The papillomavirus E6 oncoproteins are small zinc-binding proteins that share a conserved zinc-binding CXXC motif and lack intrinsic enzymatic activity. They are thought to act as adapter proteins, thereby altering the function of E6-associated cellular proteins. This model for E6 function is mostly supported by observations of human papillomavirus type 16 (HPV-16) E6 (16E6), which can alter the metabolism of the p53 tumor suppressor through association with a cellular E3 ubiquitin ligase called E6AP. HPV-16 E6 interacts with an 18-amino-acid sequence in E6AP, which leads to the formation of the E6AP-16E6 complex which in turn binds to p53, inducing the ubiquitin-dependent degradation of the trimolecular complex. It is stated that 16E6 functions as an adapter protein in the complex with p53 because E6AP does not interact with p53 in the absence of E6, hence degradation of p53 requires both E6 and E6AP PMID:26789255,PMID:15664194,PMID:17023019.