An atomic model for the human septin hexamer by cryo-EM.

In order to form functional filaments, human septins must assemble into hetero-oligomeric rod-like particles which polymerize end-to-end. The rules governing the assembly of these particles and the subsequent filaments are incompletely understood. Although crystallographic approaches have been successful in studying the separate components of the system, there has been difficulty in obtaining high resolution structures of the full particle. Here we report a first cryo-EM structure for a hexameric rod composed of human septins 2, 6 and 7 with a global resolution of ~3.6 Å and a local resolution of between ~3.0 Å and ~5.0 Å. By fitting the previously determined high-resolution crystal structures of the component subunits into the cryo-EM map, we are able to provide an essentially complete model for the particle. This exposes SEPT2 NC-interfaces at the termini of the hexamer and leaves internal cavities between the SEPT6-SEPT7 pairs. The floor of the cavity is formed by the two α0 helices including their polybasic regions. These are locked into place between the two subunits by interactions made with the α5 and α6 helices of the neighbouring monomer together with its polyacidic region. The cavity may serve to provide space allowing the subunits to move with respect to one another. The elongated particle shows a tendency to bend at its centre where two copies of SEPT7 form a homodimeric G-interface. Such bending is almost certainly related to the ability of septin filaments to recognize and even induce membrane curvature.

Schistosoma mansoni venom allergen-like protein 18 (SmVAL18) is a plasminogen-binding protein secreted during the early stages of mammalian-host infection.

Schistosomiasis is a neglected tropical disease caused by trematodes of the genus Schistosoma which have a complex life cycle characterized by an asexual multiplication phase in the snail intermediate host and a sexual reproduction phase in the mammalian definitive host. The initial steps of the human host infection involve the secretion of proteins contained in the acetabular glands of cercariae that promote parasite adhesion and proteolysis of the skin layers. Herein, we performed a functional analysis of SmVAL18, identified as one of the three SCP/TAPS proteins constituent of cercarial secretions. We evaluated the SmVAL18 binding to immobilized macromolecules of the extracellular matrix (ECM) and to plasma components. Recombinant protein, expressed in E. coli, was found to maintain an ordered secondary structure typical of the SCP/TAPS domain after purification. Expression of native SmVAL18 protein was verified to be restricted to cercariae and 3-h schistosomula stages; furthermore, the protein was observed in the corresponding secretions, confirming that SmVAL18 is secreted during the first 3 h of in vitro culture. rSmVAL18 was able to interact specifically with plasminogen (PLG) and enhance its conversion into plasmin in the presence of the urokinase-type plasminogen activator (uPA). Protein homology modelling suggested that the PLG-rSmVAL18 interaction was mediated by lysine residues of the protein. This was supported by in vitro data using the lysine analogue, 6-aminocaproic acid (ACA), which abolished the interaction. Finally, our results showed that both cercariae and 3-h schistosomula, as well as their corresponding secretions, exhibited the capacity to bind PLG and enhance its conversion into plasmin in vitro in the same way as observed for the recombinant protein. In conclusion, our findings show that SmVAL18 is a novel PLG-binding protein secreted during the early stages of the mammalian-host infection.

Schistosoma mansoni venom allergen-like protein 18 (SmVAL18) is a plasminogen-binding protein secreted during the early stages of mammalian-host infection

Schistosomiasis is a neglected tropical disease caused by trematodes of the genus Schistosoma which have a complex life cycle characterized by an asexual multiplication phase in the snail intermediate host and a sexual reproduction phase in the mammalian definitive host. The initial steps of the human host infection involve the secretion of proteins contained in the acetabular glands of cercariae that promote parasite adhesion and proteolysis of the skin layers. Herein, we performed a functional analysis of SmVAL18, identified as one of the three SCP/TAPS proteins constituent of cercarial secretions. We evaluated the SmVAL18 binding to immobilized macromolecules of the extracellular matrix (ECM) and to plasma components. Recombinant protein, expressed in E. coli, was found to maintain an ordered secondary structure typical of the SCP/TAPS domain after purification. Expression of native SmVAL18 protein was verified to be restricted to cercariae and 3-h schistosomula stages; furthermore, the protein was observed in the corresponding secretions, confirming that SmVAL18 is secreted during the first 3 h of in vitro culture. rSmVAL18 was able to interact specifically with plasminogen (PLG) and enhance its conversion into plasmin in the presence of the urokinase-type plasminogen activator (uPA). Protein homology modelling suggested that the PLG-rSmVAL18 interaction was mediated by lysine residues of the protein. This was supported by in vitro data using the lysine analogue, 6-aminocaproic acid (ACA), which abolished the interaction. Finally, our results showed that both cercariae and 3-h schistosomula, as well as their corresponding secretions, exhibited the capacity to bind PLG and enhance its conversion into plasmin in vitro in the same way as observed for the recombinant protein. In conclusion, our findings show that SmVAL18 is a novel PLG-binding protein secreted during the early stages of the mammalian-host infection.

Schistosoma mansoni venom allergen-like protein 18 (SmVAL18) is a plasminogen-binding protein secreted during the early stages of mammalian-host infection

Schistosomiasis is a neglected tropical disease caused by trematodes of the genus Schistosoma which have a complex life cycle characterized by an asexual multiplication phase in the snail intermediate host and a sexual reproduction phase in the mammalian definitive host. The initial steps of the human host infection involve the secretion of proteins contained in the acetabular glands of cercariae that promote parasite adhesion and proteolysis of the skin layers. Herein, we performed a functional analysis of SmVAL18, identified as one of the three SCP/TAPS proteins constituent of cercarial secretions. We evaluated the SmVAL18 binding to immobilized macromolecules of the extracellular matrix (ECM) and to plasma components. Recombinant protein, expressed in E. coli, was found to maintain an ordered secondary structure typical of the SCP/TAPS domain after purification. Expression of native SmVAL18 protein was verified to be restricted to cercariae and 3-h schistosomula stages; furthermore, the protein was observed in the corresponding secretions, confirming that SmVAL18 is secreted during the first 3 h of in vitro culture. rSmVAL18 was able to interact specifically with plasminogen (PLG) and enhance its conversion into plasmin in the presence of the urokinase-type plasminogen activator (uPA). Protein homology modelling suggested that the PLG-rSmVAL18 interaction was mediated by lysine residues of the protein. This was supported by in vitro data using the lysine analogue, 6-aminocaproic acid (ACA), which abolished the interaction. Finally, our results showed that both cercariae and 3-h schistosomula, as well as their corresponding secretions, exhibited the capacity to bind PLG and enhance its conversion into plasmin in vitro in the same way as observed for the recombinant protein. In conclusion, our findings show that SmVAL18 is a novel PLG-binding protein secreted during the early stages of the mammalian-host infection.