Role of FGD1, a Cdc42 Guanine Nucleotide Exchange Factor, in Epidermal Growth Factor-Stimulated c-Jun NH2-Terminal Kinase Activation and Cell Migration

Abstract

FGD1 encodes a guanine nucleotide exchange factor for Cdc42. Mutations in the FGD1 gene are responsible for an X-linked disorder known as Aarskog-Scott syndrome (AAS). While most mutations were found in the catalytic region, which consists of Dbl homology (DH) domain and adjacent pleckstrin homology (PH) domain, a missense mutation in the proline-rich domain is also found in a patient with typical clinical features as AAS. In this mutant FGD1, the serine residue at 205 is replaced with isoleucine. We recently demonstrated that FGD1 translocated to the membrane in response to extracellular stimuli such as epidermal growth factor (EGF) whereas FGD1 with S(205)/I substitution did not. Here we show that the proline-rich domain is critical for FGD1-induced directionally persistent cell migration. When inducibly expressed in HeLa Tet-Off cells, FGD1 stimulates directional migration whereas FGD1 with S(205)/I substitution does not affect it. We further demonstrate that FGD1 augments EGF-stimulated c-Jun NH(2)-terminal kinase (JNK) activation. In the presence of JNK inhibitor SP600125, motility of FGD1-expressing cells is significantly impaired, indicating a critical role of JNK in cell migration. However, FGD3, an FGD1 homologue lacking the proline-rich domain, and FGD1 with S(205)/I substitution augment EGF-stimulated JNK activation similarly to FGD1, suggesting that the proline-rich domain is not involved in the regulation of JNK. Finally, we show that FGD1, but not FGD1 with S(205)/I substitution, is phosphorylated in response to EGF, suggesting that the phosphorylation of S(205) may trigger the FGD1 translocation to the leading edge membrane and enable cells to undergo directional migration.