Seeing is believing: Ras dimers observed in live cells.

The concept that Ras proteins may function as oligomers was first described almost three decades ago when Santos et al. (1) found evidence for wild-type or mutant H-Ras multimerization using radiation inactivation assays. The idea was largely dropped when the myriad of crystal structures of H-Ras that followed over the ensuing decades suggested that Ras proteins are functional monomers. Inouye et al. (2) briefly resurrected the idea when they reported detecting H-Ras dimers by cross-linking and protein fragmentation complementation. In addition, these authors reported that forced dimerization of nonmembrane-targeted H-Ras activated Raf-1 kinase activity. Also suggestive of signaling from higher-order assemblies of Ras proteins is an extensive literature from Hancock and colleagues (3–5) who analyzed the contribution of nanoclusters of five to eight Ras molecules to signaling through the two canonical Ras effector pathways, Raf–MEK–ERK and PI3K–AKT. More recently, two groups using molecular-dynamic modeling and a variety of biophysical methods have reported dimerization of the G domains of N-Ras (6) and H-Ras (7) when tethered via C-terminal lipid modifications to artificial phospholipid bilayers. Finally, Nussinov and colleagues (8), using similar techniques, very recently reported two different modes of dimerization of bacterially expressed, and therefore unprocessed, K-Ras4B in solution, one involving α-helices 3 and 4 and the other involving β-sheet interactions of the effector binding region. Importantly, both modes of dimerization were dependent on GTP binding. This flurry of activity was fueled in part by the recent discovery that Raf kinases, the Ras effectors that signal down the mitogen-activated protein kinase pathway, function as dimers (9–11). In PNAS, Nan et al. (12) lend another voice to the growing buzz. They use superresolution photoactivated localization microscopy (PALM) to reveal K-Ras4B dimers in living cells. Nan et al. begin by showing that, in cells in which exogenous expression of PAmCherry1tagged, mutationally activated K-Ras is under the control of a doxycycline-inducible promoter such that expression levels can be RAF