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 colorectal cancer (DCC), one of the well-known substrates of Siah, and its inhibition by Siah Delta R suggest that Dab1 increases expression of DCC through inhibiting the activity of endogenous Siah. Our results suggest that Dab1 inhibits the activity of Siah.