International Journal of Molecular Sciences Review Integration of Rap1 and Calcium Signaling Ramoji Kosuru 1 and Magdalena Chrzanowska 1,2,3,* 1 Versiti Blood Research Institute, Milwaukee, WI 53201, USA; [email protected] 2 Department of Pharmacology and Toxicology, Medical College of Wisconsin, PO Box 2178, Milwaukee, WI 53201-2178, USA 3 Cardiovascular Center, Medical College of Wisconsin, PO Box 2178, Milwaukee, WI 53201-2178, USA * Correspondence: [email protected] Received: 6 February 2020; Accepted: 25 February 2020; Published: 27 February 2020 Abstract: Ca2+ is a universal intracellular signal. The modulation of cytoplasmic Ca2+ concentration regulates a plethora of cellular processes, such as: synaptic plasticity, neuronal survival, chemotaxis of immune cells, platelet aggregation, vasodilation, and cardiac excitation–contraction coupling. Rap1 GTPases are ubiquitously expressed binary switches that alternate between active and inactive states and are regulated by diverse families of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Active Rap1 couples extracellular stimulation with intracellular signaling through secondary messengers—cyclic adenosine monophosphate (cAMP), Ca2+, and diacylglycerol (DAG). Much evidence indicates that Rap1 signaling intersects with Ca2+ signaling pathways to control the important cellular functions of platelet activation or neuronal plasticity. Rap1 acts as an effector of Ca2+ signaling when activated by mechanisms involving Ca2+ and DAG-activated (CalDAG-) GEFs. Conversely, activated by other GEFs, such as cAMP-dependent GEF Epac, Rap1 controls cytoplasmic Ca2+ levels. It does so by regulating the activity of Ca2+ signaling proteins such as sarcoendoplasmic reticulum Ca2+-ATPase (SERCA). In this review, we focus on the physiological significance of the links between Rap1 and Ca2+ signaling and emphasize the molecular interactions that may offer new targets for the therapy of Alzheimer’s disease, hypertension, and atherosclerosis, among other diseases. Keywords: Rap1; calcium; CalDAG-GEF; Epac; SERCA 1. Discovery, Early and Classical Functions of Rap1: Ras Antagonism, Integrin Activation Rap1, a 21 kDa monomeric G-protein, was discovered in 1989 by Noda and his coworkers in a screen for proteins able to suppress the oncogenic effect of K-Ras (one of the mutated Ras genes) [1]. Described as Kristen-ras-revertant-1 (Krev-1), the protein was found to have high similarity to Ras proteins [2]. Simultaneously, Pizen et al. characterized two proteins, Rap1 and Rap2, as Ras homologues and proposed that Rap1, identical to Krev-1, might function as an antagonist of Ras by competing for a common target, or mediating growth inhibitory signals independently of Ras [3,4]. Since then, many groups have reported that Rap1 antagonizes Ras signaling by trapping its effector proteins, serine/threonine kinase Raf, in an inactive complex [5]. However, much research has also demonstrated the functions of Rap1 independent of Ras. The two highly conserved Rap1 isoforms, Rap1a and Rap1b, share 95% sequence identity, with a 50% sequence homology to Ras [3]. The basic structure of Rap1 is similar to Ras and consists of a catalytic domain made of a six-stranded central β-sheet (β1–β6) surrounded by five α-helices (α1–α5) and ten loops (L1–L10) [6,7]. The two regions of highest sequence similarity between Ras and Rap1 correspond to the switch 1 (amino acids 32–38) and switch 2 (amino acids 60–70) regions [7,8]. These regions adopt different conformations when bound to GTP (active) or GDP (inactive) and allow Int. J. Mol. Sci. 2020, 21, 1616; doi:10.3390/ijms21051616 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2020, 21, 1616 2 of 21 effector proteins to discriminate between the active and inactive form of small G protein. Despite the identical effector domains and a shared subset of effectors, many of Rap1’s biological functions are distinct from Ras, due to cellular and signaling differences in the utilization of the same effectors [9]. Int. J. Mol. Sci. 2020, 21, 1616 2 of 20 Furthermore,Int. J. Rap1Mol. Sci. 2020 controls, 21, 1616 cell adhesion by modulating the activity of adhesion receptors—integrins2 of 20 Int. J. Mol. Sci. 2020, 21, 1616 2 of 20 and cadherins—throughInt.Int. J.J. Mol.Mol. 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