Raf promotes dimerization of the Ras G-domain

The Ras/Raf/MEK/ERK pathway is a major regulator of cell proliferation and is highly implicated in human cancers. Ras dimerization was first proposed thirty years ago, and it is known that Ras dimerization is essential for Raf activation, yet the mechanism through which this occurs has remained elusive. Attempts to detect Ras dimers on supported membranes have led to the conclusion that Ras does not dimerize in the absence of other factors. Here we show that the Ras binding domain of Raf (Raf‐RBD) is the key element prompting Ras dimerization. A small amount of Ras/Raf‐RBD dimer is seen in solution, while the addition of Raf‐RBD to Ras on supported lipid membranes produces robust levels of dimers. On the membrane, Ras/Raf‐RBD binding and Ras dimerization appear to be concerted events that lead to a high‐affinity signaling complex. Molecular dynamics simulations of Ras and Raf‐RBD alone, in a complex, and in the dimer reveal that the proteins become increasingly connected with higher levels of allosteric networks. This is apparent in the increasing connectivity between the helices involved in dimerization, helices 4 and 5, developing more consistent communication with the phosphate binding loop in the Ras active site. These data reveal a novel mode of regulation of the mitogenic Ras/Raf/MEK/ERK pathway. Support or Funding Information NSF‐MCB‐1517295