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¹ School of Pharmacy, Tokyo University of Pharmacy and Life Science, Hachioji, Tokyo, Japan
² School of Life Science, Tokyo University of Pharmacy and Life Science, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan
³ Department of Molecular Pathology, Graduate School of Medicine, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁴ Department of Biochemistry 1, Hamamatsu University School of Medicine, 1-20-1 Handayama, Hamamatsu, 431-3192, Japan

We previously demonstrated that FGD1, the Cdc42 guanine nucleotide exchange factor (GEF) responsible for faciogenital dysplasia, is targeted by the ubiquitin ligase SCF^FWD1/β-TrCP upon phosphorylation of two serine residues in its DSGIDS motif and subsequently degraded by the proteasome. Here we show that FGD3, which was identified as a homologue of FGD1 but has been poorly characterized, has conserved the same motif and is down-regulated similarly by SCF^FWD1/β-TrCP. Although FGD3 and FGD1 share strikingly similar Dbl homology (DH) domains and adjacent pleckstrin homology (PH) domains, both of which are responsible for guanine nucleotide exchange, there also exist remarkable differences in their structures. Indeed, FGD1 and FGD3 induced significantly different morphological changes in HeLa Tet-Off cells: whereas FGD1 induced long finger-like protrusions, FGD3 induced broad sheet-like protrusions when the level of GTP-bound Cdc42 was significantly increased by the inducible expression of FGD3. Furthermore, FGD1 and FGD3 reciprocally regulated cell motility: when inducibly expressed in HeLa Tet-Off cells, FGD1 stimulated cell migration whereas FGD3 inhibited it. Thus we demonstrate that the highly homologous GEFs, FGD1 and FGD3 play different roles to regulate cellular functions but that their intracellular levels are tightly controlled by the same destruction pathway through SCF^FWD1/β-TrCP.

^* Correspondence: Email: hayakawa{at}ps.toyaku.ac.jp