Conspiracy Theory: RAS and RAF Do Not Act Alone

In the early 1990s, genetic analysis in D. melanogaster and C. elegans helped define a signaling pathway from cell surface receptors to the nucleus. Cell surface receptors with intrinsic tyrosine kinase activity (receptor tyrosine kinases or RTKs) respond to peptide ligands, growth factors, and inductive signals in development. Activation of RTKs often leads to activation of RAS, which in its GTP-bound state activates effectors, the proteins that exert its biological effect. The identification of SOS (Son of sevenless) as a guanine nucleotide exchange factor for RAS, and the adaptor protein GRB2 (in mammals)/SEM-5 (in C. elegans)/DRK (in Drosophila), coupled with the finding that these two proteins act downstream of RTKs and upstream of RAS, allowed the biochemical linking of RTKs to RAS activation. The adaptor GRB2 binds to proteins phosphorylated on tyrosine by RTKs and thereby recruits SOS to the membrane, allowing it to activate RAS. Similarly, the finding that the serine/threonine protein kinase RAF acts downstream of RAS led to its identification as a bona fide effector for RAS. Activation of RAS results in the recruitment of RAF to the plasma membrane where it is activated by a not-well-defined mechanism that involves lipid second messengers and phosphorylation. The genetic results that led to the definition of this major tyrosine kinase-RAS signaling pathway were the identification of genes with similar or identical mutant phenotypes, and the ordering of those genes into a linear pathway using double mutant analysis. For example, activated RAS bypassed the requirement for the receptor tyrosine kinase, and inactive RAS blocked signaling by an activated receptor. Thus, RAS was inferred to be a downstream target of signaling by the receptors such as EGF receptor (EGFR) and FGF receptor (FGFR). Similarly, RAF, MEK (MAP/ERK kinase), and MAP (mitogen-activated protein) kinase (together constituting one version of a MAP kinase cascade) are required for signaling by activated RAS, and GRB2 and SOS are necessary to couple the activated receptor to RAS. While these proteins would apparently be sufficient for signaling from the surface to the nucleus, over the last few years new components have been added continually, gradually burdening the simple view of this broadly used signaling pathway. For example, KSR (Kinase suppressor of RAS, a RAF-like serine/threonine protein kinase) and 14-3-3 (an abundant cytoplasmic protein named after a spot on a two-dimension