Little is known about the three-dimensional organization of rubella virus, which causes a relatively mild measles-like disease in children but leads to serious congenital health problems when contracted in utero. Although rubella virus belongs to the same family as the mosquito-borne alphaviruses, in many respects it is more similar to other aerosol-transmitted human viruses such as the agents of measles and mumps. Although the use of the triple MMR (measles, mumps and rubella) live vaccine has limited its incidence in western countries, congenital rubella syndrome remains an important health problem in the developing world. Here we report the 1.8 Å resolution crystal structure of envelope glycoprotein E1, the main antigen and sole target of neutralizing antibodies against rubella virus. E1 is the main player during entry into target cells owing to its receptor-binding and membrane-fusion functions. The structure reveals the epitope and the neutralization mechanism of an important category of protecting antibodies against rubella infection. It also shows that rubella virus E1 is a class II fusion protein, which had hitherto only been structurally characterized for the arthropod-borne alphaviruses and flaviviruses. In addition, rubella virus E1 has an extensive membrane-fusion surface that includes a metal site, reminiscent of the T-cell immunoglobulin and mucin family of cellular proteins that bind phosphatidylserine lipids at the plasma membrane of cells undergoing apoptosis. Such features have not been seen in any fusion protein crystallized so far. Structural comparisons show that the class II fusion proteins from alphaviruses and flaviviruses, despite belonging to different virus families, are closer to each other than they are to rubella virus E1. This suggests that the constraints on arboviruses imposed by alternating cycles between vertebrates and arthropods resulted in more conservative evolution. By contrast, in the absence of this constraint, the strictly human rubella virus seems to have drifted considerably into a unique niche as sole member of the Rubivirus genus.

Physicists have studied the aggregation of adhesive proteins, giving a central role to the elastic properties of membranes, whereas cell biologists have put the emphasis on the cytoskeleton. However, there is a dramatic lack of experimental studies probing both contributions on cellular systems. Here, we tested both mechanisms on living cells. We compared, for the same cell line, the growth of cadherin-GFP patterns on recombinant cadherin-coated surfaces, with the growth of vinculin-GFP patterns on extracellular matrix protein-coated surfaces by using evanescent wave microscopy. In our setup, cadherins are not linked to actin, whereas vinculins are. This property allows us to compare formation of clusters with proteins linked or not to the cytoskeleton and thus study the role of membrane versus cytoskeleton in protein aggregation. Strikingly, the motifs we obtained on both surfaces share common features: they are both elongated and located at the cell edges. We showed that a local force application can impose this symmetry breaking in both cases. However, the origin of the force is different as demonstrated by drug treatment (butanedione monoxime) and hypotonic swelling. Cadherins aggregate when membrane tension is increased, whereas vinculins (cytoplasmic proteins of focal contacts) aggregate when acto-myosin stress fibers are pulling. We propose a mechanism by which membrane tension is localized at cell edges, imposing flattening of membrane and enabling aggregation of cadherins by diffusion. In contrast, cytoplasmic proteins of focal contacts aggregate by opening cryptic sites in focal contacts under acto-myosin contractility.

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