Dual targeting of Saccharomyces cerevisiae Pso2 to mitochondria and the nucleus, and its functional relevance in the repair of DNA interstrand crosslinks

Abstract

Repair of DNA interstrand crosslinks (ICLs) involves a functional interplay among different DNA surveillance and repair pathways. Previous work has shown that ICL- inducing agents cause damage to Saccharomyces cerevisiae nuclear and mitochondrial DNA (mtDNA), and its pso2/snm1 mutants exhibit a petite phenotype followed by loss of mtDNA integrity and copy number. Complex as it is, the cause and underlying molecular mechanisms remains elusive. Here, by combining a wide range of approaches with in vitro and in vivo analyses, we assessed the subcellular localization and function of Pso2. We found evidence that the nuclear-encoded Pso2 contains one mitochondrial targeting sequence (MTS) and two nuclear localization signals (NLS1 and NLS2), although NLS1 resides within the MTS. Further analysis revealed that Pso2 is a dual-localized ICL repair protein; it can be imported into both nucleus and mitochondria, and that genotoxic agents enhance its abundance in the latter. While MTS is essential for mitochondrial Pso2 import, either NLS1 or NLS2 is sufficient for its nuclear import; this implies that the two NLS motifs are functionally redundant. Ablation of MTS abrogated mitochondrial Pso2 import, and concomitantly, raised its levels in the nucleus. Strikingly, mutational disruption of both NLS motifs blocked the nuclear Pso2 import; at the same time, they enhanced its translocation into the mitochondria, consistent with the notion that the relationship between MTS and NLS motifs is competitive. However, the nuclease activity of import-deficient species of Pso2 was not impaired. The potential relevance of dual-targeting of Pso2 into two DNA-bearing organelles is discussed.