Mitochondrial polyribonucleotide nucleotidyltransferase, also designated 3-5 RNA exonuclease, OLD35, PNPase or PNPT1, is an evolutionaly conserved protein in which the mouse protein shares 90% identity with the human version. PNPase participates in mRNA degradation and hydrolyzes single-stranded ribonucleotides in the 3 to 5 direction. PNPase forms homotrimers and is upregulated in response to interferon-beta induction. The N-terminus of PNPase contains a putative mitochondrial targeting sequence, mutation analysis confirms that N-terminal sequences of PNPase target the protein to the mitochondria. Endogenous PNPase also co-localizes with a mitochondrial marker protein in HeLa cells.
