A component of the transcriptional mediator complex inhibits RAS-dependent vulval fate specification in C. elegans

Negative regulation of receptor tyrosine kinase (RTK)/RAS signaling pathways is important for normal development and the prevention of disease in humans. We have used a genetic screen in C. elegans to identify genes that antagonize the activity of activated LET-23, a member of the EGFR family of RTKs. We identified two loss-of-function mutations in dpy-22, previously cloned as sop-1, that promote the ability of activated LET-23 to induce ectopic vulval fates. DPY-22 is a glutamine-rich protein that is most similar to human TRAP230, a component of a transcriptional mediator complex. DPY-22 has previously been shown to regulate WNT responses through inhibition of the ß-catenin-like protein BAR-1. We provide evidence that DPY-22 also inhibits RAS-dependent vulval fate specification independently of BAR-1, and probably regulates the activities of multiple transcription factors during development. Furthermore, we demonstrate that although inhibition of BAR-1-dependent gene expression has been shown to require the C-terminal glutamine-rich region, this region is dispensable for inhibition of RAS-dependent cell differentiation. Thus, the glutamine-rich region contributes to specificity of this class of mediator protein

An Activating Mutation in sos-1 Identifies Its Dbl Domain as a Critical Inhibitor of the Epidermal Growth Factor Receptor Pathway during Caenorhabditis elegans Vulval Development

Proper regulation of receptor tyrosine kinase (RTK)-Ras-mitogen-activated protein kinase (MAPK) signaling pathways is critical for normal development and the prevention of cancer. SOS is a dual-function guanine nucleotide exchange factor (GEF) that catalyzes exchange on Ras and Rac. Although the physiologic role of SOS and its CDC25 domain in RTK-mediated Ras activation is well established, the in vivo function of its Dbl Rac GEF domain is less clear. We have identified a novel gain-of-function missense mutation in the Dbl domain of Caenorhabditis elegans SOS-1 that promotes epidermal growth factor receptor (EGFR) signaling in vivo. Our data indicate that a major developmental function of the Dbl domain is to inhibit EGF-dependent MAPK activation. The amount of inhibition conferred by the Dbl domain is equal to that of established trans-acting inhibitors of the EGFR pathway, including c-Cbl and RasGAP, and more than that of MAPK phosphatase. In conjunction with molecular modeling, our data suggest that the C. elegans mutation, as well as an equivalent mutation in human SOS1, activates the MAPK pathway by disrupting an autoinhibitory function of the Dbl domain on Ras activation. Our work suggests that functionally similar point mutations in humans could directly contribute to disease

The epidermal growth factor system in Caenorhabditis elegans

The single known epidermal growth factor-like growth factor and single epidermal growth factor receptor in Caenorhabditis elegans mediate two types of processes, each via a distinct signal transduction pathway. This pathway is subject to multiple redundantly acting positive and negative modulatory signals and, in addition, virtually, every step of signaling from receptor localization to communication to RNA Pol II is regulated in a precise manner. Several instances of cell fate specification during organogenesis require the RAS–MAP kinase pathway, as well as multiple nuclear factors. In contrast, appropriate myoepithelial contractions during ovulation involve IP3-mediated signal transduction. Positive modulators of the RAS pathway include KSR, SUR-8, phosphatase PP2A, and a zinc cation diffusion facilitator. Negative regulators of the RAS pathway include homologs of CBL, GAP-1, ACK, and MAP kinase phosphatase, while negative regulators of the IP3 pathway are enzymes that modify IP3. In addition to its stimulation of RAS activity, the GRB2 homolog SEM-5 acts negatively on both signaling pathways, as does the Ack-related kinase ARK-1

sli-3 Negatively Regulates the LET-23/Epidermal Growth Factor Receptor-Mediated Vulval Induction Pathway in Caenorhabditis elegans

The LIN-3–LET-23-mediated inductive signaling pathway plays a major role during vulval development in C. elegans. Studies on the components of this pathway have revealed positive as well as negative regulators that function to modulate the strength and specificity of the signal transduction cascade. We have carried out genetic screens to identify new regulators of this pathway by screening for suppressors of lin-3 vulvaless phenotype. The screens recovered three loci including alleles of gap-1 and a new gene represented by sli-3. Our genetic epistasis experiments suggest that sli-3 functions either downstream or in parallel to nuclear factors lin-1 and sur-2. sli-3 synergistically interacts with the previously identified negative regulators of the let-23 signaling pathway and causes excessive cell proliferation. However, in the absence of any other mutation sli-3 mutant animals display wild-type vulval induction and morphology. We propose that sli-3 functions as a negative regulator of vulval induction and defines a branch of the inductive signaling pathway. We provide evidence that sli-3 interacts with the EGF signaling pathway components during vulval induction but not during viability and ovulation processes. Thus, sli-3 helps define specificity of the EGF signaling to induce the vulva