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

Vibrational Raman optical activity (ROA) spectra of tobacco rattle virus (T RV) and tobacco mosaic virus (TMV) were measured and compared with a view t o obtaining new information about the coat protein subunit structure of TRV , A sharp strong positive band observed at similar to 1344 cm(-1) in the RO A spectra of the two viruses is evidence that both contain a significant am ount of a hydrated form of alpha -helix, but more in TRV than in TMV. Altho ugh the ROA spectrum of TMV shows significant positive intensity in the ran ge similar to 1297-1312 cm(-1) characteristic of alpha -helix in a hydropho bic 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 similar to 1315 cm(-1), of much greater intensi ty 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 str ucture, plus perhaps some beta -strand judging by a prominent sharp negativ e ROA band shown by TRV at similar to 1236 cm(-1), but little alpha -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 alpha -helix, could serve to fil l the extra volume required by the larger diameter of the cylindrical TRV p articles relative to those of TMV.