Ras-Raf complexes in vitro

Purification and in vitro functional analysis of the Arabidopsis thaliana regulator of G-protein signaling-1

The model organism Arabidopsis thaliana contains a restricted set of heterotrimeric G-protein subunits, with only one canonical Galpha subunit (AtGPA1), one Gbeta subunit (AtAGB1), and two Ggamma subunits (AtAGG1 and AtGG2) identified. We have identified a novel additional component of heterotrimeric G-protein signaling in the A. thaliana genome, regulator of G-protein signaling-1 (AtRGS1). This protein has the predicted topology and structure of a G-protein-coupled receptor in that it contains seven transmembrane domains, but AtRGS1 also contains a unique C-terminal extension, namely a regulator of G-protein signaling domain (RGS box). This article describes methods for the purification and in vitro functional analysis of the RGS box of AtRGS1.

Structure and absolute stereochemistry of phormidolide, a new toxic metabolite from the marine cyanobacterium Phormidium sp

The extract from a laboratory culture of an Indonesian isolate of the cyanobacterium Phormidium sp. displayed inhibitory activity in a Ras-Raf protein interaction assay. Assay-guided fractionation led to the isolation of both active and inactive materials of novel structure. The major inactive metabolite, phormidolide, was nevertheless highly toxic to brine shrimp (LC(50) = 1.5 microM), and hence, its structure was elucidated using various spectroscopic methods, primarily NMR. A series of partial structures were developed from standard experiments and then assembled using GHMBC, 2D INADEQUATE, and ACCORD-ADEQUATE data obtained on a (13)C-enriched sample. The relative stereochemistry at phormidolide's 11 chiral centers was established using the J-based configuration analysis method in concert with the G-BIRD(R)-HSQMBC NMR experiment. Absolute stereochemistry was determined on a bis-acetonide derivative using the variable temperature Mosher ester method. The robust number of NMR restraints provided from determination of most homonuclear and heteronuclear coupling constants in phormidolide, along with an abundance of NOE information, allowed construction of a refined lowest energy three-dimensional structure in Macromodel. Phormidolide is one of only a few macrolide-type natural products to be reported from marine cyanobacteria.

Rho3 of Saccharomyces cerevisiae, which regulates the actin cytoskeleton and exocytosis, is a GTPase which interacts with Myo2 and Exo70

The Rho3 protein plays a critical role in the budding yeast Saccharomyces cerevisiae by directing proper cell growth. Rho3 appears to influence cell growth by regulating polarized secretion and the actin cytoskeleton, since rho3 mutants exhibit large rounded cells with an aberrant actin cytoskeleton. To gain insights into how Rho3 influences these events, we have carried out a yeast two-hybrid screen using an S. cerevisiae cDNA library to identify proteins interacting with Rho3. Two proteins, Exo70 and Myo2, were identified in this screen. Interactions with these two proteins are greatly reduced or abolished when mutations are introduced into the Rho3 effector domain. In addition, a type of mutation known to produce dominant negative mutants of Rho proteins abolished the interaction with both of these proteins. In contrast, Rho3 did not interact with protein kinase C (Pkc1), an effector of another Rho family protein, Rho1, nor did Rho1 interact with Exo70 or Myo2. Rho3 did interact with Bni1, another effector of Rho1, but less efficiently than with Rho1. The interaction between Rho3 and Exo70 and between Rho3 and Myo2 was also demonstrated with purified proteins. The interaction between Exo70 and Rho3 in vitro was dependent on the presence of GTP, since Rho3 complexed with guanosine 5'-O-(3-thiotriphosphate) interacted more efficiently with Exo70 than Rho3 complexed with guanosine 5'-O-(3-thiodiphosphate). Overlapping subcellular localization of the Rho3 and Exo70 proteins was demonstrated by indirect immunofluorescence. In addition, patterns of localization of both Exo70 and Rho3 were altered when a dominant active allele of RHO3, RHO3(E129,A131), which causes a morphological abnormality, was expressed. These results provide a direct molecular basis for the action of Rho3 on exocytosis and the actin cytoskeleton.

SUR-8, a conserved Ras-binding protein with leucine-rich repeats, positively regulates Ras-mediated signaling in C. elegans

We describe the identification and characterization of a novel gene, sur-8, that positively regulates Ras-mediated signal transduction during C. elegans vulval development. Reduction of sur-8 function suppresses an activated ras mutation and dramatically enhances phenotypes of mpk-1 MAP kinase and ksr-1 mutations, while increase of sur-8 dosage enhances an activated ras mutation. sur-8 appears to act downstream of or in parallel to ras but upstream of raf. sur-8 encodes a conserved protein that is composed predominantly of leucine-rich repeats. The SUR-8 protein interacts directly with Ras but not with the Ras(P34G) mutant protein, suggesting that SUR-8 may mediate its effects through Ras binding. A structural and functional SUR-8 homolog in humans specifically binds K-Ras and N-Ras but not H-Ras in vitro.

Interaction of the Grb10 adapter protein with the Raf1 and MEK1 kinases

Grb10 and its close homologues Grb7 and Grb14, belong to a family of adapter proteins characterized by a proline-rich region, a central PH domain, and a carboxyl-terminal Src homology 2 (SH2) domain. Their interaction with a variety of activated tyrosine kinase receptors is well documented, but their actual function remains a mystery. The Grb10 SH2 domain was isolated from a two-hybrid screen using the MEK1 kinase as a bait. We show that this unusual SH2 domain interacts, in a phosphotyrosine-independent manner, with both the Raf1 and MEK1 kinases. Mutation of the MEK1 Thr-386 residue, which is phosphorylated by mitogen-activated protein kinase in vitro, reduces binding to Grb10 in a two-hybrid assay. Interaction of Grb10 with Raf1 is constitutive, while interaction between Grb10 and MEK1 needs insulin treatment of the cells and follows mitogen-activated protein kinase activation. Random mutagenesis of the SH2 domain demonstrated that the Arg-betaB5 and Asp-EF2 residues are necessary for binding to the epidermal growth factor and insulin receptors as well as to the two kinases. In addition, we show that a mutation in Ser-betaB7 affects binding only to the receptors, while a mutation in Thr-betaC5 abrogates binding only to MEK1. Finally, transfection of Grb10 genes with specific mutations in their SH2 domains induces apoptosis in HTC-IR and COS-7 cells. These effects can be competed by co-expression of the wild type protein, suggesting that these mutants act by sequestering necessary signaling components.

Ras-binding domains: predicting function versus folding

Ras interacts with a number of effector molecules to achieve its prolific signalling. Based on iterative sequence profile and motif searches of databases a novel family of Ras-binding domains was recently identified (Ponting and Benjamin, Trends Biochem. Sci. 21: 422-425, 1996). Among them the rat unconventional myosin and Rho-GTPase-activating protein myr 5 was predicted to contain a Ras-binding domain at its N-terminus. Here we report that direct binding experiments between the proposed Ras-binding domain of myr 5 and Ras failed to demonstrate any interaction. Molecular modelling suggests that this domain in myr 5 adopts a similar folding topology as the Ras-binding domain of Raf kinase. However, unlike the Ras-binding domain of Raf kinase, the myr 5 domain lacks the positive surface charges necessary for binding the negatively charged Ras contact site. This result exemplifies the functional diversity of similar structures and suggests that the identified Ras-binding motif does not reliably predict Ras-binding domains.

Ras interaction with two distinct binding domains in Raf-1 may be required for Ras transformation

Although Raf-1 is a critical Ras effector target, how Ras mediates Raf-1 activation remains unresolved. Raf-1 residues 55-131 define a Ras-binding domain essential for Raf-1 activation. Therefore, our identification of a second Ras-binding site in the Raf-1 cysteine-rich domain (residues 139-184) was unexpected and suggested a more complex role for Ras in Raf-1 activation. Both Ras recognition domains preferentially associate with Ras-GTP. Therefore, mutations that impair Ras activity by perturbing regions that distinguish Ras-GDP from Ras-GTP (switch I and II) may disrupt interactions with either Raf-1-binding domain. We observed that mutations of Ras that impaired Ras transformation by perturbing its switch I (T35A and E37G) or switch II (G60A and Y64W) domain preferentially diminished binding to Raf-1-(55-131) or the Raf-1 cysteine-rich domain, respectively. Thus, these Ras-binding domains recognize distinct Ras-GTP determinants, and both may be essential for Ras transforming activity. Finally, since Ha-Ras T35A and E37G mutations prevent Ras interaction with full-length Raf-1, we suggest that Raf-Cys is a cryptic binding site that is unmasked upon Ras interaction with Raf-1-(55-131).