REGULATION OF PURIFIED HEPATIC PC-1 (PHOSPHODIESTERASE-I NUCLEOTIDE PYROPHOSPHATASE) BY THREONINE AUTO(DE)PHOSPHORYLATION AND BY BINDING OFACIDIC FIBROBLAST GROWTH-FACTOR

The plasma cell differentiation antigen PC-1 was purified to homogenei ty from rat liver membranes. Denaturing electrophoresis revealed polyp eptides of 118 and 128 kDa, which were both recognized by antibodies a gainst recombinant murine PC-1. During gel filtration PC-1 migrated as a protein of about 500 kDa, suggesting a tetrameric structure. Purifi ed PC-1 displayed a phosphodiesterase-I/nucleotide pyrophosphatase act ivity that could be completely blocked by EDTA, dithiothreitol and aci dic fibroblast growth factor (extrapolated K-i = 1.3 nM). Purified PC- 1 was also capable of threonine autophosphorylation and of phosphoryla tion of histone IIa. The autophosphorylation of PC-1 was inhibited by addition of histone IIa, and it was blocked by phosphodiesterase-I inh ibitors (acidic fibroblast growth factor, dithiothreitol), by nucleoti des (ATP, ADP, AMP), and by vanadate. When added to autophosphorylated PC-1, these compounds caused a prompt dephosphorylation. However, the same agents did not affect the (de)phosphorylation of histone IIa, wh ich is not a substrate for the PC-1 phosphatase. These data indicate t hat phosphodiesterase-I inhibitors, nucleotides and vanadate affect th e (de)phosphorylation of PC-1 by stimulating the PC-1 phosphatase and/ or by shielding the autophosphorylation site from the PC-1 kinase. The rate of dephosphorylation of PC-1 was independent of the dilution, su ggesting an autocatalytic intramolecular process. We propose that the autophosphorylation of PC-1 serves to block its nucleotide pyrophospha tase activity when extracellular ATP becomes scarce.