by mitophagy. Small GTPase Rab7 regulates the fusion of mitochondria and lysosome, while TBC1D5 inhibits Rab7 activation. However, it is not clear whether the regulation of Rab7 activity by TBC1D5 can improve mitophagy and inhibit AD progression.

Objective

To investigate the role of TBC1D5 in mitophagy and its regulatory mechanism for Rab7, and whether activation of mitophagy can inhibit the progression of AD.

Methods

Mitophagy was determined by western blot and immunofluorescence. The morphology and quantity of mitochondria were tracked by TEM. pCMV-Mito-AT1.03 was employed to detect the cellular ATP. Amyloid-β secreted by AD cells was detected by ELISA. Co-immunoprecipitation was used to investigate the binding partner of the target protein. Golgi-cox staining was applied to observe neuronal morphology of mice. The Morris water maze test and Y-maze were performed to assess spatial learning and memory, and the open field test was measured to evaluate motor function and anxiety-like phenotype of experimental animals.

Results

Mitochondrial morphology was impaired in AD models, and TBC1D5 was highly expressed. Knocking down TBC1D5 increased the expression of active Rab7, promoted the fusion of lysosome and autophagosome, thus improving mitophagy, and improved the morphology of hippocampal neurons and the impaired behavior in AD mice.

Conclusions

Knocking down TBC1D5 increased Rab7 activity and promoted the fusion of autophagosome and lysosome. Our study provided insights into the mechanisms that bring new possibilities for AD therapy targeting mitophagy.

Isolation and conformational analysis of the Gα α-helical domain

Heterotrimeric G proteins (G proteins), composed of Gα, Gβ, and Gγ subunits, are the major downstream signaling molecules of the G protein-coupled receptors. Upon activation, Gα undergoes conformational changes both in the Ras-like domain (RD) and the α-helical domain (AHD), leading to the dissociation of Gα from Gβγ and subsequent regulation of downstream effector proteins. Gα RD mediate the most of classical functions of Gα. However, the role of Gα AHD is relatively not well elucidated despite its much higher sequence differences between Gα subtypes than those between Gα RD. Here, we isolated AHD from Gαs, Gαi1, and Gαq to provide tools for examining Gα AHD. We investigated the conformational dynamics of the isolated Gα AHD compared to those of the GDP-bound Gα. The results showed higher local conformational dynamics of Gα AHD not only at the domain interfaces but also in regions further away from the domain interfaces. This finding is consistent with the conformation of Gα AHD in the receptor-bound nucleotide-free state. Therefore, the isolated Gα AHD could provide a platform for studying the functions of Gα AHD, such as identification of the Gα AHD-binding proteins.

RGS4 impacts carbohydrate and siderophore metabolism in Trichoderma reesei

BACKGROUND

Adaptation to complex, rapidly changing environments is crucial for evolutionary success of fungi. The heterotrimeric G-protein pathway belongs to the most important signaling cascades applied for this task. In Trichoderma reesei, enzyme production, growth and secondary metabolism are among the physiological traits influenced by the G-protein pathway in a light dependent manner.

RESULTS

Here, we investigated the function of the SNX/H-type regulator of G-protein signaling (RGS) protein RGS4 of T. reesei. We show that RGS4 is involved in regulation of cellulase production, growth, asexual development and oxidative stress response in darkness as well as in osmotic stress response in the presence of sodium chloride, particularly in light. Transcriptome analysis revealed regulation of several ribosomal genes, six genes mutated in RutC30 as well as several genes encoding transcription factors and transporters. Importantly, RGS4 positively regulates the siderophore cluster responsible for fusarinine C biosynthesis in light. The respective deletion mutant shows altered growth on nutrient sources related to siderophore production such as ornithine or proline in a BIOLOG phenotype microarray assay. Additionally, growth on storage carbohydrates as well as several intermediates of the D-galactose and D-arabinose catabolic pathway is decreased, predominantly in light.

CONCLUSIONS

We conclude that RGS4 mainly operates in light and targets plant cell wall degradation, siderophore production and storage compound metabolism in T. reesei.

Gαs slow conformational transition upon GTP binding and a novel Gαs regulator

G proteins are major signaling partners for G protein-coupled receptors (GPCRs). Although stepwise structural changes during GPCR-G protein complex formation and guanosine diphosphate (GDP) release have been reported, no information is available with regard to guanosine triphosphate (GTP) binding. Here, we used a novel Bayesian integrative modeling framework that combines data from hydrogen-deuterium exchange mass spectrometry, tryptophan-induced fluorescence quenching, and metadynamics simulations to derive a kinetic model and atomic-level characterization of stepwise conformational changes incurred by the β2-adrenergic receptor (β2AR)-Gs complex after GDP release and GTP binding. Our data suggest rapid GTP binding and GTP-induced dissociation of Gαs from β2AR and Gβγ, as opposed to a slow closing of the Gαs α-helical domain (AHD). Yeast-two-hybrid screening using Gαs AHD as bait identified melanoma-associated antigen D2 (MAGE D2) as a novel AHD-binding protein, which was also shown to accelerate the GTP-induced closing of the Gαs AHD.

RAB7A GTPase Is Involved in Mitophagosome Formation and Autophagosome-Lysosome Fusion in N2a Cells Treated with the Prion Protein Fragment 106-126

Failed communication between mitochondria and lysosomes causes dysfunctional mitochondria, which may induce mitochondria-related neurodegenerative diseases. Here, we show that RAB7A, a small GTPase of the Rab family, mediates the crosstalk between these two important organelles to maintain homeostasis in N2a cells treated with PrP^106-126. Specifically, we demonstrate that mitophagy deficiency in N2a cells caused by PrP^106-126 is associated with dysregulated RAB7A localization in mitochondria. Cells lacking RAB7A display decreased mitochondrial colocalization with lysosomes and significantly increased mitochondrial protein expression, resulting in inhibited mitophagy. In contrast, overexpression of GTP-bound RAB7A directly induces lysosome colocalization with mitochondria. Further study revealed that GTP-bound RAB7A protects mitochondrial homeostasis by supporting autophagosome biogenesis. Moreover, we suggest that depletion of RAB7A leads to gross morphological changes in lysosomes, which prevents autophagosome-lysosome fusion and interferes with the breakdown of autophagic cargo within lysosomes. Overexpression of GTP-bound RAB7A can also alleviate PrP^106-126-induced morphological damage and dysfunction of mitochondria, reducing neuronal apoptosis. Collectively, our data demonstrate that RAB7A successfully drives mitochondria to the autophagosomal lumen for degradation, suggesting that the communication of proteotoxic stress from mitochondria to lysosomes requires RAB7A, as a signaling molecule, to establish a link between the disturbed mitochondrial network and its remodeling. These findings indicate that small molecules regulating mitophagy have the potential to modulate cellular homeostasis and the clinical course of neurodegenerative diseases. Proposed model of mitophagy regulated by RAB7A. (1) Accumulating PrP^106-126 induced mitophagy. (2) RAB7A is recruited to mitochondria. (3) ATG5-12 and ATG9A (5) vesicles are recruited to the autophagosome formation sites in a RAB7A-dependent manner. The ATG5-12 complex recruits and anchors LC3-I to form active LC3-II (4), accelerating mitophagosomal formation. The ATG9A vesicles are thought to be a source of membranes for autophagosome assembly. The recruitment of proteins and lipids induces membrane expansion and subsequent closure to form the mitophagosome. (6) Maintenance of the normal low lysosomal PH depends on active (GTP-bound) RAB7A. (7) RAB7A recruits effector molecules responsible for tight membrane interactions, and directly or indirectly, the subsequent autophagosome merges with the lysosome, and the cargo is completely degraded.

Structural and Functional Implication of Natural Variants of Gαs

Heterotrimeric guanine nucleotide-binding proteins (G proteins) are among the most important cellular signaling components, especially G protein-coupled receptors (GPCRs). G proteins comprise three subunits, Gα, Gβ, and Gγ. Gα is the key subunit, and its structural state regulates the active status of G proteins. Interaction of guanosine diphosphate (GDP) or guanosine triphosphate (GTP) with Gα switches G protein into basal or active states, respectively. Genetic alteration in Gα could be responsible for the development of various diseases due to its critical role in cell signaling. Specifically, loss-of-function mutations of Gαs are associated with parathyroid hormone-resistant syndrome such as inactivating parathyroid hormone/parathyroid hormone-related peptide (PTH/PTHrP) signaling disorders (iPPSDs), whereas gain-of-function mutations of Gαs are associated with McCune-Albright syndrome and tumor development. In the present study, we analyzed the structural and functional implications of natural variants of the Gαs subtype observed in iPPSDs. Although a few tested natural variants did not alter the structure and function of Gαs, others induced drastic conformational changes in Gαs, resulting in improper folding and aggregation of the proteins. Other natural variants induced only mild conformational changes but altered the GDP/GTP exchange kinetics. Therefore, the results shed light on the relationship between natural variants of Gα and iPPSDs.

Effect of α-helical domain of Gi/o α subunit on GDP/GTP turnover

Heterotrimeric guanine nucleotide-binding proteins (G proteins) are composed of α, β, and γ subunits, and Gα has a GDP/GTP-binding pocket. When a guanine nucleotide exchange factor (GEF) interacts with Gα, GDP is released, and GTP interacts to Gα. The GTP-bound activated Gα dissociates from GEF and Gβγ, mediating the induction of various intracellular signaling pathways. Depending on the sequence similarity and cellular function, Gα subunits are subcategorized into four subfamilies: Gαi/o, Gαs, Gαq/11, and Gα12/13. Although the Gαi/o subtype family proteins, Gαi3 and GαoA, share similar sequences and functions, they differ in their GDP/GTP turnover profiles, with GαoA possessing faster rates than Gαi3. The structural factors responsible for these differences remain unknown. In this study, we employed hydrogen/deuterium exchange mass spectrometry and mutational studies to investigate the factors responsible for these functional differences. The Gα subunit consists of a Ras-like domain (RD) and an α-helical domain (AHD). The RD has GTPase activity and receptor-binding and effector-binding regions; however, the function of the AHD has not yet been extensively studied. In this study, the chimeric construct containing the RD of Gαi3 and the AHD of GαoA showed a GDP/GTP turnover profile similar to that of GαoA, suggesting that the AHD is the major regulator of the GDP/GTP turnover profile. Additionally, site-directed mutagenesis revealed the importance of the N-terminal part of αA and αA/αB loops in the AHD for the GDP/GTP exchange. These results suggest that the AHD regulates the nucleotide exchange rate within the Gα subfamily.

The Conformational Dynamics of Heterotrimeric G Proteins During GPCR-Mediated Activation

Heterotrimeric G proteins (G proteins) are essential cellular signaling proteins that mediate extracellular signals to achieve various cellular functions. G-protein-coupled receptors (GPCRs) are the major guanine nucleotide exchange factors (GEFs) that induce G proteins to release guanosine diphosphate and rapidly bind to guanosine triphosphate, resulting in G protein activation. G proteins undergo dynamic conformational changes during the activation/inactivation process, and the precise structural mechanism of GPCR-mediated G protein activation is of great interest. Over the last decade, a number of GPCR-G protein complex structures have been identified, yet an understanding of the mechanisms underlying allosteric conformational changes during receptor-mediated G protein activation and GPCR-G protein coupling selectivity is only now emerging. This review discusses recent studies on the dynamic conformational changes of G proteins and provides insight into the structural mechanism of GPCR-mediated G protein activation.