tokens: We conclude that GRF1 and GRF2 can form homo - and hetero - oligomers via their DH domains , that mutational inactivation of oligomer formation by GRF1 is associated with impaired biological and signaling activities , and that in 293T cells GRF1 mediates at least two pathways for Raf activation : one a constitutive signal that is mainly Ras - dependent , and one an ionomycin - induced signal that cooperates with the constitutive signal without further augmenting the level of GTP - Ras .