Structure of the ERK pathway (adapted from http://www.sabiosciences.com/). Upon ligand binding, RTK autophosphorylates on tyrosine residues, which serve as docking sites for adaptor and signalling molecules. Ras is activated by the recruitment of guanosine-nucleotide exchange factors (SOS, C3G) via adaptor proteins (Shc and Grb2; Crk). Ras can activate Raf-1 and B-Raf; Rap1 presumably can activate B-Raf. Raf proteins phosphorylate and activate MEK-1/2, which in turn activate ERK-1/2 (indicated by black arrows).
