PRESSURE DENATURATION OF THE BACTERIOPHAGE-P22 COAT PROTEIN AND ITS ENTROPIC STABILIZATION IN ICOSAHEDRAL SHELLS

The pressure stability of bacteriophage P22 coat protein in both monom eric and polymeric forms under hydrostatic pressure was examined using light scattering, fluorescence emission, polarization, and lifetime m ethodology. The monomeric protein is very unstable toward pressure and undergoes significant structural changes at pressures as low as 0.5 k bar. These structural changes ultimately lead to denaturation of the s ubunit. Comparison of the protein denatured by pressure to that in gua nidine hydrochloride suggests that pressure results in partial unfoldi ng, perhaps by a domain mechanism. Fluorescence lifetime measurements indicate that at atmospheric pressure the local environments of the tr yptophans are remarkably similar, suggesting they may be clustered. In contrast to the monomeric protein subunit, the protein when polymeriz ed into procapsid shells is very stable to applied pressure and does n ot dissociate with pressure up to 2.5 kbar. However, under applied pre ssure the procapsid shells are cold-labile, suggesting they are entrop ically stabilized. The significance of these results in terms of virus assembly are discussed.