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Lipidated small GTPases and their regulators need to bind to membranes to propagate actions in the cell, but an integrated understanding of how the lipid bilayer exerts its effect has remained elusive. Here we focused on ADP ribosylation factor (Arf) GTPases, which orchestrate a variety of regulatory functions in lipid and membrane trafficking, and their activation by the guanine-nucleotide exchange factor (GEF) Brag2, which controls integrin endocytosis and cell adhesion and is impaired in cancer and developmental diseases. Biochemical and structural data are available that showed the exceptional efficiency of Arf activation by Brag2 on membranes. We determined the high-resolution crystal structure of unbound Brag2 containing the GEF (Sec7) and membrane-binding (pleckstrin homology) domains, revealing that it has a constitutively active conformation. We used this structure to analyze the interaction of uncomplexed Brag2 and of the myristoylated Arf1/Brag2 complex with a phosphatidylinositol bisphosphate (PIP2) -containing lipid bilayer, using coarse-grained molecular dynamics. These simulations revealed that the system forms a close-packed, oriented interaction with the membrane, in which multiple PIP2 lipids bind the canonical lipid-binding site and unique peripheral sites of the PH domain, the Arf GTPase and, unexpectedly, the Sec7 domain. We cross-validated these predictions by reconstituting the binding and kinetics of Arf and Brag2 in artificial membranes. Our coarse-grained structural model thus suggests that the high efficiency of Brag2 requires interaction with multiple lipids and a well-defined orientation on the membrane, resulting in a local PIP2 enrichment, which has the potential to signal toward the Arf pathway.

The adenosine di-phosphate (ADP) ribosylation factor (Arf) small guanosine tri-phosphate (GTP)ases function as molecular switches to activate signaling cascades that control membrane organization in eukaryotic cells. In Arf1, the GDP/GTP switch does not occur spontaneously but requires guanine nucleotide exchange factors (GEFs) and membranes. Exchange involves massive conformational changes, including disruption of the core β-sheet. The mechanisms by which this energetically costly switch occurs remain to be elucidated. To probe the switch mechanism, we coupled pressure perturbation with nuclear magnetic resonance (NMR), Fourier Transform infra-red spectroscopy (FTIR), small-angle X-ray scattering (SAXS), fluorescence, and computation. Pressure induced the formation of a classical molten globule (MG) ensemble. Pressure also favored the GDP to GTP transition, providing strong support for the notion that the MG ensemble plays a functional role in the nucleotide switch. We propose that the MG ensemble allows for switching without the requirement for complete unfolding and may be recognized by GEFs. An MG-based switching mechanism could constitute a pervasive feature in Arfs and Arf-like GTPases, and more generally, the evolutionarily related (Ras-like small GTPases) Rags and Gα GTPases. [ABSTRACT FROM AUTHOR]

The FIC motif and the eukaryotic-like ankyrin repeats are found in many bacterial type IV effectors, yet little is known about how these domains enable bacteria to modulate host cell functions. Bacterial FIC domains typically bind ATP and transfer adenosine monophosphate moiety onto target proteins. The ankyrin repeat-containing protein AnkX encoded by the intracellular pathogen Legionella pneumophila is unique in that its FIC domain binds to CDP-choline and transfers a phosphocholine residue onto proteins in the Rab1 GTPase family. By determining the structures of unbound AnkX and AnkX with bound CDP-choline, CMP/phosphocholine and CMP, we demonstrate that the orientation of substrate binding in relation to the catalytic FIC motif enables this protein to function as a phosphocholinating enzyme rather than a nucleotidyl transferase. Additionally, the structure reveals that the ankyrin repeats mediate scaffolding interactions that resemble those found in protein-protein interactions, but are unprecedented in intramolecular interactions. Together with phosphocholination experiments, our structures unify a general phosphoryl transferase mechanism common to all FIC enzymes that should be conserved from bacteria to human.

Cell migration requires the generation of branched actin networks that power the protrusion of the plasma membrane in lamellipodia (1,2). The actin-related proteins 2 and 3 (Arp2/3) complex is the molecular machine that nucleates these branched actin networks (3). This machine is activated at the leading edge of migrating cells by Wiskott-Aldrich syndrome protein (WASP)-family verprolin-homologous protein (WAVE, also known as SCAR). The WAVE complex is itself directly activated by the small GTPase Rac, which induces lamellipodia (4-6). However, how cells regulate the directionality of migration is poorly understood. Here we identify a new protein, Arpin, that inhibits the Arp2/3 complex invitro, and show that Rac signalling recruits and activates Arpin at the lamellipodial tip, like WAVE. Consistently, after depletion of the inhibitory Arpin, lamellipodia protrude faster and cells migrate faster. A major role of this inhibitory circuit, however, is to control directional persistence of migration. Indeed, Arpin depletion in both mammalian cells and Dicttyostelium discoideum amoeba resulted in straighter trajectories, whereas Arpin microinjection in fish keratocytes, one of the most persistent systems of cell migration, induced these cells to turn. The coexistence of the Rac-Arpin-Arp2/3 inhibitory circuit with the Rac-WAVE-Arp2/3 activatory circuit can account for this conserved role of Arpin in steering cell migration., The Arp2/3 complex is activated at different cellular locations by different nucleation promoting factors (NPFs), WAVE at lamellipo dia, neural WASP (N-WASP) at clathrin-coated pits, and WASP and SCAR homologue [...]

One of nature's strategies for interfering with molecular interactions is to trap macromolecules in transition states with their partners in dead-end complexes that are unable to complete their biological function. This type of inhibition, which we refer to as 'interracial inhibition', is illustrated by two natural inhibitors, brefeldin A (BFA) and camptothecin (CPT), whose modes of action have been elucidated fully in structural studies. Interfacial inhibition occurs at the protein-protein interface in the case of BFA and at the protein-DNA interface in the case of CPT. In both systems, the drugs take advantage of transient structural and energetic conditions created by the macromolecular complex, which give rise to 'hot-spots' for drug binding. In addition to these examples, several natural compounds such as forskolin, tubulin inhibitors and immunophilins target protein interfaces. We propose that interfacial inhibition is a paradigm for the discovery of drugs that interfere with macromolecular complexes.

The structural principles for the unique property, called multispecificity, of the small GTP-binding proteins (G proteins) are investigated by a database-implemented comparison of their protein-protein interfaces. The multiplicity of the interactions of amino acids inside and outside the switch regions is quantified and the physicochemical nature of their multiple interactions and the contribution of structural plasticity are characterized.

Author(s): Louis Renault; Bernard Guibert; Jacqueline Cherfils (corresponding author) GEFs, which activate small G proteins by stimulating GDP/GTP exchange, are emerging as pivotal regulators in collecting the signals that lead [...]

Activation of the G[alpha] subunit of heterotrimeric GTP-binding proteins by transmembrane receptors requires the propagation of structural signals from the receptor-binding site to the nucleotide-binding site at the opposite side of the protein. In a previous model, it was suggested that the G[beta]-G[gamma] dimer is tilted away from G[alpha] by a lever-arm motion of the G[alpha] N-terminal helix. Here, we propose that the motion occurs in the opposite direction, close-packing the G[alpha]-G[beta] interface and creating a novel interface between the helical domain of G[alpha] and the N terminus of G[gamma], which determines the specificity of activation.