E6-AP is a founding member of HECT (homologous to E6-AP C terminus) domain subfamily of E3 ubiquitin ligases. It degrades tumor suppressor p53 in association with the E6 oncoprotein of the human papilloma virus. However, there are conflicting reports on its role in the degradation of p53 in the absence of E6 oncoprotein. Here, we studied the role of E6-AP in regulation of p53 in mouse neuro 2a cells. Overexpression of E6-AP in neuro 2a cells increased the ubiquitylation and degradation of p53, which could be prevented upon deletion of HECT domain. E6-AP also directly ubiquitylated p53 in an in vitro ubiquitylation assay. Partial knockdown of E6-AP increased the levels of p53 and p53-dependent transcription. Partial knockdown also increased neuronal cell death, which may be mediated partly via p53. Our result suggests that E6-AP not only enhances the degradation of p53 but also regulates the neuronal cell growth. © 2008 Birkhaueser.