Thermal Denaturation Equilibria of Tmv Coat Proteins

Thermal denaturation of RNA free coat proteins of tobacco mosaic virus (TMV) was studied for wildtype TMV (vulgare) and the temperature-sensitive mutant, Ni 118. The ability to form soluble aggregates as well as the optical properties (ORD, UV-difference spectra), and the sedimen­ tation behavior were used as criteria for the native state. At pH 7.5 , 7 = 0 .0 2 denaturation is reversible for both proteins. The ORD data indicate that the denatured proteins contain residual secondary structure. The “melting temperatures”, as defined by ORD measurements (cp = 0 .02 mM), are 3 9 .5 + 1 °C for vulgare and 27 + 1 °C for Ni 118 at pH 7.5 , 7 = 0 .0 2 . By means of the aggregation test (cp = 0 .05m M ) somewhat lower melting temperatures were found under the same solvent conditions. The difference between the primary structures of vulgare and Ni 118 proteins being a proline —*■ leucine (pos. 20) replacement, the results suggest the cyclic structure of proline (20) to have a specific stabilizing function in the three dimensional protein structure. This conclusion is supported by tive mutant with a threonine residue in pos. 20.