The SOS1 protein is responsible for stimulating the conversion of RAS from the inactive GDP-bound form (RAS(OFF)) to the active GTP-bound form (RAS(ON)) in response to growth factor receptor signaling. SOS1 directly activates RAS proteins by promoting the release of the bound GDP and thereby facilitating the binding of GTP, which is present within a cell in great excess to GDP, to generate RAS(ON).
SOS1 itself is activated by RAS through the binding of RAS(ON) to an allosteric site on the SOS1 protein. As a result, there is a positive feedback loop between SOS1 and RAS that increases RAS signaling. The activation of RAS by SOS1 is “processive”; that is, once a single molecule of SOS1 is activated it can sequentially activate multiple RAS molecules. As a result, the potential for amplification of RAS signals by SOS1 is considerable. Therefore, we believe that inhibition of SOS1 may represent a viable approach for targeting RAS-driven tumors.