Structural basis for activation of trimeric Gi proteins by multiple growth factor receptors via GIV/Girdin.
Bottom Line: We discovered a unifying mechanism that allows GIV/Girdin, a bona fide metastasis-related protein and a guanine-nucleotide exchange factor (GEF) for Gαi, to serve as a direct platform for multiple RTKs to activate Gαi proteins.Using a combination of homology modeling, protein-protein interaction, and kinase assays, we demonstrate that a stretch of ∼110 amino acids within GIV C-terminus displays structural plasticity that allows folding into a SH2-like domain in the presence of phosphotyrosine ligands.Expression of a SH2-deficient GIV mutant (Arg 1745→Leu) that cannot bind RTKs impaired all previously demonstrated functions of GIV-Akt enhancement, actin remodeling, and cell migration.
Affiliation: Department of Medicine, University of California, San Diego, School of Medicine, CA 92093.Show MeSH
Related in: MedlinePlus
Mentions: Besides this unique coexistence of GEF and SH2-like domains, GIV contains a growing list of many other of domains/motifs, cooperation between which could also be critical for its function as signal transducer (Figure 8). Here we demonstrated that GIV's GEF and SH2-like domains cooperate in the formation of RTK-GIV-Gαi ternary complexes and subsequent activation of Gαi. We also provided evidence that cooperation between GIV's SH2-like domain and its phosphotyrosines is essential for the formation of RTK-GIV-PI3K ternary complexes and subsequent activation of the PI3K/Akt pathway. That cells expressing SH2-deficient GIV mutants poorly remodel actin also suggests that SH2-like and actin-binding domains of GIV cooperate in linking EGF signaling to actin remodeling. It is likely that there are many more interdomain collaborations with the new discovered SH2-like domain—for example, the observed recruitment of GIV to the PM after growth factor stimulation is likely to be cooperatively mediated by its phosphatidylinositol 4-phosphate (PI4P) binding (Enomoto et al., 2005) and the SH2 domains. The coiled-coil domain, which mediates homo-oligomerization (Enomoto et al., 2005), could cooperate with the SH2-like domain to enhance clustering of RTK-GIV complexes at the PM. We propose that the SH2-like domain is key to achieve targeting and proximity of GIV to RTKs, which allows GIV to integrate incoming signals from these receptors and modulate them via activation of G proteins in the vicinity of the receptors. Examples of such interdomain collaborations were reported in the case of other multimodular signal transducers that bridge RTKs and small G proteins, such as Ras-GAP (Schlessinger and Lemmon, 2003), which inactivates Ras in the vicinity of ligand-activated receptors.
Affiliation: Department of Medicine, University of California, San Diego, School of Medicine, CA 92093.