Human Raf1 is a proto-oncogene-encoded 74 kDa kinase that serves as an upstream activator of Mek1 by phosphorylating Ser-218 and Ser-222 in the MAP kinase kinase (Mek1). It has also been implicated in the phosphorylation and activation of the cdc2 tyrosine phosphatase cdc25. Raf1 binds to the GTP-bound form of the Ras G-protein via an N-terminal domain in Raf-1. Raf-1 is activated upon phosphorylation by protein kinase C, Src and ceramide-activated protein kinase. Raf-1 is highly related to the A-Raf and B-Raf isoforms within the N-terminal G-protein binding domain (CR1), phosphorylation site domain (CR2) and the catalytic domain (CR3). Raf-1 physically interacts with Ras, RapIB, MEK1, the 14-3-3 family of proteins and the molecular chaperones hsp90 and p50.
