Mutations in
faciogenital dysplasia protein (Fgd1) result in the human disease
faciogenital dysplasia (FGDY). Fgd1 contains a
RhoGEF domain specific for Cdc42. Fgd1 also contains a Src homology (SH3) binding domain (SH3-BD) that binds directly to the SH3 domain of
cortactin, which promotes actin assembly by actin-related
protein (Arp)2/3 complex. Here, we report the effect of
ligation of
cortactin's SH3 domain by the Fgd1 SH3-BD on actin polymerization in vitro.
Glutathione S-transferase (GST)-fused Fgd1 SH3-BD enhanced the ability of
cortactin to stimulate Arp2/3-mediated actin polymerization. However, a synthetic
peptide containing only the SH3-BD sequence had no effect. The SH3-BD
peptide bound to
cortactin and inhibited the effect of GST-Fgd1 SH3-BD, suggesting that GST dimerization was responsible for the stimulating effect of GST-Fgd1 SH3-BD. When GST-Fgd1 SH3-BD was prepared as a heterodimer with a control GST fusion
protein (GST-Pac1), no stimulatory effect on actin polymerization was observed. In addition, when
cortactin was dimerized via its N-terminus, away from the C-terminal SH3 domain, actin polymerization with
Arp2/3 complex increased markedly, compared to free
cortactin. Thus,
cortactin ligated by Fgd1 is fully active, indicating that the cell can use Fgd1 to target actin assembly. Moreover, if Fgd1 is multimerized, then
cortactin's activity should be enhanced. Fgd1 and
cortactin may participate as scaffolds and signal transducers in a positive feedback cycle to promote actin assembly at the cell cortex.