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Electron Cryomicroscopy Reveals Different F1+F2 Protein States in Intact Parainfluenza Virions

Forschungszentrum für Elektronenmikroskopie, Freie Universität Berlin, Fabeckstrasse 36a, D-14195 Berlin, Germany,¹ Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, Institut für Biologie/Biophysik, Invalidenstrasse 42, D-10115 Berlin, Germany,² Institut für Immunologie und Molekularbiologie, Fachbereich Veterinärmedizin, Freie Universität Berlin, Philippstrasse 13, D-10115 Berlin, Germany³

Received 2 October 2007/ Accepted 13 January 2008

Electron cryomicrographs of intact parainfluenza virus 5 (PIV5) virions revealed two different surface structures, namely, a continuous layer and distinct individual spikes. The structure of these spikes reconstructed from intact virions was compared with known F ectodomain structures and was found to be different from the prefusion PIV5 F0 structure but, surprisingly, very similar to the human PIV3 F postfusion structure. Hence, we conclude that the individual F1+F2 spikes in intact PIV5 virions also correspond to the postfusion state. Since the observed fusion activity of PIV5 virions has to be associated with prefusion F1+F2 proteins, they have necessarily to be localized in the continuous surface structure. The data therefore strongly suggest that the prefusion state of the F1+F2 protein requires stabilization, most probably by the association with hemagglutinin-neuraminidase. The conversion of F1+F2 proteins from the prefusion toward the postfusion state while embedded in the virus membrane is topologically difficult to comprehend on the basis of established models and demands reconsideration of our current understanding.

Published ahead of print on 23 January 2008.