Characterization of an internal ribosomal entry site in the coding sequence of tyrosyl-DNA phosphodiesterase 2
Tyrosyl DNA-phosphodiesterase 2 (TDP2) is a multifunctional protein that has been implicated in a myriad of cellular pathways. While most well-known for its phosphodiesterase activity removing stalled topoisomerase 2 from DNA, TDP2 has been shown to interact with both survival and apoptotic MAP kinase signaling cascades, facilitate enterovirus replication, and has been genetically linked to neurological diseases ranging from Parkinson's to dyslexia. To accurately evaluate TDP2 as a therapeutic target, it is important to understand how TDP2 acts in these disparate settings. Here we show that TDP2 expression is regulated at the translational level by the use of an internal ribosomal entry site (IRES) that initiates translation at codon 54, the second in-frame methionine of the TDP2 coding sequence. This IRES drives expression of a shorter, N-terminally truncated isoform of TDP2, ∆N-TDP2, which omits a nuclear localization sequence rendering it diffusely located throughout the cell. ∆N-TDP2 retains phosphodiesterase activity and is protective against etoposide-induced cell death, but co-immunoprecipitates with fewer high-molecular weight ubiquitinated species, suggesting partial loss-of-function of TDP2's ubiquitin association domain.