Unusual segmental flexibility in a region of tobacco mosaic virus coat protein

ASSEMBLY of tobacco mosaic virus (TMV) is known to occur by threading in the RNA on the inner side of the growing hollow rodlet of viral coat protein, and is thought to involve a pre-assembled double disk as an intermediate1–3. Given the structure of the subunit4, such a mechanism requires that it be possible to move the segment of the polypeptide chain which separates the nucleic acid groove from the lumen of the cylinder so that it is out of the way in the assembly process. Evidence suggesting that the segment in question has some flexibility comes from X-ray diffraction studies of the virus and its components. In the intact virus studied by the Heidelberg group this segment (residues 102–112) apparently exists largely as an a-helix (the V-helix)4; on the other hand, in the crystals of the isolated coat protein disk studied by the Cambridge group, residues 102–112 and adjacent segments (88–100, 112–114) do not give a well-defined electron density, reflecting a high degree of disorder5. However, X-ray evidence is inconclusive on this point, as it does not distinguish between thermal disorder indicating flexibility and static disorder indicating irregular packing of the segment in the crystal, de Wit et al.6,7 have pointed out that there must be internal mobility in the TMV coat protein, based on the results of 13C NMR of various TMV protein aggregates. Unfortunately, their results do not show whether or not there is flexibility in a particular segment of the protein. In an effort to distinguish between the thermal disorder and the static disorder in the V-helix segment, we describe here our investigation of the problem by high-resolution proton nuclear magnetic resonance (NMR) studies, and report a flexibility due to thermal motional disorder