A comparison of the solution structures of tobacco rattle and tobacco mosaic viruses from Raman optical activity

Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK 1 Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK 2 Author for correspondence: Laurence Barron. Fax +44 141 330 4888. e-mail laurence{at}chem.gla.ac.uk Vibrational Raman optical activity (ROA) spectra of tobacco rattle virus (TRV) and tobacco mosaic virus (TMV) were measured and compared with a view to obtaining new information about the coat protein subunit structure of TRV. A sharp strong positive band observed at 1344 cm -1 in the ROA spectra of the two viruses is evidence that both contain a significant amount of a hydrated form of -helix, but more in TRV than in TMV. Although the ROA spectrum of TMV shows significant positive intensity in the range 1297–1312 cm -1 characteristic of -helix in a hydrophobic environment, as expected from the helix interface residues in the four-helix bundles that constitute the basic motif of the TMV coat protein fold, that of TRV shows little positive ROA intensity here. Instead TRV shows a strong positive ROA band at 1315 cm -1 , of much greater intensity than bands shown here by TMV, that is characteristic of polyproline II (PPII) helix. This suggests that the additional long central and C-terminal sequences of the TRV coat proteins contain a significant amount of PPII structure, plus perhaps some -strand judging by a prominent sharp negative ROA band shown by TRV at 1236 cm -1 , but little -helix. The open flexible hydrated nature of PPII helical structure is consistent with the earlier suggestions that the additional sequences are exposed and, together with a larger amount of hydrated -helix, could serve to fill the extra volume required by the larger diameter of the cylindrical TRV particles relative to those of TMV.