Assembly-dependent conformational changes in a viral capsid protein

Inter- and intra-subunit bonding within the surface lattice of the capsid of bacteriophage T4 has been investigated by differential scanning calorimetry of polyheads, in conjunction with electron microscopy, limited proteolysis and sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The bonding changes corresponding to successive stages of assembly of the major capsid protein gp23, including its maturation cleavage, were similarly characterized. The uncleaved/unexpanded surface lattice exhibits two endothermic transitions. The minor event, at 46 degrees C, does not visibly affect the surface lattice morphology and probably represents denaturation of the N-terminal domain of gp23. The major endotherm, at 65 degrees C, represents denaturation of the gp23 polymers. Soluble gp23 from dissociated polyheads is extremely unstable and exhibits no endotherm. Cleavage of gp23 to gp23* and the ensuing expansion transformation effects a major stabilization of the surface lattice of polyheads, with single endotherms whose melting temperatures (t*m) range from 73 to 81 degrees C, depending upon the mutant used and the fraction of gp23 that is cleaved to gp23* prior to expansion. Binding of the accessory proteins soc and hoc further modulates the thermograms of cleaved/expanded polyheads, and their effects are additive. hoc binding confers a new minor endotherm at 68 degrees C corresponding to at least partial denaturation of hoc. Denatured hoc nevertheless remains associated with the surface lattice, although in an altered, protease-sensitive state which correlates with delocalization of hoc subunits visualized in filtered images. While hoc binding has little effect on the thermal stability of the gp23* matrix, soc binding further stabilizes the surface lattice (delta Hd approximately +50%; delta t*m = +5.5 degrees C). It is remarkable that in all states of the surface lattice, the inter- and intra-subunit bonding configurations of gp23 appear to be co-ordinated to be of similar thermal stability. Thermodynamically, the expansion transformation is characterized by delta H much less than 0; delta Cp approximately 0, suggesting enhancement of van der Waals' and/or H-bonding interactions, together with an increased exposure to solvent of hydrophobic residues of gp23* in the expanded state. These findings illuminate hypotheses of capsid assembly based on conformational properties of gp23: inter alia, they indicate a role for the N-terminal portion of gp23 in regulating polymerization, and force a reappraisal of models of capsid swelling based on the swivelling of conserved domains.