Figure 6.

(Upper) A mechanism accounting for increased exchange protection in the expanded shell (right) vis-à-vis the procapsid shell (left). The effective increase in the subunit exchange-protected core with expansion is due to domain interchange between neighboring subunits. The shell lattice (upper diagram) is represented as a cluster of six subunits (hexon) in which the protected core is indicated by dark shading and the two coat protein domains are shown as unshaded and lightly shaded. The enlargement (lower diagram) depicts rearrangement of contacts and domains, including Cys 405 sulfhydryls, between two neighboring subunits. (Lower) Energy landscape representation of coupling between subunit folding and capsid assembly in P22. A transition state containing partially exposed hydrophobic surfaces (dark gray) is proposed for the heat-induced expansion.