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

Abstract

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.