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

Abstract

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 α-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 α-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 β-strand judging by a prominent sharp negative ROA band shown by TRV at ∼1236 cm−1, but little α-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 α-helix, could serve to fill the extra volume required by the larger diameter of the cylindrical TRV particles relative to those of TMV.