E3 Ubiquitin Ligase CHIP Drives Monoubiquitination-mediated Nuclear Import of Tumor Suppressor PTEN

Abstract

Nuclear localization of Phosphatase and Tensin Homolog Deleted on Chromosome TEN (PTEN) is crucial for its tumor suppressive activities. Monoubiquitination has been established as a principal mechanism driving nuclear translocation of PTEN. In this study, we describe a novel mechanism wherein Carboxy terminus of Hsc70 Interacting Protein (CHIP) mediates monoubiquitination of PTEN, leading to the latters nuclear import. Analysis of subcellular fractions of multiple cell lines revealed a rise in both nuclear and total cellular PTEN levels under monoubiquitination-promoting conditions, an effect that was abrogated when CHIP expression was silenced. We established a time-point kinetics of CHIP-mediated nuclear translocation of PTEN using immunocytochemistry. We identified a role of Karyopherin 1 (KPNA1) in facilitating nuclear transport of monoubiquitinated PTEN. We further established a direct interaction between CHIP and PTEN inside the nuclear compartment, where CHIP could participate in either polyubiquitination or monoubiquitination of the latter. CHIP was found to monoubiquitinate PTEN on K13 and K289 residues. Finally, we showed that CHIP-mediated monoubiquitination of PTEN resulted in increased apoptosis, decreased cell viability and proliferation. This enhancement of PTENs tumor suppressive abilities by CHIP involved modulation of transcriptional activities of PTEN. To the best of our knowledge, this is the first report elucidating CHIPs non-canonical roles on PTEN, which is also established here as a nuclear interacting partner of CHIP.