Capsid-deficient alphaviruses generate propagative infectious microvesicles at the plasma membrane.

Alphavirus budding is driven by interactions between nucleocapsids assembled in the cytoplasm and envelope proteins present at the plasma membrane. So far, the expression of capsid and envelope proteins in infected cells has been considered an absolute requirement for alphavirus budding and propagation. In the present study, we show that Semliki Forest virus and Sindbis virus lacking the capsid gene can propagate in mammalian and insect cells. This propagation is mediated by the release of infectious microvesicles (iMVs), which are pleomorphic and have a larger size and density than wild-type virus. iMVs, which contain viral RNA inside and viral envelope proteins on their surface, are released at the plasma membrane and infect cells using the endocytic pathway in a similar way to wild-type virus. iMVs are not pathogenic in immunocompetent mice when injected intravenously, but can infect different organs like lungs and heart. Finally, we also show that alphavirus genomes without capsid can mediate the propagation of heterologous genes, making these vectors potentially interesting for gene therapy or vaccination studies. The minimalist infectious system described in this study shows that a self-replicating RNA able to express membrane proteins with binding and fusion properties is able to propagate, providing some insights into virus evolution.

Archaeal RNA polymerase: the influence of the protruding stalk in crystal packing and preliminary biophysical analysis of the Rpo13 subunit.

We review recent results on the complete structure of the archaeal RNAP (RNA polymerase) enzyme of Sulfolobus shibatae. We compare the three crystal forms in which this RNAP packs (space groups P212121, P21212 and P21) and provide a preliminary biophysical characterization of the newly identified 13-subunit Rpo13. The availability of different crystal forms for this RNAP allows the analysis of the packing degeneracy and the intermolecular interactions that determine this degeneracy. We observe the pivotal role played by the protruding stalk composed of subunits Rpo4 and Rpo7 in the lattice contacts. Aided by MALLS (multi-angle laser light scattering), we have initiated the biophysical characterization of the recombinantly expressed and purified subunit Rpo13, a necessary step towards the understanding of Rpo13's role in archaeal transcription.

Rules of engagement between αvß6 integrin and foot-and-mouth disease virus.

Foot-and-mouth disease virus (FMDV) mediates cell entry by attachment to an integrin receptor, generally αvß6, via a conserved arginine-glycine-aspartic acid (RGD) motif in the exposed, antigenic, GH loop of capsid protein VP1. Infection can also occur in tissue culture adapted virus in the absence of integrin via acquired basic mutations interacting with heparin sulphate (HS); this virus is attenuated in natural infections. HS interaction has been visualized at a conserved site in two serotypes suggesting a propensity for sulfated-sugar binding. Here we determined the interaction between αvß6 and two tissue culture adapted FMDV strains by cryo-electron microscopy. In the preferred mode of engagement, the fully open form of the integrin, hitherto unseen at high resolution, attaches to an extended GH loop via interactions with the RGD motif plus downstream hydrophobic residues. In addition, an N-linked sugar of the integrin attaches to the previously identified HS binding site, suggesting a functional role.