EGF receptor signaling — a quantitative view Ben-Zion Shilo, Naama Barkai  Current Biology  Volume 17, Issue 24, Pages R1038-R1041 (December 2007) DOI: 10.1016/j.cub.2007.10.062 Copyright © 2007 Elsevier Ltd Terms and Conditions

Figure 1 The EGF receptor pathway in Drosophila. Four activating ligands have been identified; three are produced as inactive transmembrane precursors, which generate an active secreted ligand upon processing. Spitz is the major ligand and functions in most cases where the pathway is activated, Keren plays only a minor redundant role, and Gurken is usd exclusively during oogenesis. The fourth ligand, Vein, is produced as a secreted molecule, and is a weaker activating ligand used either to enhance signaling by other ligands or in distinct situations such as muscle patterning. The three membrane-anchored ligands are retained in the ER, and processed, following trafficking by the dedicated chaperone protein Star, to a late compartment in the secretory pathway. Here, the ligands encounter Rhomboids, serine proteases which cleave the ligand precursors within the transmembrane domain to release the active, secreted form. Rhomboids also cleave and inactivate Star, attenuating the level of cleaved ligand that is produced. Within a receiving cell, the ligands encounter the EGF receptor, which upon dimerization triggers the canonical Sos/Ras/Raf/MEK/MAP kinase pathway. The cardinal transcriptional output of the pathway is mediated by the ETS protein Pointed, which is either phosphorylated by MAP kinase to produce an active transcriptional activator (PointedP2), or transcriptionally induced by MAP kinase to produce a constitutive transcriptional activator (PointedP1). The ETS protein Yan acts as a constitutive repressor, which competes for Pointed-binding sites and can be removed from the nucleus and degraded upon phosphorylation by MAP kinase. Several negative regulators keep the pathway in check. Especially important is a group of inducible repressive elements, which constitute a negative feedback loop. Argos is a secreted molecule, which sequesters the ligand Spitz, Sprouty attenuates signaling within the receiving cell, and Kekkon is a transmembrane protein forming a non-functional heterodimer with the receptor. Current Biology 2007 17, R1038-R1041DOI: (10.1016/j.cub.2007.10.062) Copyright © 2007 Elsevier Ltd Terms and Conditions

Figure 2 Quantitative description of morphogen profile. (A) A model of morphogen in one dimension. The morphogen is secreted by a localized source, diffuses away from it and is subject to linear degradation. Shown is the exponential profile attained at steady state (note the logarithmic scale). Distinct cell-fates at a distance Δx are defined according to two concentration thresholds (L1, and L2). In response to perturbation in the morphogen production rate, the profile shifts by a constant distance δx (dashed line). This shift is proportional to the decay rate of the profile. Accordingly, the quantitative shape of the profile can be predicted by simply measuring this shift. (B) A gradient of EGF receptor (EGFR) activation in the Drosophila egg chamber. The kekkon gene is expressed at high levels of activation in the follicle cells, whereas pipe is repressed at high and intermediate levels of EGF receptor activation, and expressed only in regions of lower EGF receptor activation. The activation and repression thresholds are predicted to correspond to 60% and 20% of the maximal EGF receptor activation, respectively. (C) Cross-section through the egg, showing the different expression domains of kekkon and pipe. The oocyte nucleus is located at the dorsal-anterior corner. (D) Side view of the expression domains of kekkon and pipe. Current Biology 2007 17, R1038-R1041DOI: (10.1016/j.cub.2007.10.062) Copyright © 2007 Elsevier Ltd Terms and Conditions

Figure 3 Generating threshold responses by zero-order ultrasensitivity. (A) In a wild-type embryo (stage 10), the activating ligand Spi emanates from the ventral midline (arrowhead), triggering EGF receptor in the adjacent cells, and leading to graded activation of MAP kinase that is detected with dpERK antibodies (red). (B) Within the domain of MAP kinase activation (dashed white line), the degradation pattern of Yan (green, full line) at the same stage shows a much more restricted and sharp response. (C) A classical zero-order hypersensitivity model, showing the reversible conversion of a substrate between two states, and the dependence of the final product only on the difference between the rate of opposing enzymatic reactions. (D) In the case of the Yan degradation network, in addition to phosphorylation by MAP kinase (MAPK) and dephosphorylation by unknown proteases, aspects such as synthesis and degradation have to be considered. Similar to the classical model, a switch-like behavior is generated when the substrate is in excess with respect to the dissociation constants for the two opposing enzymes. Current Biology 2007 17, R1038-R1041DOI: (10.1016/j.cub.2007.10.062) Copyright © 2007 Elsevier Ltd Terms and Conditions