SUMO stimulates the dephosphorylation of an inhibitory phosphoserine residue in Raf

Small ubiquitin-related modifier (SUMO) becomes covalently conjugated to many proteins regulating a variety of cellular and development processes. A previous study demonstrated that multiple components of the Ras/MAPK signaling pathway are directly conjugated to SUMO in the early Drosophila embryo, and that the Ras/MAPK signaling cascade requires SUMO for activation in insulin or Spitz-stimulated Drosophila S2 cells. SUMO knockdown leads to reduced MAPK phosphorylation in response to either signal. Here, we elucidate a mechanism by which SUMO positively regulates the Ras/MAPK signaling pathway by showing that the dephosphorylation of Raf at serine 346 to activate Ras/MAPK signaling requires SUMO. Furthermore, site-directed mutagenesis of serine 346 to alanine, preventing its phosphorylation, bypasses the requirement for SUMO in the activation of the Ras/MAPK pathway. Earlier studies have shown that serine 346 dephosphorylation is required for membrane localization of Raf, which is a pre-requisite for Raf activation. We show that fusing the Raf catalytic domain to the transmembrane and extracellular domains of Torso artificially tethering Raf to the plasma membrane restores Ras signaling in SUMO knockdown cells leading to SUMO independent Ras/MAPK signaling. We conclude that SUMO stimulates Ras/MAPK signaling by promoting the removal of an inhibitory phosphate group, which is a pre-requisite for Raf membrane localization and activation. While there is no evidence that Raf itself is sumoylated, several Raf interacting proteins, such as 14-3-3ζ and PP2A are members of the SUMO-ome suggesting an indirect role for SUMO in Raf localization.