Inhibition of ubiquitin ligase Siah-1A by disabled-1

Disabled-1 (Dab1) is a cytosolic adaptor protein that plays critical roles in cortical development. However, a detailed mechanism of action has not yet been clearly defined. Through yeast two-hybrid screening, we observed that mouse Siah-1A, an E3 ubiquitin ligase containing a RING finger motif, interacts with Dab1. Co-immunoprecipitation experiments and in vitro binding experiments both indicated direct interaction between Siah and Dab1. Steady-state expression of Siah was enhanced by the presence of Dab1 or lactacystin, a representative proteasomal inhibitor. Auto-ubiquitination of Siah was inhibited by the presence of Dab1, suggesting inhibition of Siah activity and subsequent increase of Siah expression by Dab1. Both Dab1-induced increase of steady-state expression of Deleted in cololectal cancer (DCC), one of the well-known substrates of Siah, and its inhibition by SiahΔR suggest that Dab1 increases expression of DCC through inhibiting the activity of endogenous Siah. Our results suggest that Dab1 inhibits the activity of Siah.