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Plasmodium sporozoites make a remarkable journey from the mosquito midgut to the mammalian liver. The sporozoite's major surface protein, circumsporozoite protein (CSP), is a multifunctional protein required for sporozoite development and likely mediates several steps of this journey. In this study, we show that CSP has two conformational states, an adhesive conformation in which the C-terminal cell-adhesive domain is exposed and a nonadhesive conformation in which the N terminus masks this domain. We demonstrate that the cell-adhesive domain functions in sporozoite development and hepatocyte invasion. Between these two events, the sporozoite must travel from the mosquito midgut to the mammalian liver, and N-terminal masking of the cell-adhesive domain maintains the sporozoite in a migratory state. In the mammalian host, proteolytic cleavage of CSP regulates the switch to an adhesive conformation, and the highly conserved region I plays a critical role in this process. If the CSP domain architecture is altered such that the cell-adhesive domain is constitutively exposed, the majority of sporozoites do not reach their target organs, and in the mammalian host, they initiate a blood stage infection directly from the inoculation site. These data provide structure-function information relevant to malaria vaccine development.

About 500 million cases of malaria occur annually. However, a substantial number of patients who actually have relapsing fever (RF) Borrelia can be misdiagnosed with malaria due to similar manifestations and geographic distribution of the two diseases. More alarmingly, high prevalence of concomitant infections with malaria and RF Borrelia has been reported. Therefore, we used a mouse model to study the effects of such mixed infection. We observed a 21-fold increase in spirochete titers, whereas the numbers of parasitized erythrocytes were reduced 15-fold. This may be explained by polarization of the host immune response towards the intracellular malaria parasite, resulting in unaffected extracellular spirochetes and hosts that succumb to sepsis. Mixed infection also resulted in severe malaria anemia with low hemoglobin levels, even though the parasite counts were low. Overall, co-infected animals had higher fatality rate and shorter time to death than both malaria and RF single infection. Furthermore, secondary malaria infection reactivated a quiescent RF brain infection, which is the first evidence of a clinically and biologically relevant cue for reactivation of RF Borrelia infection. Our study highlights the importance of investigating concomitant infections in vivo to elucidate the immune responses that are involved in the clinical outcome.

The protein tyrosine phosphatase YopH, produced by the pathogen Yersinia pseudotuberculosis, is an essential virulence determinant involved in antiphagocytosis. Upon infection, YopH is translocated into the target cell, where it recognizes focal complexes. Genetic analysis revealed that YopH harbours a region that is responsible for specific localization of this PTPase to focal complexes in HeLa cells and professional phagocytes. This region is a prerequisite for blocking an immediate-early Yersinia-induced signal within target cells. The region is also essential for antiphagocytosis and virulence, illustrating the biological significance of localization of YopH to focal complexes during Yersinia infection. These results also indicate that focal complexes play a role in the general phagocytic process.

The tyrosine phosphatase YopH is an essential virulence effector of pathogenic Yersinia spp. YopH, which is translocated from extracellularly located bacteria into interacting target cells, blocks phagocytosis by professional phagocytes. We show here that immunoprecipitation of YopH from lysates of J774 cells infected with Y. pseudotuberculosis expressing an inactive form of YopH resulted in co-precipitation of certain phosphotyrosine proteins. The association between the inactive YopH and phosphotyrosine proteins in the 120 kDa range was rapid and could be detected after 2 min of infection. The proteins were identified as the docking proteins Cas and Fyn-binding protein (FYB). Upon infection of J774 cells with Y. pseudotuberculosis lacking YopH expression both of these proteins became tyrosine phosphorylated. Moreover, this infection caused recruitment of Cas to peripheral focal complexes, and FYB was relocalized to areas surrounding these structures. Both Cas and FYB became dephosphorylated upon infection with Y. pseudotuberculosis expressing active YopH, and this was associated with disruption of focal complexes. With regard to the previous identification of Cas and focal complexes as targets of YopH in HeLa cells, the present study supports an important role for these targets in a general mechanism of bacterial uptake. 

Preventing the early host immune defense allows pathogenic Yersinia to proliferate in lymphatic tissue. This ability depends on signaling that occurs between the bacteria and the host cells. Following intimate contact with the target cell a signal is generated within the bacterium that results in increased expression of virulence-associated proteins that are subsequently delivered into the infected cell. These proteins, designated Yops, interfere with the host-cell signaling pathways that are normally activated to eliminate infectious agents.

Pathogenic Yersinia resist uptake by eukaryotic cells by a mechanism involving the virulence protein YopH, a protein tyrosine phosphatase. We show that p130^Cas and FAK are phosphorylated and recruited to peripheral focal complexes during bacterial uptake in HeLa cells. The inactive form of YopH interacts with the tyrosine phosphorylated forms of FAK and p130^Cas and co-localizes with these proteins in focal adhesions. On the other hand, the presence of active YopH results in inhibition of uptake, dephosphorylation of p130^Cas and FAK, and disruption of peripheral focal complexes. We suggest that p130^Cas and FAK are substrates for YopH and that the dephosphorylation of these proteins impairs the uptake of Yersinia pseudotuberculosis into HeLa cells.

YopH is translocated by cell-surface-bound bacteria through the plasma membrane to the cytosol of the HeLa cell. The transfer mechanism is contact dependent and polarizes the translocation to only occur at the contact zone between the bacterium and the target cell. More than 99% of the PTPase activity is associated with the HeLa cells. In contrast to the wild-type strain, the yopBD mutant cannot deliver YopH to the cytosol. Instead YopH is deposited in localized areas in the proximity of cell-associated bacteria. A yopN mutant secretes 40% of the total amount of YopH to the culture medium, suggesting a critical role of YopN in regulation of the polarized translocation. Evidence for a region in YopH important for its translocation through the plasma membrane of the target cell but not for secretion from the pathogen is provided.

Malaria is one of the most devastating diseases of the developing world responsible for approximately two million deaths annually. The high mortality together with the fact that resistance to available antimalarial drugs has increased, highlights the necessity of finding new chemotherapeutics against the parasite. Polyamines play a critical role in the regulation of cell proliferation and differentiation in most organisms including the malaria parasite. Therefore, targeting enzymes in the polyamine synthesis could be a possible approach to combat malaria. In order to evaluate the curative potential of the polyamine biosynthesis inhibitors S-adenosyl-3-thio-1,8-diaminooctane (AdoDATO) and trans-4-methylcyclohexylamine (4MCHA), which both target spermidine synthase, we took the advantage of an accessible mouse model using the rodent malaria parasite, P. berghei. Despite the promising inhibitory potential of AdoDATO, this drug was inefficient against malaria infection in mice. In contrast, 4MCHA restrained the parasite infection, which subsequently led to clearance within 24 days. This curative effect was not synergistically enhanced by combination treatment with the ornithine decarboxylase inhibitor, α-difluoromethylornithine (DFMO) and neither did a prophylactic treatment of 4MCHA increase the antimalarial effect. Interestingly, mice that received 4MCHA treatment gained a protective immunity towards malaria infection. The nature of this protective immunity is not established.