The novel protein phosphatase PphA from Synechocystis PCC 6803 controls dephosphorylation of the signalling protein P-II

摘要

The family of P signal transduction proteins consists of one of the most highly conserved signalling proteins in nature. The cyanobacterial P homologue transmits signals on the nitrogen and carbon status of the cells through phosphorylation of a seryl residue. Recently, we identified a protein phosphatase 2C (PP2C) homologue from the cyanobacterium Synechocystis PCC 6803, termed PphA, to be the cellular phospho-P-II (P-II-P) phosphatase. In this investigation, we characterized the enzymatic properties of PphA and investigated the regulation of its catalytic activity towards P-II-P. PphA dephosphorylates phosphocasein and P-II-P with similar efficiency in a strictly Mg2+- or Mn2+-dependent reaction. Low-molecular-weight phosphorylated molecules are poor substrates for PphA. Its reactivity towards P-II-P, but not towards phosphocasein, is inhibited by various nucleotides, suggesting that this effect is based on specific properties of the P-II protein. The inhibitory effect of ATP can be strongly enhanced by the addition of 2-oxoglutarate or oxaloacetate. At low concentrations of 2-oxoglutarate, changes in the ATP levels within the physiological range affect the degree of P-II-Pase inhibition, whereas at 2-oxoglutarate levels beyond 0.1 mM, inhibition is almost complete at very low ATP levels. This suggests that P dephosphorylation is not only sensitive to 2-oxoglutarate and oxaloacetate levels, it also integrates signals from the energy charge of the cells under specific cellular conditions. [References: 33]