Regulatory input at the level of MEK, PTP-ER, and MAPK expression adds another layer to the network of factors that control RAS/MAPK signaling.

<p>Schematic model of proteins associated with RAS/MAPK signal transmission discussed in this work. Components used in secondary screens (GAP1, NF1) are also depicted. <i>Sur-8</i>, <i>PP1-87B</i>, and the five STRIPAK complex components were positioned between RAS and RAF in our epistasis assays. Their position would be consistent with a role in the RAF activation process. As it has been previously shown in mammalian models, SUR-8 and PP1 may be acting on RAF activation by dephosphorylating the N-terminal 14-3-3 binding site. Because PP2A is also known to dephosphorylate this site and because STRIPAK has been characterized as a PP2A-associated complex, STRIPAK may be involved in facilitating PP2A binding to RAF. GFZF was positioned at the level of MEK and was found to impact MEK expression, presumably by regulating <i>mek</i> transcription. CG4936 was found to impact expression of the MAPK phosphatase, PTP-ER, and also probably acts at the level of transcriptional regulation. CG1603, FIP1, and CDK12 were found to act on MAPK expression, most likely acting as transcriptional regulators (CG1603) or involved in transcript maturation/processing (FIP1 and CDK12). Finally, components of the spliceosome, splicing factors and the EJC were found to modulate MAPK expression by altering the splicing of the <i>mapk</i> transcript. The particular sensitivity of MAPK to disruption of these spliceosome components may be due to their involvement in recruiting specific mRNA processing factors such as the EJC. Alternatively, the reason why <i>mapk</i> displays an increased requirement for this set of spliceosome components may be due to a feature in <i>mapk</i>'s gene structure. For example, intron length is correlated with sensitivity of transcripts to EJC depletion <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001809#pbio.1001809-AshtonBeaucage1" target="_blank">[41]</a>.