Christopher A. Johnston

RIC-8 Is Required for GPR-1/2-Dependent Gα Function during Asymmetric Division of C. elegans Embryos

Heterotrimeric G proteins are crucial for asymmetric cell division, but the mechanisms of signal activation remain poorly understood. Here, we establish that the evolutionarily conserved protein RIC-8 is required for proper asymmetric division of one-cell stage C. elegans embryos. Spindle severing experiments demonstrate that RIC-8 is required for generation of substantial pulling forces on astral microtubules. RIC-8 physically interacts with GOA-1 and GPA-16, two Galpha subunits that act in a partially redundant manner in one-cell stage embryos. RIC-8 preferentially binds to GDP bound GOA-1 and is a guanine nucleotide exchange factor (GEF) for GOA-1. Our analysis suggests that RIC-8 acts before the GoLoco protein GPR-1/2 in the sequence of events leading to Galpha activation. Furthermore, coimmunoprecipitation and in vivo epistasis demonstrate that inactivation of the Gbeta subunit GPB-1 alleviates the need for RIC-8 in one-cell stage embryos. Our findings suggest a mechanism in which RIC-8 favors generation of Galpha free from Gbetagamma and enables GPR-1/2 to mediate asymmetric cell division.

GTPase acceleration as the rate-limiting step in Arabidopsis G protein-coupled sugar signaling

Heterotrimeric G protein signaling is important for cell-proliferative and glucose-sensing signal transduction pathways in the model plant organism Arabidopsis thaliana. AtRGS1 is a seven-transmembrane, RGS domain-containing protein that is a putative membrane receptor for d-glucose. Here we show, by using FRET, that d-glucose alters the interaction between the AtGPA1 and AtRGS1 in vivo. AtGPA1 is a unique heterotrimeric G protein α subunit that is constitutively GTP-bound given its high spontaneous nucleotide exchange coupled with slow GTP hydrolysis. Analysis of a point mutation in AtRGS1 that abrogates GTPase-accelerating activity demonstrates that the regulation of AtGPA1 GTP hydrolysis mediates sugar signal transduction during Arabidopsis development, in contrast to animals where nucleotide exchange is the limiting step in the heterotrimeric G protein nucleotide cycle.

Receptor-Mediated Activation of Heterotrimeric G-Proteins: Current Structural Insights

G-protein-coupled receptors (GPCRs) serve as catalytic activators of heterotrimeric G-proteins (Gαβγ) by exchanging GTP for the bound GDP on the Gα subunit. This guanine nucleotide exchange factor activity of GPCRs is the initial step in the G-protein cycle and determines the onset of various intracellular signaling pathways that govern critical physiological responses to extracellular cues. Although the structural basis for many steps in the G-protein nucleotide cycle have been made clear over the past decade, the precise mechanism for receptor-mediated G-protein activation remains incompletely defined. Given that these receptors have historically represented a set of rich drug targets, a more complete understanding of their mechanism of action should provide further avenues for drug discovery. Several models have been proposed to explain the communication between activated GPCRs and Gαβγ leading to the structural changes required for guanine nucleotide exchange. This review is focused on the structural biology of G-protein signal transduction with an emphasis on the current hypotheses regarding Gαβγ activation. We highlight several recent results shedding new light on the structural changes in Gα that may underlie GDP release.

Clathrin Adaptor AP2 Regulates Thrombin Receptor Constitutive Internalization and Endothelial Cell Resensitization

ABSTRACT Protease-activated receptor 1 (PAR1), a G protein-coupled receptor for the coagulant protease thrombin, is irreversibly activated by proteolysis. Unactivated PAR1 cycles constitutively between the plasma membrane and intracellular stores, thereby providing a protected receptor pool that replenishes the cell surface after thrombin exposure and leads to rapid resensitization to thrombin signaling independent of de novo receptor synthesis. Here, we show that AP2, a clathrin adaptor, binds directly to a tyrosine-based motif in the cytoplasmic tail of PAR1 and is essential for constitutive receptor internalization and cellular recovery of thrombin signaling. Expression of a PAR1 tyrosine mutant or depletion of AP2 by RNA interference leads to significant inhibition of PAR1 constitutive internalization, loss of intracellular uncleaved PAR1, and failure of endothelial cells and other cell types to regain thrombin responsiveness. Our findings establish a novel role for AP2 in direct regulation of PAR1 trafficking, a process critically important to the temporal and spatial aspects of thrombin signaling.

Regulators of G-protein signaling accelerate GPCR signaling kinetics and govern sensitivity solely by accelerating GTPase activity

G-protein heterotrimers, composed of a guanine nucleotide-binding Gα subunit and an obligate Gβγ dimer, regulate signal transduction pathways by cycling between GDP- and GTP-bound states. Signal deactivation is achieved by Gα-mediated GTP hydrolysis (GTPase activity) which is enhanced by the GTPase-accelerating protein (GAP) activity of “regulator of G-protein signaling” (RGS) proteins. In a cellular context, RGS proteins have also been shown to speed up the onset of signaling, and to accelerate deactivation without changing amplitude or sensitivity of the signal. This latter paradoxical activity has been variably attributed to GAP/enzymatic or non-GAP/scaffolding functions of these proteins. Here, we validated and exploited a Gα switch-region point mutation, known to engender increased GTPase activity, to mimic in cis the GAP function of RGS proteins. While the transition-state, GDP·AlF4 −-bound conformation of the G202A mutant was found to be nearly identical to wild-type, Gαi1(G202A)·GDP assumed a divergent conformation more closely resembling the GDP·AlF4 −-bound state. When placed within Saccharomyces cerevisiae Gα subunit Gpa1, the fast-hydrolysis mutation restored appropriate dose–response behaviors to pheromone signaling in the absence of RGS-mediated GAP activity. A bioluminescence resonance energy transfer (BRET) readout of heterotrimer activation with high temporal resolution revealed that fast intrinsic GTPase activity could recapitulate in cis the kinetic sharpening (increased onset and deactivation rates) and blunting of sensitivity also engendered by RGS protein action in trans. Thus Gα-directed GAP activity, the first biochemical function ascribed to RGS proteins, is sufficient to explain the activation kinetics and agonist sensitivity observed from G-protein–coupled receptor (GPCR) signaling in a cellular context.

Canoe binds RanGTP to promote PinsTPR/Mud-mediated spindle orientation

The scaffolding protein Canoe regulates spindle orientation by binding to RanGTP and recruiting RanGTP and Mud to the cell cortex.

Structural basis for nucleotide exchange on Gαi subunits and receptor coupling specificity

Heterotrimeric G proteins are molecular switches that relay information intracellularly in response to various extracellular signals. How ligand-activated G protein-coupled receptors act at a distance to exert exchange activity on the Gα nucleotide binding pocket is poorly understood. Here we describe the synergistic action of two peptides: one from the third intracellular loop of the D2 dopamine receptor (D2N), and a second, Gα·GDP-binding peptide (KB-752) that mimics the proposed role of Gβγ in receptor-promoted nucleotide exchange. The structure of both peptides in complex with Gαi1 suggests that conformational changes in the β3/α2 loop and β6 strand act in concert for efficient nucleotide exchange. Two key residues in the α4 helix were found to define a receptor/Gαi coupling specificity determinant.

A Point Mutation to Gαi Selectively Blocks GoLoco Motif Binding DIRECT EVIDENCE FOR Gα·GoLoco COMPLEXES IN MITOTIC SPINDLE DYNAMICS

Heterotrimeric G-protein Gα subunits and GoLoco motif proteins are key members of a conserved set of regulatory proteins that influence invertebrate asymmetric cell division and vertebrate neuroepithelium and epithelial progenitor differentiation. GoLoco motif proteins bind selectively to the inhibitory subclass (Gαi) of Gα subunits, and thus it is assumed that a Gαi·GoLoco motif protein complex plays a direct functional role in microtubule dynamics underlying spindle orientation and metaphase chromosomal segregation during cell division. To address this hypothesis directly, we rationally identified a point mutation to Gαi subunits that renders a selective loss-of-function for GoLoco motif binding, namely an asparagine-to-isoleucine substitution in the αD-αE loop of the Gα helical domain. This GoLoco-insensitivity (“GLi”) mutation prevented Gαi1 association with all human GoLoco motif proteins and abrogated interaction between the Caenorhabditis elegans Gα subunit GOA-1 and the GPR-1 GoLoco motif. In contrast, the GLi mutation did not perturb any other biochemical or signaling properties of Gαi subunits, including nucleotide binding, intrinsic and RGS protein-accelerated GTP hydrolysis, and interactions with Gβγ dimers, adenylyl cyclase, and seven transmembrane-domain receptors. GoLoco insensitivity rendered Gαi subunits unable to recruit GoLoco motif proteins such as GPSM2/LGN and GPSM3 to the plasma membrane, and abrogated the exaggerated mitotic spindle rocking normally seen upon ectopic expression of wild type Gαi subunits in kidney epithelial cells. This GLi mutation should prove valuable in establishing the physiological roles of Gαi·GoLoco motif protein complexes in microtubule dynamics and spindle function during cell division as well as to delineate potential roles for GoLoco motifs in receptor-mediated signal transduction.

Multiple tail domain interactions stabilize nonmuscle myosin II bipolar filaments

Contractile force transduction by myosin II derives from its assembly into bipolar filaments. The coiled-coil tail domain of the myosin II heavy chain mediates filament assembly, although the mechanism is poorly understood. Tail domains contain an alternating electrostatic repeat, yet only a small region of the tail (termed the “assembly domain”) is typically required for assembly. Using computational analysis, mutagenesis, and electron microscopy we discovered that the assembly domain does not function through self-interaction as previously thought. Rather, the assembly domain acts as a unique, positively charged interaction surface that can stably contact multiple complementary, negatively charged surfaces in the upstream tail domain. The relative affinities of the assembly domain to each complementary interaction surface sets the characteristic molecular staggers observed in myosin II filaments. Together these results explain the relationship between the charge repeat and assembly domain in stabilizing myosin bipolar filaments.

Formin-mediated actin polymerization cooperates with Mushroom body defect (Mud)–Dynein during Frizzled–Dishevelled spindle orientation

Summary To position the mitotic spindle, cytoskeletal components must be coordinated to generate cortical forces on astral microtubules. Although the dynein motor is common to many spindle orientation systems, ‘accessory pathways’ are often also required. In this work, we identified an accessory spindle orientation pathway in Drosophila that functions with Dynein during planar cell polarity, downstream of the Frizzled (Fz) effector Dishevelled (Dsh). Dsh contains a PDZ ligand and a Dynein-recruiting DEP domain that are both required for spindle orientation. The Dsh PDZ ligand recruits Canoe/Afadin and ultimately leads to Rho GTPase signaling mediated through RhoGEF2. The formin Diaphanous (Dia) functions as the Rho effector in this pathway, inducing F-actin enrichment at sites of cortical Dsh. Chimeric protein experiments show that the Dia–actin accessory pathway can be replaced by an independent kinesin (Khc73) accessory pathway for Dsh-mediated spindle orientation. Our results define two ‘modular’ spindle orientation pathways and show an essential role for actin regulation in Dsh-mediated spindle orientation.