N-(cyclohexanecarboxyl)-O-phospho-l-serine, a minimal substrate for the dual-specificity protein phosphatase IphP

A low molecular weight protein tyrosine phosphatase from Synechocystis sp. strain PCC 6803: enzymatic characterization and identification of its potential substrates

The predicted protein product of open reading frame slr0328 from Synechocystis sp. PCC 6803, SynPTP, possesses significant amino acid sequence similarity with known low molecular weight protein tyrosine phosphatases (PTPs). To determine the functional properties of this hypothetical protein, open reading frame slr0328 was expressed in Escherichia coli. The purified recombinant protein, SynPTP, displayed its catalytic phosphatase activity towards several tyrosine, but not serine, phosphorylated exogenous protein substrates. The protein phosphatase activity of SynPTP was inhibited by sodium orthovanadate, a known inhibitor of tyrosine phosphatases, but not by okadaic acid, an inhibitor for many serine/threonine phosphatases. Kinetic analysis indicated that the K(m) and V(max) values for SynPTP towards p-nitrophenyl phosphate are similar to those of other known bacterial low molecular weight PTPs. Mutagenic alteration of the predicted catalytic cysteine of PTP, Cys(7), to serine abolished enzyme activity. Using a combination of immunodetection, mass spectrometric analysis and mutagenically altered Cys(7)SerAsp(125)Ala-SynPTP, we identified PsaD (photosystem I subunit II), CpcD (phycocyanin rod linker protein) and phycocyanin-α and -β subunits as possible endogenous substrates of SynPTP in this cyanobacterium. These results indicate that SynPTP might be involved in the regulation of photosynthesis in Synechocystis sp. PCC 6803.

A new family of phosphoinositide phosphatases in microorganisms: identification and biochemical analysis

Background

Phosphoinositide metabolism is essential to membrane dynamics and impinges on many cellular processes, including phagocytosis. Modulation of phosphoinositide metabolism is important for pathogenicity and virulence of many human pathogens, allowing them to survive and replicate in the host cells. Phosphoinositide phosphatases from bacterial pathogens are therefore key players in this modulation and constitute attractive targets for chemotherapy. MptpB, a virulence factor from Mycobacterium tuberculosis, has phosphoinositide phosphatase activity and a distinct active site P-loop signature HCXXGKDR that shares characteristics with eukaryotic lipid phosphatases and protein tyrosine phosphatases. We used this P-loop signature as a "diagnostic motif" to identify related putative phosphatases with phosphoinositide activity in other organisms.

Results

We found more than 200 uncharacterised putative phosphatase sequences with the conserved signature in bacteria, with some related examples in fungi and protozoa. Many of the sequences identified belong to recognised human pathogens. Interestingly, no homologues were found in any other organisms including Archaea, plants, or animals. Phylogenetic analysis revealed that these proteins are unrelated to classic eukaryotic lipid phosphatases. However, biochemical characterisation of those from Listeria monocytogenes and Leishmania major, demonstrated that, like MptpB, they have phosphatase activity towards phosphoinositides. Mutagenesis studies established that the conserved Asp and Lys in the P-loop signature (HCXXGKDR) are important in catalysis and substrate binding respectively. Furthermore, we provide experimental evidence that the number of basic residues in the P-loop is critical in determining activity towards poly-phosphoinositides.

Conclusion

This new family of enzymes in microorganisms shows distinct sequence and biochemical characteristics to classic eukaryotic lipid phosphatases and they have no homologues in humans. This study provides a foundation for examining the biological role of this new family of phosphatases and their potential as pharmaceutical targets against infectious diseases.

Prodrug approaches to improving the oral absorption of antiviral nucleotide analogues

Nucleotide analogues have been well accepted as therapeutic agents active against a number of viruses. However, their use as antiviral agents is limited by the need for phosphorylation by endogenous enzymes, and if the analogue is orally administered, by low bioavailability due to the presence of an ionizable diacid group. To circumvent these limitations, a number of prodrug approaches have been proposed. The ideal prodrug achieves delivery of a parent drug by attachment of a non-toxic moiety that is stable during transport and delivery, but is readily cleaved to release the parent drug once at the target. Here, a brief overview of several promising prodrug strategies currently under development is given.

MptpB, a virulence factor from Mycobacterium tuberculosis, exhibits triple-specificity phosphatase activity

Bacterial pathogens have developed sophisticated mechanisms of evading the immune system to survive in infected host cells. Central to the pathogenesis of Mycobacterium tuberculosis is the arrest of phagosome maturation, partly through interference with PtdIns signalling. The protein phosphatase MptpB is an essential secreted virulence factor in M. tuberculosis. A combination of bioinformatics analysis, enzyme kinetics and substrate-specificity characterization revealed that MptpB exhibits both dual-specificity protein phosphatase activity and, importantly, phosphoinositide phosphatase activity. Mutagenesis of conserved residues in the active site signature indicates a cysteine-based mechanism of dephosphorylation and identifies two new catalytic residues, Asp165, essential in catalysis, and Lys164, apparently involved in substrate specificity. Sequence similarities with mammalian lipid phosphatases and a preference for phosphoinositide substrates suggests a potential novel role of MptpB in PtdIns metabolism in the host and reveals new perspectives for the role of this phosphatase in mycobacteria pathogenicity.

Myotubularin, a protein tyrosine phosphatase mutated in myotubular myopathy, dephosphorylates the lipid second messenger, phosphatidylinositol 3-phosphate

The lipid second messenger phosphatidylinositol 3-phosphate [PI(3)P] plays a crucial role in intracellular membrane trafficking. We report here that myotubularin, a protein tyrosine phosphatase required for muscle cell differentiation, is a potent PI(3)P phosphatase. Recombinant human myotubularin specifically dephosphorylates PI(3)P in vitro. Overexpression of a catalytically inactive substrate-trapping myotubularin mutant (C375S) in human 293 cells increases PI(3)P levels relative to that of cells overexpressing the wild-type enzyme, demonstrating that PI(3)P is a substrate for myotubularin in vivo. In addition, a Saccharomyces cerevisiae strain in which the myotubularin-like gene (YJR110w) is disrupted also exhibits increased PI(3)P levels. Both the recombinant yeast enzyme and a human myotubularin-related protein (KIAA0371) are able to dephosphorylate PI(3)P in vitro, suggesting that this activity is intrinsic to all myotubularin family members. Mutations in the MTM1 gene that cause human myotubular myopathy dramatically reduce the ability of the phosphatase to dephosphorylate PI(3)P. Our findings provide evidence that myotubularin exerts its effects during myogenesis by regulating cellular levels of the inositol lipid PI(3)P.