COLD DENATURATION OF AN ICOSAHEDRAL VIRUS - THE ROLE OF ENTROPY IN VIRUS ASSEMBLY

Assembly of icosahedral viruses is not completely understood at the mo lecular level. The main puzzle is to answer how chemically identical p rotein subunits take up unique positionally dependent conformations du ring the process of assembly. The stability of the ribonucleoprotein p articles of cowpea mosaic virus (CPMV) to pressures and subzero temper atures has been studied. At room temperature, reversible pressure dena turation of CPMV is obtained only in the presence of 5.0 M urea. On th e other hand, when the temperature is decreased to -15 degrees C, the ribonucleoprotein components denature, at 2.5 kbar, in the presence of 1.0 M urea. At temperatures close to -20 degrees C, denaturation is o btained even in the absence of urea. Whereas the denaturation promoted by pressure and urea at room temperature is reversible, virus particl es denatured when the temperature is decreased under pressure cannot r eassemble. Bis-ANS binding data suggest that this irreversibility may be related to protein release from RNA, which probably does not occur under denaturating conditions at room temperature. The contributions o f enthalpy (Delta H) and entropy (Delta S*) for the free energy of as sociation of CPMV are calculated from the cold denaturation curves und er pressure. The entropy change is positive and large, making the asse mbly of ribonucleoprotein components an entropy-driven process, sugges ting that the burial of nonpolar side chains during the process of ass embly is the structural foundation for CPMV assembly.