Exoerythrocytic Plasmodium parasites secrete a cysteine protease inhibitor involved in sporozoite invasion and capable of blocking cell death of host hepatocytes.

Plasmodium parasites must control cysteine protease activity that is critical for hepatocyte invasion by sporozoites, liver stage development, host cell survival and merozoite liberation. Here we show that exoerythrocytic P. berghei parasites express a potent cysteine protease inhibitor (PbICP, P. berghei inhibitor of cysteine proteases). We provide evidence that it has an important function in sporozoite invasion and is capable of blocking hepatocyte cell death. Pre-incubation with specific anti-PbICP antiserum significantly decreased the ability of sporozoites to infect hepatocytes and expression of PbICP in mammalian cells protects them against peroxide- and camptothecin-induced cell death. PbICP is secreted by sporozoites prior to and after hepatocyte invasion, localizes to the parasitophorous vacuole as well as to the parasite cytoplasm in the schizont stage and is released into the host cell cytoplasm at the end of the liver stage. Like its homolog falstatin/PfICP in P. falciparum, PbICP consists of a classical N-terminal signal peptide, a long N-terminal extension region and a chagasin-like C-terminal domain. In exoerythrocytic parasites, PbICP is posttranslationally processed, leading to liberation of the C-terminal chagasin-like domain. Biochemical analysis has revealed that both full-length PbICP and the truncated C-terminal domain are very potent inhibitors of cathepsin L-like host and parasite cysteine proteases. The results presented in this study suggest that the inhibitor plays an important role in sporozoite invasion of host cells and in parasite survival during liver stage development by inhibiting host cell proteases involved in programmed cell death.

The "SARS-unique domain" (SUD) of SARS coronavirus is an oligo(G)-binding protein.

Caused by a new coronavirus, severe acute respiratory syndrome (SARS) is a highly contagious disease associated with significant fatality that emerged in 2003. The molecular cause of the unusually high human pathogenicity of the SARS coronavirus (SARS-CoV) is still unknown. In an effort to characterize molecular components of the virus that are absent in other coronaviruses, all of which are considerably less pathogenic for humans, we recombinantly produced the SARS-unique domain (SUD) within non-structural protein 3 (Nsp3) of SARS-CoV and characterized its nucleic-acid binding properties. Zone-interference gel electrophoresis and electrophoretic mobility shift assays revealed a specific affinity of SUD for oligo(G)-strings. A few such segments are present in the SARS-CoV genome, but also in mRNAs of host proteins involved in the regulation of signaling pathways. A putative role of SUD in virus-induced apoptosis or survival of host cells is discussed.

Structural and functional characterization of Falcipain-2, a hemoglobinase from the malarial parasite Plasmodium falciparum.

Malaria is caused by protozoan erythrocytic parasites of the Plasmodium genus, with Plasmodium falciparum being the most dangerous and widespread disease-causing species. Falcipain-2 (FP-2) of P. falciparum is a papain-family (C1A) cysteine protease that plays an important role in the parasite life cycle by degrading erythrocyte proteins, most notably hemoglobin. Inhibition of FP-2 and its paralogues prevents parasite maturation, suggesting these proteins may be valuable targets for the design of novel antimalarial drugs, but lack of structural knowledge has impeded progress toward the rational discovery of potent, selective, and efficacious inhibitors. As a first step toward this goal, we present here the crystal structure of mature FP-2 at 3.1 A resolution, revealing novel structural features of the FP-2 subfamily proteases including a dynamic beta-hairpin hemoglobin binding motif, a flexible N-terminal alpha-helical extension, and a unique active-site cleft. We also demonstrate by biochemical methods that mature FP-2 can proteolytically process its own precursor in trans at neutral to weakly alkaline pH, that the binding of hemoglobin to FP-2 is strictly pH-dependent, and that FP-2 preferentially binds methemoglobin over hemoglobin. Because the specificity and proteolytic activity of FP-2 toward its multiple targets appears to be pH-dependent, we suggest that environmental pH may play an important role in orchestrating FP-2 function over the different life stages of the parasite. Moreover, it appears that selectivity of FP-2 for methemoglobin may represent an evolutionary adaptation to oxidative stress conditions within the host cell.