The 12 A structure of trypsin-treated measles virus N-RNA.

Recombinant measles virus nucleoprotein (N) was produced in insect cells where it bound to cellular RNA to form helical N-RNA structures. These structures were observed by electron microscopy but were too flexible for high-resolution image analysis. Removal of the C-terminal tail of N by trypsin treatment resulted in structures that were much more rigid and seemed more regular. Several methods of image analysis were employed in order to make a helical reconstruction of the digested N-RNA. During this analysis, it became clear that the apparently regular coils of digested N-RNA consisted of a series of closely related helical states. The iterative helical real space reconstruction method allowed the identification of two helical states for which a reconstruction could be calculated. The model with the highest resolution shows N monomers that consist of three domains and that are connected to their neighbours by two narrow connections, one close to the helical axis and another toward the middle of the monomers. There are no connections between N molecules in subsequent helical turns. After labelling the RNA in the structure with cis-platinum, the connection closest to the helical axis increased in density, suggesting the position of the RNA. The shapes of the monomers of the nucleoproteins of influenza virus, rabies virus (both determined before) and that of measles virus (determined here) are all similar, whereas the overall shapes of their respective N-RNAs (nucleocapsids) is very different. This is probably due to the position and number of the connections between the N subunits in the N-RNA, one for influenza virus allowing much flexibility, two for rabies virus at either end of the N molecules leading to ribbons and two for measles virus leading to the typical paramyxovirus helical nucleocapsid.