## 1 Introduction
In addition to the N-terminal Ras-binding conserved region (CR1) (residues 52-194, CRaf numbering), the three Raf isoforms (ARaf, BRaf, and CRaf) share two regions with high similarity that are connected through unstructured linker regions [5]. The second conserved region (CR2) (residues 254-269) contains a serine/threonine-rich domain with key phosphorylation sites [29; 30; 31] and the third conserved region (CR3) comprises the C-terminal kinase domain (residues 349-609) (Figure 1a). Prior to its recruitment to the membrane and interaction with Ras, Raf is held in an autoinhibited state through the aid of 14-3-3 scaffold proteins that bind motifs located in the CR2 and CR3 regions [32; 33; 34]. Recruitment of Raf to the membrane through Ras results in dimerization of the Raf kinase domain, leading to its activation and subsequent phosphorylation of MEK to propagate signaling [35; 36]. Recent cryo-EM structures of inactive and active BRaf/14-3-3 and BRaf/MEK1/14-3-3 complexes have shed light into this mechanism revealing a critical role for the Raf-CRD in stabilizing the autoinhibited state [37; 32]. It has been noted that the Raf-RBD is exposed in the complex with Ras, allowing its interaction with Ras to displace the CRD from the kinase/14-3-3 complex [20]. From there, the proposed model for Ras activation of Raf is built on the premise that the CRD must both bind Ras and insert into the membrane upon release from its autoinhibition role, leading to a signaling state in which Ras is monomeric on the membrane [20]. Figure 1: _Cuts._  
Here, we present the crystal structure of HRas bound to a construct containing both the CRaf-RBD and CRD at 2.8 A resolution (PDB ID 7 JHP) (Figure 1b,c and Supplementary Material Table S1) and build on our recently proposed model for Ras activation of Raf [8]. Our model is based on the discovery that, on supported lipid bilayers, Ras dimerizes upon binding to Raf, and allows for the possibility that previously observed insertion of the CRD into the membrane is an artifact of experimental conditions. Our crystal structure of the HRas-CRaf-RBD_CRD is nearly identical to that of the recently published KRas-CRaf-RBD_CRD structure [20], adding to evidence that the Ras isoforms activate Raf kinase through a common mechanism [8]. This structure reveals that the Raf-CRD interacts with Ras residues at a surface contiguous to the Raf-RBD along \(\beta\)2, stretching to loop 3 at the interswitch region. As the dimer is not present in our crystals (Figure S1a), we used this structure to build a model of the HRas-CRaf-RBD_CRD dimer based on our HRas-CRaf-RBD structure, which places the Raf-CRD at the base of the two Ras protomers  
Figure 1: Structure of the HRas-CRaf-RBD_CRD complex. (**a**) Schematic diagrams of HRas and CRaf provide structural boundaries for the Ras G domain and hypervariable region, and the three conserved regions found in Raf kinases. (**b**) Cartoon and surface representations of the HRas–CRaf-RBD_CRD complex reveal a continuous binding surface spanning Ras (pink), residues 23 through 48 for both the Raf-RBD (teal), and Raf-CRD (green). (**c**) The Raf-CRD interacts with residues in Ras \(\alpha\)1, \(\alpha\)5 and the interswitch region through hydrogen bonds and hydrophobic contacts. The symbol \(\blacklozenge\) identifies residues that show chemical shift perturbations in the analysis described by [PERSON] et al. [24]. opposite the C-terminal end of \(\alpha\)5 and the HVR, facing away from the expected location of the membrane. In this model, the CRD is poised to have an impact on both the Ras active site and allosteric connections in the signaling complex. Significantly, although not noted in the publication [20], this dimer is present through a two-fold crystallographic symmetry axis in the higher resolution crystal structure of the KRas-CRaf-RBD_CRD complex (PDB ID 6 XI7) (Figure S1b), consistent with our model and the analysis presented here.