Involvement of Rabring7 in EGF receptor degradation as an E3 ligase

Rab7, a member of the Rab family small G proteins, is involved in the late stage of the endocytic pathway. We previously identified a Rab7 target protein, Rabring7, which contains a RING finger domain at its C termini, but the precise role of Rabring7 remains unknown. In this study, we demonstrate using an in vitro ubiquitination assay with recombinant E1 and E2 proteins that Rabring7 has E3 ligase activity and that it preferentially reacts with Ubc4 and Ubc5 as its E2 proteins. Rabring7 ubiquitinated itself but not Rab7, and a mutation at Cys-229 in the RING finger domain (Rabring7C229S) completely diminished its E3 ligase activity. In the ligand-induced degradation of EGF receptor (EGFR), Rabring7 accelerated the degradation of EGFR, whereas Rabring7C229S inhibited the degradation induced by cCbl, another E3 ligase. These results suggest that Rabring7 is involved in the endocytic trafficking of EGFR through its E3 ligase activity.