# Ras Multimers on the Membrane: Many Ways for a Heart-to-Heart Conversation
## 3 Ras Dimerization at Variable G-domain Interfaces
Although their computational predictions indicated that the \(\beta\) homodimer is more stable than the \(\alpha\) homodimer, they commented that the relative populations and stability of the interfaces may change in full-length membrane anchored KRas. Later in 2015, [PERSON] et al. characterized the \(\beta\)-sheet dimer interface for KRas using structure matching and all-atom MD simulations [119]. In the study, the authors suggested that, although formation of \(\beta\)-interface dimers is possible, it has a weak binding free-energy and it is unlikely to play significant roles in cellular KRas function. The region constituting \(\beta\)2-loop-\(\beta\)3 was previously recognized as a 'constitutive' effector (or so-called 'Ec') region since mutations in many of the residues in this region, such as K42 and L53 (at this dimer interface) or I46, G48, and E49 (in the loop) were found to significantly impact Ras-mediated Raf activation with minimal effects on nucleotide binding, hydrolysis, or Raf binding [120]. [PERSON] et al. named the combination of Ec and \(\alpha\)5 a novel, switch III region. Some mutations in this region (such as D47A and E49A) occur in human cancers and augment HRas nanoclustering and signaling [121; 122]. How these mutations alter Ras dimerization remains to be further elucidated, so do the functional roles of the \(\beta\)2-\(\beta\)3 interface. These dimer interfaces may just be a subset of what may exist, as revealed in a recent collaborative, large-scale modeling effort [116]. With a Multiscale Machine-Learned Modeling Infrastructure (MuMMI), the authors ran an ensemble of over 100,000 simulations of active WT KRas on lipid bilayers with a variety of lipid compositions. Of these simulations, 10,939 showed Ras-Ras contacts. While the above dimerization interfaces were significantly populated in the simulations, a much broader range of Ras G-domain contacts was observed, suggesting that membrane-tethered Ras could dimerize at more G-domain interfaces than what we have experimentally captured. Even just considering the three dimer interfaces shown in Figure 3A, the residues involved in these dimer interfaces are distributed all around the G-domain (Figure 3B). In addition, Ras may use variations to the  
Figure 3: Various Ras G-domain dimer interfaces. (**A**) Three common interfaces identified by computational and experimental studies. These major interfaces can be classified as the \(\alpha\)4-\(\alpha\)5 interface (left panel), the \(\alpha\)3-\(\alpha\)4 interface (middle panel), and the \(\beta\)2-\(\beta\)3 interface (right panel). Blue regions indicate dimerization interfaces, two identical KRas monomers are colored with two shades of orange for the sake of visual clarity. (**B**) Mapping interface residues for all three dimer interfaces shown in (**A**) on the Ras G-domain, with the left showing those for the \(\alpha\)4-\(\alpha\)5 and \(\alpha\)3-\(\alpha\)4 interfaces and the right showing those for the \(\beta\)2-\(\beta\)3. (**C**) Predicted heterodimer interfaces. Left and middle panels are two predicted HRas-GTP/HRas-GDP heterodimer structures, featuring symmetric \(\beta\)-interface (left) and an asymmetric \(\alpha\)-\(\beta\) interface (middle). On the right is a predicted structure for a KRas G12D-GTP/ KRas WT-GTP heterodimer. Predictions are made based on the methods described in [PERSON] et al. [80]. 'typical' dimer interfaces in certain scenarios, such as heterodimer formation (Figure 3C). Together, these data and analyses suggest that Ras dimerization may indeed be able to proceed with flexibility in the G-domain dimer interface. Among its other ramifications, the presence of multiple dimer interfaces suggests a potential mechanism for Ras to assemble into nanoclusters. [PERSON] et al. reported trimers of fully processed, guanosine-5\({}^{\prime}\)-(\(\beta\gamma\)-imino) triphosphate (GppNHp)-bound KRas (farnegylated and carboxymethyl) on artificial membranes via negative staining EM.