The Number of Protein Subunits Per Helix Turn in Narcissus Mosaic Virus Particles

Abstract

SUMMARYBy combining the results of Fourier transform calculations from digitized electron micrographs and molecular volume calculations based on the amino acid composition and RNA content of narcissus mosaic virus particles, firm evidence that there are 8-8 protein subunits per turn of the helix in the virus particles was obtained.Narcissus mosaic virus (NMV) is a potexvirus and has elongated flexuous particles, with a length of about 550 nm and a diameter of about 13 nm (Tollin et al., 1967). X-ray diffraction studies of orientated virus particles can be interpreted in terms of a helical arrangement of protein sabunits, with 5q -1 subunits in five turns of the helix, where q is an integer lying in the range 7 <~ q <~ 10 ( Tollin et al., 1968). The pitch of the helix varies from 3.6 nm when the water content of the specimen is high to 3.3 nm in the dry specimen.Optical diffraction patterns from electron micrographs of NMV particles can also be interpreted on the basis of a helical arrangement of protein subunits. The first studies (ToUin et al., 1975) suggested that the value of q was 7, but a re-evaluation of the optical diffraction patterns suggested that q was 9 (Bancroft et al., 1980). Although the X-ray diffraction evidence (Tollin et al., 1968) also slightly favoured a q value of 9, we have now combined the results of Fourier transform calculations based on digitization of the electron micrographs (Low, 1982), and molecular volume calculations based on the amino acid composition of the NMV protein and the RNA content of the virus which give unambiguous evidence in favour of q = 9.The optical diffraction patterns only give information about the amplitudes in the Fourier transform whereas the calculated transform from digitized images gives the phases of the transform as well. This makes it possible to distinguish between an even and an odd Bessel function because the phase difference across the meridian (see Fig.

Structure of Flexible Filamentous Plant Viruses

“…One apparent exception to these predictions might be the very different reported helical symmetries of the rigid tobamoviruses and tobraviruses (23,28). However, reports of the tobravirus symmetry (about 25 subunits per turn) all appear to depend on a single early electron microscopic observation, whereas stoichiometric calculations similar to those that have been used for the potexviruses (38) and potyviruses (51) suggest that the tobraviruses have only about 16 subunits per turn, similar to the tobamoviruses. The existence of a common protein fold will greatly facilitate the design of modified coat proteins for use in peptide expression (12,54) and conferral of resistance on hosts (39,57) and has important implications for the taxonomy, evolution, and further structural study of filamentous viruses.…”