Dissociation of RalA from synaptic membranes by Ca2+/calmodulin

Calmodulin extracts the Ras family protein RalA from lipid bilayers by engagement with two membrane-targeting motifs

RalA is a small GTPase and a member of the Ras family. This molecular switch is activated downstream of Ras and is widely implicated in tumor formation and growth. Previous work has shown that the ubiquitous Ca2+-sensor calmodulin (CaM) binds to small GTPases such as RalA and K-Ras4B, but a lack of structural information has obscured the functional consequences of these interactions. Here, we have investigated the binding of CaM to RalA and found that CaM interacts exclusively with the C terminus of RalA, which is lipidated with a prenyl group in vivo to aid membrane attachment. Biophysical and structural analyses show that the two RalA membrane-targeting motifs (the prenyl anchor and the polybasic motif) are engaged by distinct lobes of CaM and that CaM binding leads to removal of RalA from its membrane environment. The structure of this complex, along with a biophysical investigation into membrane removal, provides a framework with which to understand how CaM regulates the function of RalA and sheds light on the interaction of CaM with other small GTPases, including K-Ras4B.

The RAL signaling network: Cancer and beyond

The RAL proteins RALA and RALB belong to the superfamily of small RAS-like GTPases (guanosine triphosphatases). RAL GTPases function as molecular switches in cells by cycling through GDP- and GTP-bound states, a process which is regulated by several guanine exchange factors (GEFs) and two heterodimeric GTPase activating proteins (GAPs). Since their discovery in the 1980s, RALA and RALB have been established to exert isoform-specific functions in central cellular processes such as exocytosis, endocytosis, actin organization and gene expression. Consequently, it is not surprising that an increasing number of physiological functions are discovered to be controlled by RAL, including neuronal plasticity, immune response, and glucose and lipid homeostasis. The critical importance of RAL GTPases for oncogenic RAS-driven cellular transformation and tumorigenesis still attracts most research interest. Here, RAL proteins are key drivers of cell migration, metastasis, anchorage-independent proliferation, and survival. This chapter provides an overview of normal and pathological functions of RAL GTPases and summarizes the current knowledge on the involvement of RAL in human disease as well as current therapeutic targeting strategies. In particular, molecular mechanisms that specifically control RAL activity and RAL effector usage in different scenarios are outlined, putting a spotlight on the complexity of the RAL GTPase signaling network and the emerging theme of RAS-independent regulation and relevance of RAL.

Pleiotropic Roles of Calmodulin in the Regulation of KRas and Rac1 GTPases: Functional Diversity in Health and Disease

Calmodulin is a ubiquitous signalling protein that controls many biological processes due to its capacity to interact and/or regulate a large number of cellular proteins and pathways, mostly in a Ca²⁺-dependent manner. This complex interactome of calmodulin can have pleiotropic molecular consequences, which over the years has made it often difficult to clearly define the contribution of calmodulin in the signal output of specific pathways and overall biological response. Most relevant for this review, the ability of calmodulin to influence the spatiotemporal signalling of several small GTPases, in particular KRas and Rac1, can modulate fundamental biological outcomes such as proliferation and migration. First, direct interaction of calmodulin with these GTPases can alter their subcellular localization and activation state, induce post-translational modifications as well as their ability to interact with effectors. Second, through interaction with a set of calmodulin binding proteins (CaMBPs), calmodulin can control the capacity of several guanine nucleotide exchange factors (GEFs) to promote the switch of inactive KRas and Rac1 to an active conformation. Moreover, Rac1 is also an effector of KRas and both proteins are interconnected as highlighted by the requirement for Rac1 activation in KRas-driven tumourigenesis. In this review, we attempt to summarize the multiple layers how calmodulin can regulate KRas and Rac1 GTPases in a variety of cellular events, with biological consequences and potential for therapeutic opportunities in disease settings, such as cancer.

Both the C-terminal polylysine region and the farnesylation of K-RasB are important for its specific interaction with calmodulin

Background

Ras protein, as one of intracellular signal switches, plays various roles in several cell activities such as differentiation and proliferation. There is considerable evidence showing that calmodulin (CaM) binds to K-RasB and dissociates K-RasB from membrane and that the inactivation of CaM is able to induce K-RasB activation. However, the mechanism for the interaction of CaM with K-RasB is not well understood.

Methodology/Principal Findings

Here, by applying fluorescence spectroscopy and isothermal titration calorimetry, we have obtained thermodynamic parameters for the interaction between these two proteins and identified the important elements of K-RasB for its interaction with Ca²⁺/CaM. One K-RasB molecule interacts with one CaM molecule in a GTP dependent manner with moderate, micromolar affinity at physiological pH and physiologic ionic strength. Mutation in the polybasic domain of K-Ras decreases the binding affinity. By using a chimera in which the C-terminal polylysine region of K-RasB has been replaced with that of H-Ras and vice versa, we find that at physiological pH, H-Ras-(KKKKKK) and Ca²⁺/CaM formed a 1∶1 complex with an equilibrium association constant around 10⁵ M⁻¹, whereas no binding reaction of K-RasB-(DESGPC) with Ca²⁺/CaM is detected. Furthermore, the interaction of K-RasB with Ca²⁺/CaM is found to be enhanced by the farnesylation of K-RasB.

Conclusions/Significance

We demonstrate that the polylysine region of K-RasB not only contributes importantly to the interaction of K-RasB with Ca²⁺/CaM, but also defines its isoform specific interaction with Ca²⁺/CaM. The farnesylation of K-RasB is also important for its specific interaction with Ca²⁺/CaM. Information obtained here can enhance our understanding of how CaM interacts with K-RasB in physiological environments.

Calmodulin binding to the small GTPase Ral requires isoprenylated Ral

Ral, a member of the Ras-p21 superfamily of small GTPases, has been shown to require the calcium-signaling protein calmodulin (CaM) for activation. In the present work, we investigated the properties of the Ral-CaM interaction. Using CaM affinity binding assay with lysates from mammalian cells overexpressing various Ral mutants, we found that RalB(V23, DeltaCAAX) lacking the C-terminal isoprenylation region bound significantly less efficiently to CaM. Binding of other mutants containing critical amino acid changes in the nucleotide or substrate binding regions (residues 23, 28, and 49) was not affected. In addition, all mutants bound significantly better in the presence of calcium versus the calcium chelator EGTA. Using in vitro transcription-translation in the presence of geranylgeranyl pyrophosphate, we demonstrate enhanced Ral binding to CaM. Inhibition of isoprenylation in cells in culture with lovastatin resulted in decreased binding of CaM to Ral. The present results show that post-translational isoprenylation of Ral is important in Ral-CaM interaction.

Calmodulin binds RalA and RalB and is required for the thrombin-induced activation of Ral in human platelets

Ral GTPases may be involved in calcium/calmodulin-mediated intracellular signaling pathways. RalA and RalB are activated by calcium, and RalA binds calmodulin in vitro. It was examined whether RalA can bind calmodulin in vivo, whether RalB can bind calmodulin, and whether calmodulin is functionally involved in Ral activation. Yeast two-hybrid analyses demonstrated both Rals interact directly but differentially with calmodulin. Coimmunoprecipitation experiments determined that calmodulin and RalB form complexes in human platelets. In vitro pull-down experiments in platelets and in vitro binding assays showed endogenous Ral and calmodulin interact in a calcium-dependent manner. Truncated Ral constructs determined in vitro and in vivo that RalA has an additional calmodulin binding domain to that previously described, that although RalB binds calmodulin, its C-terminal region is involved in partially inhibiting this interaction, and that in vitro RalA and RalB have an N-terminal calcium-independent and a C-terminal calcium-dependent calmodulin binding domain. Functionally, in vitro Ral-GTP pull-down experiments determined that calmodulin is required for the thrombin-induced activation of Ral in human platelets. We propose that differential binding of calmodulin by RalA and RalB underlies possible functional differences between the two proteins and that calmodulin is involved in the regulation of the activation of Ral-GTPases.

A role for phospholipase D1 in neurotransmitter release

Phosphatidic acid produced by phospholipase D (PLD) as a result of signaling activity is thought to play a role in membrane vesicle trafficking, either as an intracellular messenger or as a cone-shaped lipid that promotes membrane fusion. We recently described that, in neuroendocrine cells, plasma membrane-associated PLD1 operates at a stage of Ca(2+)-dependent exocytosis subsequent to cytoskeletal-mediated recruitment of secretory granules to exocytotic sites. We show here that PLD1 also plays a crucial role in neurotransmitter release. Using purified rat brain synaptosomes subjected to hypotonic lysis and centrifugation, we found that PLD1 is associated with the particulate fraction containing the plasma membrane. Immunostaining of rat cerebellar granule cells confirmed localization of PLD1 at the neuronal plasma membrane in zones specialized for neurotransmitter release (axonal neurites, varicosities, and growth cone-like structures). To determine the potential involvement of PLD1 in neurotransmitter release, we microinjected catalytically inactive PLD1(K898R) into Aplysia neurons and analyzed its effects on evoked acetylcholine (ACh) release. PLD1(K898R) produced a fast and potent dose-dependent inhibition of ACh release. By analyzing paired-pulse facilitation and postsynaptic responses evoked by high-frequency stimulations, we found that the exocytotic inhibition caused by PLD1(K898R) was not the result of an alteration in stimulus-secretion coupling or in vesicular trafficking. Analysis of the fluctuations in amplitude of the postsynaptic responses revealed that the PLD1(K898R) blocked ACh release by reducing the number of active presynaptic-releasing sites. Our results provide evidence that PLD1 plays a major role in neurotransmission, most likely by controlling the fusogenic status of presynaptic release sites.

The brain exocyst complex interacts with RalA in a GTP-dependent manner: identification of a novel mammalian Sec3 gene and a second Sec15 gene

Ral is a small GTPase involved in critical cellular signaling pathways. The two isoforms, RalA and RalB, are widely distributed in different tissues, with RalA being enriched in brain. The best characterized RalA signaling pathways involve RalBP1 and phospholipase D. To investigate RalA signaling in neuronal cells we searched for RalA-binding proteins in brain. We found at least eight proteins that bound RalA in a GTP-dependent manner. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) identified these as the components of the exocyst complex. The yeast exocyst is a regulator of polarized secretion, docking vesicles to regions of the plasma membrane involved in active exocytosis. We identified the human FLJ10893 protein as the mammalian homologue of the yeast exocyst protein Sec3p. The exocyst complex did not contain the previously identified exocyst component rSec15, but a new homologue of both yeast Sec15p and rSec15, called KIAA0919. Western blots confirmed that two rat exocyst proteins, rSec6 and rSec8, bound active RalA in nerve terminals, as did RalBP1. Phospholipase D bound RalA in a nucleotide-independent manner. This places the RalA signaling system in mammalian nerve terminals, where the exocyst may act as an effector for activated RalA in directing sites of exocytosis.

Multiple aspects of Rab protein action in the secretory pathway: focus on Rab3 and Rab6

Rab proteins form the largest branch of the Ras superfamily of GTPases. They are localized to the cytoplasmic face of organelles and vesicles involved in the biosynthetic/secretory and endocytic pathways in eukaryotic cells. It is now well established that Rab proteins play an essential role in the processes that underlie the targeting and fusion of transport vesicles with their appropriate acceptor membranes. They perform this task through interactions with a wide variety of effector molecules. In this review, we illustrate recent advances in the field of Rab GTPases, taking as examples two proteins involved in the biosynthetic pathway, Rab3 and Rab6.