Fadd
High risk
strains of human papillomavirus (HPV), such as HPV
16, cause human cervical carcinoma. The E6 protein of HPV 16 mediates the rapid
degradation of the tumor suppressor p53, although this is not the only function
of E6 and cannot completely explain its transforming potential. Previous work
in our laboratory has demonstrated that E6 can protect cells from TNF-induced
apoptosis by binding to the C-terminal end of TNF R1, thus blocking apoptotic
signal transduction. In this study, E6 was shown to also protect cells from
apoptosis induced via the Fas pathway. Furthermore,
use of an inducible E6-expression system demonstrated that this protection is
dose-dependent, with higher levels of E6 leading to greater protection. While
E6 suppresses activation of both caspase 3 and caspase 8, it does not affect apoptotic signaling through
the mitochondrial pathway. Mammalian two-hybrid and in vitro pull-down assays
were then used to demonstrate that E6 binds directly to the DED of FADD, with
deletion and site-directed mutants enabling the
localization of the E6 binding site to the N-terminal end of the FADD DED. E6
is produced in two forms: a full-length version of approximately 16 kD and a smaller version of about
half that size corresponding to the N-terminal half of the full-length protein.
Pull-down and functional assays demonstrated that the full-length, but not the
small version of E6 was able to bind to FADD and to protect cells from Fas-induced apoptosis. In addition, binding to E6 leads to
degradation of FADD, with the loss of cellular FADD proportional to the amount
of E6 expressed. These results support a model in which E6-mediated degradation
of FADD prevents transmission of apoptotic signals via the Fas
pathway.
PMID: 15073179 [PubMed - as supplied by publisher]