Three different Raf (EC 2.7.11.1) isoforms originating from 3 independent genes can be distinguished in mammals: Raf-1/c-Raf, B-Raf, and A-Raf. They are bona fide Ras (a membrane-associated guanine nucleotide-binding protein) effectors and upstream activators of the ubiquitous ERK pathway, which has drawn the attention to these proteins as potential targets in cancer therapy. All Raf isoforms share a common modular structure consisting of 3 conserved regions (CR) with distinct functions. CR1 contains a Ras-binding domain (RBD), which is necessary for the interaction with Ras and with membrane phospholipids required for membrane recruitment, and a cysteine-rich domain (CRD), which is a secondary Ras-binding site and also necessary for the interaction of CR1 with the kinase domain for Raf auto-inhibition. CR2 contains important inhibitory phosphorylation sites participating in the negative regulation of Ras binding and Raf activation. CR3 features the kinase domain, including the activation segment, whose phosphorylation is crucial for kinase activation. The common and key step in the activation of all 3 Raf isoforms is membrane recruitment by a Ras family protein. In turn, activated Raf kinases phosphorylate both MEK isoforms MEK1 and MEK2 on 2 residues in the activation loop, which in turn can bind, phosphorylate, and activate ERK[1].
Raf kinase inhibitor protein (RKIP) is a member of the phosphatidylethanolamine-binding protein (PEBP) family that  interacts with a number of different proteins and regulates multiple signaling pathways. PEBP was identified as a physiologically relevant inhibitor of Raf-MEK-ERK and renamed RKIP. It binds specifically to the Raf-1 kinase, although it is not a direct substrate of Raf. RKIP inhibits the kinase activity of Raf-1 by dissociating the Raf-1/MEK complex and acting as a competitive inhibitor of MEK phosphorylation. What’s more, RKIP can bind to the N-region of the Raf-1 kinase domain therebye inhibiting its activation. Besides its role in the Raf-MEK-ERK signaling cascade, it has been shown that  (1) RKIP also antagonizes NF–κB signaling by interacting with several upstream kinases that regulate the IκB protein, (2) has a positive effect on heterotrimeric G protein-dependent and GSK signaling, (3) inhibits the activation phosphorylation of the transcriptional factor STAT3, (4) and activates Nrf2 by destabilizing the BTB domain containing protein Keap1[2].