 Adaptor proteins can modulate GEF activity by interacting with other domains besides the catalytic domain.For example, SOS1, the Ras GEF in the MAPK/ERK pathway, is recruited by the adaptor protein GRB2 in response to EGF receptor activation.The binding of SOS1 to GBR2 localizes it to the plasma membrane, where it can activate the membrane bound Ras.