Cyclic phosphodiesterase having 3'-nucleotidase activity from Bacillus subtilis. Purification and some properties of the enzyme

The Characterization of Escherichia coli CpdB as a Recombinant Protein Reveals that, besides Having the Expected 3´-Nucleotidase and 2´,3´-Cyclic Mononucleotide Phosphodiesterase Activities, It Is Also Active as Cyclic Dinucleotide Phosphodiesterase

Endogenous cyclic diadenylate phosphodiesterase activity was accidentally detected in lysates of Escherichia coli BL21. Since this kind of activity is uncommon in Gram-negative bacteria, its identification was undertaken. After partial purification and analysis by denaturing gel electrophoresis, renatured activity correlated with a protein identified by fingerprinting as CpdB (cpdB gene product), which is annotated as 3´-nucleotidase / 2´,3´-cyclic-mononucleotide phosphodiesterase, and it is synthesized as a precursor protein with a signal sequence removable upon export to the periplasm. It has never been studied as a recombinant protein. The coding sequence of mature CpdB was cloned and expressed as a GST fusion protein. The study of the purified recombinant protein, separated from GST, confirmed CpdB annotation. The assay of catalytic efficiencies (k_cat/K_m) for a large substrate set revealed novel CpdB features, including very high efficiencies for 3´-AMP and 2´,3´-cyclic mononucleotides, and previously unknown activities on cyclic and linear dinucleotides. The catalytic efficiencies of the latter activities, though low in relative terms when compared to the major ones, are far from negligible. Actually, they are perfectly comparable to those of the ‘average’ enzyme and the known, bona fide cyclic dinucleotide phosphodiesterases. On the other hand, CpdB differs from these enzymes in its extracytoplasmic location and in the absence of EAL, HD and DHH domains. Instead, it contains the domains of the 5´-nucleotidase family pertaining to the metallophosphoesterase superfamily, although CpdB lacks 5´-nucleotidase activity. The possibility that the extracytoplasmic activity of CpdB on cyclic dinucleotides could have physiological meaning is discussed.

Ecto-nucleotidases and Ecto-phosphatases from Leishmania and Trypanosoma parasites

Ecto-enzymes can be defined as membrane-bound proteins that have their active site facing the extracellular millieu. In trypanosomatids, the physiological roles of these enzymes remain to be completed elucidated; however, many important events have already been related to them, such as the survival of parasites during their complex life cycle and the successful establishment of host infection. This chapter focuses on two remarkable classes of ecto-enzymes: ecto-nucleotidases and ecto-phosphatases, summarizing their occurrence and possible physiological roles in Leishmania and Trypanosoma genera. Ecto-nucleotidases are characterized by their ability to hydrolyze extracellular nucleotides, playing an important role in purinergic signaling. By the action of these ecto-enzymes, parasites are capable of modulating the host immune system, which leads to a successful parasite infection. Furthermore, ecto-nucleotidases are also involved in the purine salvage pathway, acting in the generation of nucleosides that are able to cross plasma membrane via specialized transporters. Another important ecto-enzyme present in a vast number of pathogenic organisms is the ecto-phosphatase. These enzymes are able to hydrolyze extracellular phosphorylated substrates, releasing free inorganic phosphate that can be internalized by the cell, crossing the plasma membrane through a Pi-transporter. Ecto-phosphatases are also involved in the invasion and survival of parasite in the host cells. Several alternative functions have been suggested for these enzymes in parasites, such as participation in their proliferation, differentiation, nutrition and protection. In this context, the present chapter provides an overview of recent discoveries related to the occurrence of ecto-nucleotidase and ecto-phosphatase activities in Leishmania and Trypanosoma parasites.

Enzymatic and mutational analyses of a class II 3',5'-cyclic nucleotide phosphodiesterase, PdeE, from Myxococcus xanthus

Myxococcus xanthus PdeE, an enzyme homologous to class II 3',5'-cyclic nucleotide phosphodiesterases, hydrolyzed cyclic AMP (cAMP) and cGMP with K(m) values of 12 μM and 25 μM, respectively. A pdeE mutant exhibited delays in fruiting body and spore formation compared with the wild type when cultured on starvation medium.

Gene cloning and characterization of a novel extracellular ribonuclease of Bacillus subtilis

An extracellular nuclease gene of Bacillus subtilis was cloned in the same organism by detecting the amplified enzyme activity, which was secreted from the transformant cells on an RNA-containing agar medium. An open reading frame encoding 289 amino acids was identified within the cloned fragment. The transcriptional initiation site was determined by nuclease S1 mapping and the promoter region showed similarity to the conserved recognition sequences for the E sigma A and/or E sigma E RNA polymerases. The production of the nuclease by the B. subtilis transformants greatly depends on the liquid medium used. SDS/PAGE analysis of the purified enzyme showed two adjoining bands of molecular mass about 32 kDa, and the NH2-terminal amino acid sequence analysis suggested that the NH2-terminal portion of the nuclease was subjected to a limited proteolysis after or during secretion. The nuclease was uniquely characterized as a Mg(2+)-activated ribonuclease which hydrolyzes RNA apparently nonspecifically into oligonucleotides with 5'-terminal phosphate. The deduced amino acid sequence of this enzyme shows no obvious similarity with other nuclease sequences.

Purification and properties of 2'-nucleotidase from mammalian brain

A nucleotidase specific for 2'-nucleotides was localized in both the soluble and the synaptosomal fractions of rat brain. The enzyme was partially purified from the soluble fraction of bovine brain. The s20,w was 4.9 S with an estimated molecular weight of about 70 000. The optimum pH was 8.0 and Km value for 2'-AMP was 5.5 . 10(-4) M. The substrate and inhibitor specificities of the enzyme were examined. The nucleotidase has an absolute requirement for Mg2+ but neither Fe3+ nor Ca2+ acted as replacement ions. In fact, Mn2+ and Ca2+ inhibited the Mg2+-dependent 2'-nucleotidase.