Enveloped viruses have evolved complex glycoprotein machinery that drives the fusion of viral and cellular membranes, permitting entry of the viral genome into the cell. For the paramyxoviruses, the fusion (F) protein catalyses this membrane merger and entry step, and it has been postulated that the F protein undergoes complex refolding during this process. Here we report the crystal structure of the parainfluenza virus 5 F protein in its prefusion conformation, stabilized by the addition of a carboxy-terminal trimerization domain. The structure of the F protein shows that there are profound conformational differences between the pre- and postfusion states, involving transformations in secondary and tertiary structure. The positions and structural transitions of key parts of the fusion machinery, including the hydrophobic fusion peptide and two helical heptad repeat regions, clarify the mechanism of membrane fusion mediated by the F protein.

Class I viral fusion proteins share common mechanistic and structural features but little sequence similarity. Structural insights into the protein conformational changes associated with membrane fusion are based largely on studies of the influenza virus hemagglutinin in pre-and postfusion conformations. Here, we present the crystal structure of the secreted, uncleaved ectodomain of the paramyxovirus, human parainfluenza virus 3 fusion (F) protein, a member of the class I viral fusion protein group. The secreted human parainfluenza virus 3 F forms a trimer with distinct head, neck, and stalk regions. Unexpectedly, the structure reveals a six-helix bundle associated with the postfusion form of F, suggesting that the anchor-minus ectodomain adopts a conformation largely similar to the postfusion state. The transmembrane anchor domains of F may therefore profoundly influence the folding energetics that establish and maintain a metastable, prefusion state.atomic structure ͉ class I fusion protein ͉ membrane fusion

High quality factors are essential for vibratory microsensors. Therefore, the vibrating structure of the sensors is often encapsulated in a housing where the air is evacuated for reduced air damping. However, the vacuum is usually low and the quality factor is still mainly determined by the energy losses to the surrounding air molecules. Air damping in low vacuum is usually estimated using the free molecular model proposed by Christian (Christian R 1966 Vacuum 16 175–8). The major drawback of the model is that the effect of the nearby objects (e.g. the electrodes for electrostatic driving) and the dimensions of the plate cannot be considered. Therefore, the damping effect is often significantly underestimated for real structures.This paper proposes a new model for air damping of microstructures in low vacuum. In this model, the damping effect is calculated by using an energy transfer mechanism instead of the momentum transfer mechanism in Christian's model. For an isolated oscillating plate, the calculated quality factor by the model is the same as that by Christian's model. However, for an oscillating plate with a neighboring object, the damping effect by the new model is related to the dimensions of the vibrating plate and the gap between the plate and the nearby object. The quality factors calculated agree with experimental data better than with Christian's model by about an order of magnitude.