The adaptor protein soc-1/Gab1 modifies growth factor receptor output in Caenorhabditis elegans

Abstract

Previous genetic analysis has shown that dos/soc-1/Gab1 functions positively in receptor tyrosine kinase (RTK)-stimulated Ras/Map kinase signaling through the recruitment of csw/ptp-2/Shp2. Using sensitized assays in Caenorhabditis elegans for let-23/Egfr and daf-2/InsR (insulin receptor-like) signaling, it is shown that soc-1/Gab1 inhibits phospholipase C-gamma (PLCgamma) and phosphatidylinositol 3'-kinase (PI3K)-mediated signaling. Furthermore, as well as stimulating Ras/Map kinase signaling, soc-1/Gab1 stimulates a poorly defined signaling pathway that represses class 2 daf-2 phenotypes. In addition, it is shown that SOC-1 binds the C-terminal SH3 domain of SEM-5. This binding is likely to be functional as the sem-5(n2195)G201R mutation, which disrupts SOC-1 binding, behaves in a qualitatively similar manner to a soc-1 null allele in all assays for let-23/Egfr and daf-2/InsR signaling that were examined. Further genetic analysis suggests that ptp-2/Shp2 mediates the negative function of soc-1/Gab1 in PI3K-mediated signaling, as well as the positive function in Ras/Map kinase signaling. Other effectors of soc-1/Gab1 are likely to inhibit PLCgamma-mediated signaling and stimulate the poorly defined signaling pathway that represses class 2 daf-2 phenotypes. Thus, the recruitment of soc-1/Gab1, and its effectors, into the RTK-signaling complex modifies the cellular response by enhancing Ras/Map kinase signaling while inhibiting PI3K and PLCgamma-mediated signaling.

Figure 1.

Schematic showing the effect of Gab1 on Ras/Map kinase-mediated signaling as revealed by genetic studies. Additional effects on PLCγ-, PI3K-, and DAF-2B-mediated signaling are derived from this study (thick lines). Arrows represent a positive/activating interaction. Lines ending in a perpendicular represent a negative/inhibitory interaction. IP₃ receptor, inositol trisphosphate receptor. Dashed arrow shows that there is evidence that Gab1 can bind the Met RTK, but not other RTKs directly (Weidner et al. 1996).

Figure 2.

The C-terminal SH3 domain of SEM-5/Grb2 binds an atypical proline-rich motif within SOC-1/Gab1. (A) Alignment of atypical proline-rich motifs of Gab1 and Dos with a similar sequence found in SOC-1. Gab1 (337–351) constitutes a weak Grb2-binding site, and Gab1(514–528) binds strongly (Lock et al. 2000). Both Dos sites shown contribute to Drk/Grb2 binding equally (Feller et al. 2002). Residues conserved in three of the four Gab1/Dos sites are in boldface type, as are the corresponding residues in SOC-1. (B) Domain structures of SOC-1 and SEM-5 showing location of mutations used in this study. Thick horizontal line shows regions of SOC-1 used in the yeast two-hybrid assay. Bubbles highlight proline-rich motifs. Vertical lines indicate positions of tyrosines. (C) Yeast two-hybrid analysis of the interaction between SEM-5 and SOC-1. Bait two-hybrid constructs were cloned into pGBKT7 and prey proteins were cloned into pGAD424 (CLONTECH) and two-hybrid analysis was performed as described previously (Hopper et al. 2000). SOC-1 (179–268) contains two PxxP motifs, most notably PPVPPPR at residues 236–242. The SEM-5 G201R construct bears the same mutation as found in the sem-5(n2195) allele.