A cascade involving the enzymes Raf, MEK1, ERK results in the phosphorylation of Jun, conformational change allows Jun to bind to Fos and hence AP-1 to form. AP-1 then acts as transcription factor. Raf is activated via the second messenger DAG, SOS, and Ras. DAG recruits among other proteins the RAS guanyl nucleotide-releasing protein (RasGRP), a guanine nucleotide exchange factor (GEF), to the membrane. RasGRP activates the small GTPase Ras by exchanging Guanosine diphosphate (GDP) bound to Ras against Guanosine triphosphate (GTP). Ras can also be activated by the guanine nucleotide exchange factor SOS which binds to the LAT signalosom. Ras then initiates the MAPK cascade.