Transmission of cutaneous leishmaniasis by sand flies is enhanced by regurgitation of fPPG

Sand flies are the exclusive vectors of the protozoan parasite Leishmania, but the mechanism of transmission by fly bite has not been determined nor incorporated into experimental models of infection. In sand flies with mature Leishmania infections the anterior midgut is blocked by a gel of parasite origin, the promastigote secretory gel. Here we analyse the inocula from Leishmania mexicana-infected Lutzomyia longipalpis sand flies. Analysis revealed the size of the infectious dose, the underlying mechanism of parasite delivery by regurgitation, and the novel contribution made to infection by filamentous proteophosphoglycan (fPPG), a component of promastigote secretory gel found to accompany the parasites during transmission. Collectively these results have important implications for understanding the relationship between the parasite and its vector, the pathology of cutaneous leishmaniasis in humans and also the development of effective vaccines and drugs. These findings emphasize that to fully understand transmission of vector-borne diseases the interaction between the parasite, its vector and the mammalian host must be considered together.     

Mechanisms of gene silencing by double-stranded RNA

Double-stranded RNA (dsRNA) is an important regulator of gene expression in many eukaryotes. It triggers different types of gene silencing that are collectively referred to as RNA silencing or RNA interference. A key step in known silencing pathways is the processing of dsRNAs into short RNA duplexes of characteristic size and structure. These short dsRNAs guide RNA silencing by specific and distinct mechanisms. Many components of the RNA silencing machinery still need to be identified and characterized, but a more complete understanding of the process is imminent.     

Pilus chaperones represent a new type of protein-folding catalyst

Adhesive type 1 pili from uropathogenic Escherichia coli strains have a crucial role during infection by mediating the attachment to and potentially the invasion of host tissue. These filamentous, highly oligomeric protein complexes are assembled by the 'chaperone-usher' pathway, in which the individual pilus subunits fold in the bacterial periplasm and form stoichiometric complexes with a periplasmic chaperone molecule that is essential for pilus assembly. The chaperone subsequently delivers the subunits to an assembly platform (usher) in the outer membrane, which mediates subunit assembly and translocation to the cell surface. Here we show that the periplasmic type 1 pilus chaperone FimC binds non-native pilus subunits and accelerates folding of the subunit FimG by 100-fold. Moreover, we find that the FimC-FimG complex is formed quantitatively and very rapidly when folding of FimG is initiated in the presence of both FimC and the assembly-competent subunit FimF, even though the FimC-FimG complex is thermodynamically less stable than the FimF-FimG complex. FimC thus represents a previously unknown type of protein-folding catalyst, and simultaneously acts as a kinetic trap preventing spontaneous subunit assembly in the periplasm.     

The star-formation history of the Universe from the stellar populations of nearby galaxies

The determination of the star-formation history of the Universe is a key goal of modern cosmology, as it is crucial to our understanding of how galactic structures form and evolve. Observations of young stars in distant galaxies at different times in the past have indicated that the stellar birthrate peaked some eight billion years ago before declining by a factor of around ten to its present value. Here we report an analysis of the 'fossil record' of the current stellar populations of 96,545 nearby galaxies, from which we obtained a complete star-formation history. Our results broadly support those derived from high-redshift galaxies. We find, however, that the peak of star formation was more recent--around five billion years ago. We also show that the bigger the stellar mass of the galaxy, the earlier the stars were formed, which indicates that high- and low-mass galaxies have very different histories.     

Absence of S6K1 protects against age- and diet-induced obesity while enhancing insulin sensitivity

Elucidating the signalling mechanisms by which obesity leads to impaired insulin action is critical in the development of therapeutic strategies for the treatment of diabetes. Recently, mice deficient for S6 Kinase 1 (S6K1), an effector of the mammalian target of rapamycin (mTOR) that acts to integrate nutrient and insulin signals, were shown to be hypoinsulinaemic, glucose intolerant and have reduced beta-cell mass. However, S6K1-deficient mice maintain normal glucose levels during fasting, suggesting hypersensitivity to insulin, raising the question of their metabolic fate as a function of age and diet. Here, we report that S6K1-deficient mice are protected against obesity owing to enhanced beta-oxidation. However on a high fat diet, levels of glucose and free fatty acids still rise in S6K1-deficient mice, resulting in insulin receptor desensitization. Nevertheless, S6K1-deficient mice remain sensitive to insulin owing to the apparent loss of a negative feedback loop from S6K1 to insulin receptor substrate 1 (IRS1), which blunts S307 and S636/S639 phosphorylation; sites involved in insulin resistance. Moreover, wild-type mice on a high fat diet as well as K/K A(y) and ob/ob (also known as Lep/Lep) mice-two genetic models of obesity-have markedly elevated S6K1 activity and, unlike S6K1-deficient mice, increased phosphorylation of IRS1 S307 and S636/S639. Thus under conditions of nutrient satiation S6K1 negatively regulates insulin signalling.     

Membrane structure and interactions with protein and DNA in bacteriophage PRD1

Membranes are essential for selectively controlling the passage of molecules in and out of cells and mediating the response of cells to their environment. Biological membranes and their associated proteins present considerable difficulties for structural analysis. Although enveloped viruses have been imaged at about 9 A resolution by cryo-electron microscopy and image reconstruction, no detailed crystallographic structure of a membrane system has been described. The structure of the bacteriophage PRD1 particle, determined by X-ray crystallography at about 4 A resolution, allows the first detailed analysis of a membrane-containing virus. The architecture of the viral capsid and its implications for virus assembly are presented in the accompanying paper. Here we show that the electron density also reveals the icosahedral lipid bilayer, beneath the protein capsid, enveloping the viral DNA. The viral membrane contains about 26,000 lipid molecules asymmetrically distributed between the membrane leaflets. The inner leaflet is composed predominantly of zwitterionic phosphatidylethanolamine molecules, facilitating a very close interaction with the viral DNA, which we estimate to be packaged to a pressure of about 45 atm, factors that are likely to be important during membrane-mediated DNA translocation into the host cell. In contrast, the outer leaflet is enriched in phosphatidylglycerol and cardiolipin, which show a marked lateral segregation within the icosahedral asymmetric unit. In addition, the lipid headgroups show a surprising degree of order.     

Immune recognition of a human renal cancer antigen through post-translational protein splicing

Cytotoxic T lymphocytes (CTLs) detect and destroy cells displaying class I molecules of the major histocompatibility complex (MHC) that present oligopeptides derived from aberrant self or foreign proteins. Most class I peptide ligands are created from proteins that are degraded by proteasomes and transported, by the transporter associated with antigen processing, from the cytosol into the endoplasmic reticulum, where peptides bind MHC class I molecules and are conveyed to the cell surface. C2 CTLs, cloned from human CTLs infiltrating a renal cell carcinoma, kill cancer cells overexpressing fibroblast growth factor-5 (FGF-5). Here we show that C2 cells recognize human leukocyte antigen-A3 MHC class I molecules presenting a nine-residue FGF-5 peptide generated by protein splicing. This process, previously described strictly in plants and unicellular organisms, entails post-translational excision of a polypeptide segment followed by ligation of the newly liberated carboxy-terminal and amino-terminal residues. The occurrence of protein splicing in vertebrates has important implications for the complexity of the vertebrate proteome and for the immune recognition of self and foreign peptides.