The highly conserved region within exonuclease III-like in PML-I regulates the cytoplasmic localization of PML-NBs.

The sub-nuclear protein structure PML-NB regulates a wide range of important cellular functions, while its abnormal cytoplasmic localization may have pathological consequences. However, the nature of this aberrant localization remains poorly understood. In this study, we unveil that PML-I, the most conserved and abundant structural protein of PML-NB, possesses potent cytoplasmic targeting ability within the N-terminal half of the exonuclease III-like domain encoded by its unique exon 9, independent of the known nuclear localization signal. Fusion of this region to PML-VI can relocate PML-VI from the nucleus to the cytosol. Structural and deletion analysis revealed that the cytoplasmic targeting ability of this domain was restrained by the sequences encoded by exon 8a and the 3' portion of exon 9 in PML-I. Deletion of either of these regions relocates PML-I to the cytosol. Furthermore, we observed a potential interaction between the ER-localized TREX1 and the cytoplasmic-located PML-I mutants. Our results suggest that perturbation of the EXO-like domain of PML-I may represent an important mode to translocate PMLs from the nucleus to the cytosol, thereby interfering with the normal nuclear functions of PML-NBs.