Human homologs of BVP and Cdc14: Characterization of new dual specificity phosphatases

Originally characterized as a dual specificity protein phosphatase (DSP) from the Autographa californica nuclear polyhedrosis virus, BVP is now known to hydrolyze the γ and β phosphates from the 5 ′-end of RNA. In a screen for human homolog(s) of BVP, we isolated a 1.6 kb cDNA that encodes a protein identical to PIR1, a nuclear protein that can associate with RNA or ribonucleoprotein complexes [Yuan et al., J. Biol. Chem. (1998) 272, 20347–20353]. We demonstrate that like BVP, human PIR1 uses an enzymatic mechanism that is similar to the protein tyrosine phosphatases (PTPs) to remove two phosphates from the 5 ′-triphosphate end of RNA, but not from mononucleotide triphosphates. The specific activity of PIR1 with RNA is three to four orders of magnitude greater than that with the best protein substrates examined, suggesting that PIR1 is involved in RNA metabolism rather than protein dephosphorylation. The RNA phosphatase activities of PIR1 are inhibited by sodium vanadate, an inhibitor of the PTPs. A 120 amino acid C-terminal segment of PIR1 is not required for RNA phosphatase activity but may play a role in targeting this enzyme to the nucleus. Based on similarities in sequence, we propose that human PIR1, BVP and their homologs constitute a unique group of RNA 5 ′-phosphatases. The highly conserved essential CDC14 gene of the budding yeast, Saccharomyces cerevisiae, encodes a DSP that is required for exit from mitosis. Two human homologs of the yeast protein designated HCdc14A1 and HCdc14B1 have been previously identified. Our laboratory has isolated two additional variants for each of the HCdc14A1 and HCdc14B1 isoforms. We have examined the expression of transcripts corresponding to these variants and present evidence suggesting that three distinct variants are generated from each of two human Cdc14 genes. We have confirmed the localization of the HCdc14A gene to Chromosome 1 p21 and present data that suggest the HCdc14B gene is present on human chromosome 9q22. The six human homologs identified to date are unable to carry out the critical cell cycle function of the yeast protein as shown by their inability to complement a yeast cdc14 null mutant. We have also examined the potential modes of regulation of PTP 1B, a tyrosine-specific PTP. Our in vitro studies demonstrate that the proline-rich region located within the C-terminal noncatalytic domain of PTP 1B, but not the closely related TCPTP, binds to Src homology 3 domains from three proteins, expressed in bacteria as glutathione S-transferase fusion proteins. The binding of these domains did not appear to have a significant impact on the activity of PTP 1B. We also isolated a particulate fraction-enriched protein of 50 kDa that interacts with the proline-rich region of PTP 1B