Fgd1, the Cdc42 guanine nucleotide exchange factor responsible for faciogenital dysplasia, is localized to the subcortical actin cytoskeleton and Golgi membrane

Abstract

FGD1, the gene responsible for the inherited disease faciogenital dysplasia, encodes a guanine nucleotide exchange factor (GEF) that specifically activates the p21 GTPase Cdc42. In order, FGD1 is composed of a proline-rich N-terminal region, adjacent GEF and pleckstrin homology (PH) domains, a FYVE-finger domain and a second C-terminal PH domain (PH2), structural motifs involved in signaling and subcellular localization. Fgd1, the mouse FGD1 ortholog, is expressed in regions of active bone formation within osteoblasts and in the osteoblast-like cell line MC3T3-E1, a finding consistent with its role in skeletal formation. Here, we use subcellular fractionation studies to show that endogenous Fgd1 protein is localized in the cytosolic and Golgi and plasma membrane fractions of mouse calvarial cells. Immunocytochemical studies performed with osteoblast-like MC3T3-E1 cells and other mammalian cell lines confirm the localization of Fgd1 and show that the proline-rich N-terminal region is necessary and sufficient for Fgd1 subcellular localization to the plasma membrane and Golgi complex. In contrast, the FYVE-finger and PH2 domains do not appear to direct the localization of Fgd1 or the activation of Cdc42. In addition, microinjection studies indicate that the N-terminal Fgd1 domain inhibits filopodia formation, suggesting that this region down-regulates GEF function. These results characterize the function of the Fgd1 domains for both protein localization and Cdc42 activation and indicate that the Fgd1 Cdc42GEF protein is involved in the regulation of Cdc42 activity at the subcortical actin cytoskeleton and Golgi complex.

Faciogenital Dysplasia Protein (FGD1) Regulates Export of Cargo Proteins from the Golgi Complex via Cdc42 Activation

“…FGD1 has been hypothesized to coordinate membrane transport and the actin cytoskeleton during embryogenesis (Estrada et al, 2001), and it has been implicated in skeletal development, with mutations in FGD1 leading to faciogenital dysplasia (FGDY; Aarskog-Scott syndrome). FGDY is an X-linked developmental disorder that is characterized by a disproportionately short stature and by facial, skeletal, cardiac, ocular and urogenital anomalies (Aarskog, 1970;Scott, 1971).…”

“…FGD1 comprises (in order): a proline-rich N-terminal region; adjacent GEF (Dbl-homology, DH) and pleckstrin homology (PH) domains; a FYVE (Fab1p, YOTB, Vac1p, and EEA1)-finger domain; and a second C-terminal PH domain (PH2; Estrada et al, 2001). Most of these structural motifs are known to be involved in signaling and/or subcellular localization.…”

“…Most of these structural motifs are known to be involved in signaling and/or subcellular localization. Indeed, most of the FGD1 mutations identified to date are in the DH/PH region (Orrico et al, 2000), the portion of FGD1 that is responsible for the specific activation of the Rho GTPase Cdc42 (the DH domain) and for membrane binding (the PH domain; Estrada et al, 2001).…”

“…Given that FGD1 has been localized to the organelles of secretory pathway, including the Golgi complex (Estrada et al, 2001), it might be involved in the regulation of intracellular membrane transport during cell differentiation and therefore be required for normal morphogenesis of bone tissue. Importantly, the main FGD1 target, Cdc42, has a fundamental role in the coordination of membrane transport events, via reorganization of the cytoskeleton and other signaling events (Erickson and Cerione, 2001).…”

Traffic Jams II: An Update of Diseases of Intracellular Transport

“…Genetic analysis of the yeast OS-9 homolog, YOS-9, suggests that the protein specifically supports the ER export of GPIanchored proteins and has been shown genetically to interact with the Sec34/Sec35 complex which is involved in targeting of ER vesicles to the Golgi complex. (43) localized to the subcortical actin cytoskeleton and the Golgi, suggesting that it may be involved in regulating Cdc42 activity in these locations.…”