Human papillomavirus type 16 E6-induced degradation of E6TP1 correlates with its ability to immortalize human mammary epithelial cells

Recent analyses have identified a number of binding partners for E6, includ ing E6AP, ERC55, paxillin, hDlg, p300, interferon regulatory factor 3, hMCM 7, Bak, and E6TP1. Notably, association with E6 targets p53, E6TP1, myc, hM CM7, and Bak for degradation. However, the relative importance of the vario us E6 targets in cellular transformation remains unclear. E6 alone can domi nantly immortalize normal human mammary epithelial cells (MECs), permitting an assessment of the importance of various E6 targets in cellular transfor mation. Studies in this system indicate that E6-induced degradation of p53 and E6 binding to ERC55 or hDlg do not correlate with efficient immortaliza tion. Here, we have examined the role of E6TP1, a Rap GTPase-activating pro tein, in E6-induced immortalization of MECs. We tested a large set of human papillomavirus type 16 E6 mutants for their ability to bind and target E6T P1 for degradation in vitro and in vivo. We observed a strict correlation b etween the ability of E6 protein to target E6TP1 for degradation and its ab ility to immortalize MECs. Recent studies have identified telomerase as a t arget of E6 protein. Previous analyses of E6 mutants have revealed this tra it to closely correlate with MEC immortalization. We examined our entire pa nel of E6 mutants for rapid induction of telomerase activity and found in g eneral a strong correlation with immortalizing ability. The tight correlati on between E6TP1 degradation and MEC immortalization strongly supports a cr itical role of functional inactivation of E6TP1 in E6-induced cellular immo rtalization.