The ubiquitin-protein ligase E6-associated protein (E6-AP) serves as its own substrate

Recognition of substrate proteins by the ubiquitin-conjugation system is a highly specific and regulated event and involves the action of ubiquitin-conjugating enzymes (E2) and ubiquitin-protein ligases (E3). However, the E2 and E3 involved in the recognition of particular substrates have been identified in only a few cases. The ubiquitin-protein ligase E6-associated protein (E6-AP) was originally identified as a protein involved in the human papillomavirus E6-oncoprotein-induced degradation of p53. The substrate proteins of E6-AP in the absence of the E6 oncoprotein, however, have not been identified. We show here that E6-AP can target itself for ubiquitination in vitro and provide evidence that, under conditions of overexpression, E6-AP efficiently promotes its own degradation in vivo. Autoubiquitination of E6-AP is mediated mainly by intermolecular transfer of ubiquitin. In addition, highly ubiquitinated forms of E6-AP cannot bind to p53 in the presence of the E6 oncoprotein and, conversely, binding of E6-AP to p53 interferes with ubiquitination of E6-AP. These results suggest that autoubiquitination and subsequent degradation of E6-AP represents a mechanism to control intracellular E6-AP levels by inactivating E6-AP molecules that are not bound to substrate proteins.