# Ras Multimers on the Membrane: Many Ways for a Heart-to-Heart Conversation
## 1 Introduction  
The Ras small GTPases are well established as key regulators of cell physiology [1; 2]. Residing on the inner leaflet of the membrane, Ras functions as a molecular switch that receives signals from numerous extracellular inputs and relays them to an array of intracellular effectors. A wide variety of cell surface receptors, such as growth factor receptors [3], integrins [4], G-protein coupled receptors [5], and immune receptors [6; 7] can signal to Ras. Canonical Ras effector pathways including Raf-Mitogen Activated Protein Kinase (MAPK) [8; 9; 10], phosphatidylinositol-3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) [11; 12], Ral guanine nucleotide dissociation stimulator (RalGDS) [13; 14], Phospholipase C (PLC) [15], and many others [16] have been well studied. Depending on the upstream input and the cellular context, activated Ras can recruit specific subsets of effectors and execute appropriate cellular programs accordingly. Through these effector pathways, Ras controls nearly all aspects of cell physiology: growth, proliferation, apoptosis, metabolism, motility, to name a few [1]. For the central roles of Ras in cell physiology, it is not surprising that deregulated Ras activities are common in human diseases. As much as one third of all human cancers are associated with mutationally activated Ras, and mutations in Ras and its regulators are also direct causes in Rasopathies and neuronal developmental disorders [17; 18; 19]. Four decades after _RAS_ was identified as a proto-oncogene, however, our success in targeting mutant Ras has been limited. Except for the recent progress in targeting Ras G12C [20; 21] and potentially G12D [22], direct inhibition of mutant Ras has proven difficult. Efforts to target downstream effector pathways, most notably Raf-MAPK [23; 24] and PI3K-Akt [25], are often met with tumor resistance that can frequently be attributed to activation of alternative Ras pathways [24; 26]. To overcome these issues and effectively manipulate mutant Ras in human diseases, it is imperative to further understand how Ras operates on the cell membrane and interacts with its diverse signaling partners. The capacity of Ras to participate in broad cellular processes reflects its ability to interface with variable input sources and generate context-specific outputs with consistency. Such versatility and robustness require that Ras activities--both the competence and the specificity of signaling--be modulated over multiple facets. As such, Ras could adapt to different cellular environments and engage specific subsets of signaling partners to invoke the desired cellular processes accordingly to the biological context. A multitude of mechanisms have been identified to regulate Ras activities in cells. Of these, GTP-loading [27; 28], post-translational modifications [29] and membrane localization [19; 30] have been well established. More recently, formation of Ras multimers, including Ras dimers and nanoclusters, has emerged as a new mechanism of regulation [31; 32; 33]. These mechanisms pertain to distinct aspects of Ras biology, although they are not entirely independent of each other. GTP-loading is a core mechanism for regulating all G-proteins, including the Ras small GTPases [27]. The first \(\sim\)166 residues of Ras form a heart-shaped, G-domain that contains the GTPase and effector-binding motifs (Figure 1, left) [34; 35]. Wild-type (WT) Ras cycles between an inactive, GDP-bound state and an active, GTP-bound state via counteracting enzymes including guanine exchange factors (GEFs) and GTPase-activating proteins (GAPs). Normally, Ras becomes GTP-loaded in response to upstream events that recruit Ras GEFs to the membrane, for example by Grb2 upon receptor tyrosine kinase activation [36; 37]. GTP-loading causes conformational changes in the switch I and II regions that enable Ras to recruit cytosolic effectors (Figure 1, left) to the membrane for subsequent activation.