Crystal Structure of the DH/PH Fragment of Dbs without Bound GTPase

Dbl proteins are guanine nucleotide exchange factors for Rho GTPases, containing adjacent Dbl homology (DH) and pleckstrin homology (PH) domains. This domain architecture is virtually invariant and typically required for full exchange potential. Several structures of DH/PH fragments bound to GTPases implicate the PH domain in nucleotide exchange. To more fully understand the functional linkage between DH and PH domains, we have determined the crystal structure of the DH/PH fragment of Dbs without bound GTPase. This structure is generally similar to previously determined structures of Dbs bound to GTPases albeit with greater apparent mobility between the DH and PH domains. These comparisons suggest that the DH and PH domains of Dbs are spatially primed for binding GTPases and small alterations in intradomain conformations that may be elicited by subtle biological responses, such as altered phosphoinositide levels, are sufficient to enhance exchange by facilitating interactions between the PH domain and GTPases

A negative-feedback loop regulating ERK1/2 activation and mediated by RasGPR2 phosphorylation

The dynamic regulation of ERK1 and −2 (ERK1/2) is required for precise signal transduction controlling cell proliferation, differentiation, and survival. However, the underlying mechanisms regulating the activation of ERK1/2 are not completely understood. In this study, we show that phosphorylation of RasGRP2, a guanine nucleotide exchange factor (GEF), inhibits its ability to activate the small GTPase Rap1 that ultimately leads to decreased activation of ERK1/2 in cells. ERK2 phosphorylates RasGRP2 at Ser394 located in the linker region implicated in its autoinhibition. These studies identify RasGRP2 as a novel substrate of ERK1/2 and define a negative-feedback loop that regulates the BRaf–MEK–ERK signaling cascade. This negative-feedback loop determines the amplitude and duration of active ERK1/2

Electrostatic and Functional Analysis of the Seven-Bladed WD β-Propellers

β-propeller domains composed of WD repeats are highly ubiquitous and typically used as multi-site docking platforms to coordinate and integrate the activities of groups of proteins. Here, we have used extensive homology modelling of the WD40-repeat family of seven-bladed β-propellers coupled with subsequent structural classification and clustering of these models to define subfamilies of β-propellers with common structural, and probable, functional characteristics. We show that it is possible to assign seven-bladed WD β-propeller proteins into functionally different groups based on the information gained from homology modelling. We examine general structural diversity within the WD40-repeat family of seven-bladed β-propellers and demonstrate that seven-bladed β-propellers composed of WD-repeats are structurally distinct from other seven-bladed β-propellers. We further provide some insights into the multifunctional diversity of the seven-bladed WD β-propeller surfaces. This report once again reinforces the importance of structural data and the usefulness of homology models in functional classification

Role of the C-Terminal SH3 Domain and N-Terminal Tyrosine Phosphorylation in Regulation of Tim and Related Dbl-Family Proteins †

Dbl-related oncoproteins are guanine nucleotide exchange factors (GEFs) specific for Rho-family GTPases and typically possess tandem Dbl (DH) and pleckstrin homology (PH) domains that act in concert to catalyze exchange. Although the exchange potential of many Dbl-family proteins is constitutively activated by truncation, the precise mechanisms of regulation for many Dbl-family proteins are unknown. Tim and Vav are distantly related Dbl-family proteins that are similarly regulated; their Dbl homology (DH) domains interact with N-terminal helices to exclude and prevent activation of Rho GTPases. Phosphorylation, substitution, or deletion of the blocking helices relieves this autoinhibition. Here we show that two other Dbl-family proteins, Ngef and Wgef, which like Tim contain a C-terminal SH3 domain, are also activated by tyrosine phosphorylation of a blocking helix. Consequently, basal autoinhibition of DH domains by direct steric exclusion using short N-terminal helices likely represents a conserved mechanism of regulation for the large family of Dbl-related proteins. N-Terminal truncation or phosphorylation of many other Dbl-family GEFs leads to their activation; similar autoinhibition mechanisms could explain some of these events. In addition, we show that the C-terminal SH3 domain binding to a polyproline region N-terminal to the DH domain of the Tim subgroup of Dbl-family proteins provides a unique mechanism of regulated autoinhibition of exchange activity that is functionally linked to the interactions between the autoinhibitory helix and the DH domain

Activation of Human Phospholipase C-η2 by Gβγ †

Phospholipase C-η2 (PLC-η2) was recently identified as a novel broadly expressed phosphoinositide-hydrolyzing isozyme [Zhou, Y., et al. (2005) Biochem. J. 391, 667–676; Nakahara, M., et al. (2005) J. Biol. Chem. 280, 29128–29134]. In this study, we investigated the direct regulation of PLC-η2 by Gβγ subunits of heterotrimeric G proteins. Coexpression of PLC-η2 with Gβ1γ2, as well as with certain other Gβγ dimers, in COS-7 cells resulted in increases in inositol phosphate accumulation. Gβ1γ2-dependent increases in phosphoinositide hydrolysis also were observed with a truncation mutant of PLC-η2 that lacks the long alternatively spliced carboxy-terminal domain of the isozyme. To begin to define the enzymatic properties of PLC-η2 and its potential direct activation by Gβγ, a construct of PLC-η2 encompassing the canonical domains conserved in all PLCs (PH domain through C2 domain) was purified to homogeneity after expression from a baculovirus in insect cells. Enzyme activity of purified PLC-η2 was quantified after reconstitution with PtdIns(4,5)P2-containing phospholipid vesicles, and values for Km (14.4 µM) and Vmax [12.6 µmol min−1 (mg of protein)−1] were similar to activities previously observed with purified PLC-β or PLC-ε isozymes. Moreover, purified Gβ1γ2 stimulated the activity of purified PLC-η2 in a concentration-dependent manner similar to that observed with purified PLC-β2. Activation was dependent on the presence of free Gβ1γ2 since its sequestration in the presence of Gαi1 or GRK2-ct reversed Gβ1γ2-promoted activation. The PH domain of PLC-η2 is not required for Gβ1γ2-mediated regulation since a purified fragment encompassing the EF-hand through C2 domains but lacking the PH domain nonetheless was activated by Gβ1γ2. Taken together, these studies illustrate that PLC-η2 is a direct downstream effector of Gβγ and, therefore, of receptor-activated heterotrimeric G proteins

Crystal Structures of the Type III Effector Protein AvrPphF and Its Chaperone Reveal Residues Required for Plant Pathogenesis

The avrPphF locus from Pseudomonas syringae pv. phaseolicola, the causative agent of bean halo-blight disease, encodes proteins which either enhance virulence on susceptible hosts or elicit defense responses on hosts carrying the R1 resistance gene. Here we present the crystal structures of the two proteins from the avrPphF operon. The structure of AvrPphF ORF1 is strikingly reminiscent of type III chaperones from bacterial pathogens of animals, indicating structural conservation of these specialized chaperones, despite high sequence divergence. The AvrPphF ORF2 effector adopts a novel "mushroom"-like structure containing "head" and "stalk" subdomains. The head subdomain possesses limited structural homology to the catalytic domain of bacterial ADP-ribosyltransferases (ADP-RTs), though no ADP-RT activity was detected for AvrPphF ORF2 in standard assays. Nonetheless, this structural similarity identified two clusters of conserved surface-exposed residues important for both virulence mediated by AvrPphF ORF2 and recognition of this effector by bean plants expressing the R1 resistance gene

A Fluorogenic, Small Molecule Reporter for Mammalian Phospholipase C Isozymes

Phospholipase C isozymes (PLCs) catalyze the conversion of the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2) into two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This family of enzymes are key signaling proteins that regulate the physiological responses of many extracellular stimuli such as hormones, neurotransmitters, and growth factors. Aberrant regulation of PLCs has been implicated in various diseases including cancer and Alzheimer’s disease. How, when, and where PLCs are activated under different cellular contexts are still largely unknown. We have developed a fluorogenic PLC reporter, WH-15, that can be cleaved in a cascade reaction to generate fluorescent 6-aminoquinoline. When applied in enzymatic assays with either pure PLCs or cell lysates, this reporter displays more than a 20-fold fluorescence enhancement in response to PLC activity. Under assay conditions, WH-15 has comparable Km and Vmax with the endogenous PIP2. This novel reporter will likely find broad applications that vary from imaging PLC activity in live cells to high throughput screening of PLC inhibitors

Fluorescent Phosphatidylinositol 4,5-Bisphosphate Derivatives with Modified 6-Hydroxy Group as Novel Substrates for Phospholipase C

The capacity to monitor spatiotemporal activity of phospholipase C (PLC) isozymes with a PLC-selective sensor would dramatically enhance understanding of the physiological function and disease relevance of these signaling proteins. Previous structural and biochemical studies defined critical roles for several of the functional groups of the endogenous substrate of PLC isozymes, phosphatidylinositol 4,5-bisphosphate (PIP2), indicating that these sites cannot be readily modified without compromising interactions with the lipase active site. However, the role of the 6-hydroxy group of PIP2 for interaction and hydrolysis by PLC has not been explored, possibly due to challenges in synthesizing 6-hydroxy derivatives. Here, we describe an efficient route for the synthesis of novel, fluorescent PIP2 derivatives modified at the 6-hydroxy group. Two of these derivatives were used in assays of PLC activity in which the fluorescent PIP2 substrates were separated from their diacylglycerol products and reaction rates quantified by fluorescence. Both PIP2 analogues effectively function as substrates of PLC-δ1, and the KM and Vmax values obtained with one of these are similar to those observed with native PIP2 substrate. These results indicate that the 6-hydroxy group can be modified to develop functional substrates for PLC isozymes, thereby serving as the foundation for further development of PLC-selective sensors

The DH and PH Domains of Trio Coordinately Engage Rho GTPases for their Efficient Activation

Rho-family GTPases are activated by the exchange of bound GDP for GTP, a process that is catalyzed by Dbl-family guanine nucleotide exchange factors (GEFs). The catalytic unit of Dbl-family GEFs consists of a Dbl-homology (DH) domain followed almost invariantly by a pleckstrin-homology (PH) domain. The majority of the catalytic interface forms between the switch regions of the GTPase and the DH domain, but full catalytic activity often requires the associated PH domain. Although PH domains are usually characterized as lipid binding regions, they also participate in protein-protein interactions. For example, the DH-associated PH domain of Dbs must contact its cognate GTPases for efficient exchange. Similarly, the N-terminal DH/PH fragment of Trio, which catalyzes exchange on both Rac1 and RhoG, is four-fold more active in vitro than the isolated DH domain. Given continued uncertainty regarding functional roles of DH-associated PH domains, we have undertaken structural and functional analyses of the N-terminal DH/PH cassette of Trio. The crystal structure of this fragment of Trio bound to nucleotide-depleted Rac1 highlights the engagement of the PH domain with Rac1 and substitution of residues involved in this interface substantially diminishes activation of Rac1 and RhoG. Also, these mutations significantly reduce the ability of full-length Trio to induce neurite outgrowth dependent on RhoG activation in PC-12 cells. Overall, these studies substantiate a general role for DH-associated PH domains in directly engaging Rho GTPases for efficient guanine nucleotide exchange and support a parsimonious explanation for the essentially invariant linkage between DH and PH domains