Background: Pneumococci have developed multiple strategies to infect the host. Results: PepO is a ubiquitously expressed pneumococcal protein that interacts with host proteins and facilitates host cell invasion and evasion of innate immunity. Conclusion: PepO is a plasminogen-and fibronectin-binding pneumococcal invasin. Significance: Understanding the mechanism of pneumococcal interaction with host aids designing better therapeutical strategies and gaining control over the pathogen.

The major membrane phospholipid classes, described thus far, include phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn), phosphatidylserine (PtdSer), and phosphatidylinositol (PtdIns). Here, we demonstrate the natural occurrence and genetic origin of an exclusive and rather abundant lipid, phosphatidylthreonine (PtdThr), in a common eukaryotic model parasite, Toxoplasma gondii. The parasite expresses a novel enzyme PtdThr synthase (TgPTS) to produce this lipid in its endoplasmic reticulum. Genetic disruption of TgPTS abrogates de novo synthesis of PtdThr and impairs the lytic cycle and virulence of T. gondii. The observed phenotype is caused by a reduced gliding motility, which blights the parasite egress and ensuing host cell invasion. Notably, the PTS mutant can prevent acute as well as yet-incurable chronic toxoplasmosis in a mouse model, which endorses its potential clinical utility as a metabolically attenuated vaccine. Together, the work also illustrates the functional speciation of two evolutionarily related membrane phospholipids, i.e., PtdThr and PtdSer.

Toxoplasma gondii is an obligate intracellular parasite, which inflicts acute as well as chronic infections in a wide range of warm-blooded vertebrates. Our recent work has demonstrated the natural occurrence and autonomous synthesis of an exclusive lipid phosphatidylthreonine in T. gondii. Targeted gene disruption of phosphatidylthreonine synthase impairs the parasite virulence due to unforeseen attenuation of the consecutive events of motility, egress and invasion. However, the underlying basis of such an intriguing phenotype in the parasite mutant remains unknown. Using an optogenetic sensor (gene-encoded calcium indicator, GCaMP6s), we show that loss of phosphatidylthreonine depletes calcium stores in intracellular tachyzoites, which leads to dysregulation of calcium release into the cytosol during the egress phase of the mutant. Consistently, the parasite motility and egress phenotypes in the mutant can be entirely restored by ionophore-induced mobilization of calcium. Collectively, our results suggest a novel regulatory function of phosphatidylthreonine in calcium signaling of a prevalent parasitic protist. Moreover, our application of an optogenetic sensor to monitor subcellular calcium in a model intracellular pathogen exemplifies its wider utility to other entwined systems.

The PI3K/Akt pathway is central for numerous cellular functions and is frequently deregulated in human cancers. The catalytic subunits of PI3K, p110, are thought to have a potential oncogenic function, and the regulatory subunit p85 exerts tumor suppressor properties. The fruit fly, Drosophila melanogaster, is a highly suitable system to investigate PI3K signaling, expressing one catalytic, Dp110, and one regulatory subunit, Dp60, and both show strong homology with the human PI3K proteins p110 and p85. We recently showed that p37δ, an alternatively spliced product of human PI3K p110δ, displayed strong proliferation-promoting properties despite lacking the catalytic domain completely. Here we functionally evaluate the different domains of human p37δ in Drosophila. The N-terminal region of Dp110 alone promotes cell proliferation, and we show that the unique C-terminal region of human p37δ further enhances these proliferative properties, both when expressed in Drosophila, and in human HEK-293 cells. Surprisingly, although the N-terminal region of Dp110 and the C-terminal region of p37δ both display proliferative effects, over-expression of full length Dp110 or the N-terminal part of Dp110 decreases survival in Drosophila, whereas the unique C-terminal region of p37δ prevents this effect. Furthermore, we found that the N-terminal region of the catalytic subunit of PI3K p110, including only the Dp60 (p85)-binding domain and a minor part of the Ras binding domain, rescues phenotypes with severely impaired development caused by Dp60 over-expression in Drosophila, possibly by regulating the levels of Dp60, and also by increasing the levels of phosphorylated Akt. Our results indicate a novel kinase-independent function of the PI3K catalytic subunit.

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