Re-evaluation of PTEN as an ADP-ribosylated tankyrase binding partner.

Abstract

Tankyrases establish an intricate network of protein interactions through their ankyrin repeat cluster domains (ARCs), which bind protein partners containing a characteristic peptide defined as a tankyrase-binding motif (TBM). Once the protein complex has been formed, the proteins bound to ARCs can either undergo ADP-ribosylation by tankyrases or stay unmodified. In the past years, this web of tankyrase-centered interactions has grown as new partners have been discovered. Since the catalytic and scaffolding functions of tankyrases are extensively studied and tankyrases are targets for inhibition for therapeutic purposes, it is fundamental to explore and validate which proteins are regulated by tankyrases. In this study, we analyzed the tumor suppressor phosphatidylinositol 3,4,5-trisphosphate 3-phosphatase and dual-specificity protein phosphatase PTEN as a previously reported tankyrase binding protein and substrate. Inhibition of tankyrase could stabilize PTEN, and this has been implied as a possible therapeutic strategy for using tankyrase inhibitors in cancer. However, we reveal that the described PTEN putative TBM (pTBM) does not bind tankyrase ARCs. This result is consistent with evolutionary analysis, which indicates that pTBM originated before tankyrases, unlike other validated TBMs. We employ pull-down and ADP-ribosylation assays to demonstrate that PTEN does not form a complex with tankyrase in vitro and that PTEN is not ADP-ribosylated by tankyrase. Finally, we show that, in contrast to what was previously reported, catalytic inhibition of tankyrases does not have an impact on PTEN endogenous protein levels, excluding in this way any direct connection between PTEN and tankyrases.