title: A thermally induced phase transition in a viral capsid transforms the hexamers, leaving the pentamers unchanged.
abstract: Scanning calorimetry combined with cryo-electron microscopy affords a powerful approach to investigating hierarchical interactions in multi-protein complexes. Calorimetry can detect the temperatures at which certain interactions are disrupted and cryo-EM can reveal the accompanying structural changes. The procapsid of bacteriophage HK97 (Prohead I) is a 450A-diameter shell composed of 60 hexamers and 12 pentamers of gp5, organized with icosahedral symmetry. Gp5 consists of the N-terminal Delta-domain (11kDa) and gp5* (31 kDa): gp5* forms the contiguous shell from which clusters of Delta-domains extend inwards. At neutral pH, Prohead I exhibits an endothermic transition at 53 degrees C with an enthalpy change of 14 kcal/mole (of gp5 monomer). We show that this transition is reversible. To capture its structural expression, we incubated Prohead I at 60 degrees C followed by rapid freezing and, by cryo-EM, observed a capsid species 10% larger than Prohead I. At 11A resolution, visible changes are confined to the gp5 hexamers. Their Delta-domain clusters have disappeared and are presumably disordered, either by unfolding or dispersal. The gp5* hexamer rings are thinned and flattened as they assume the conformation observed in Expansion Intermediate I, a transition state of the normal, proteolysis-induced, maturation pathway. We infer that, at ambient temperatures, the hexamer Delta-domains restrain their gp5* rings from switching to a lower free energy, EI-I-like, state; above 53 degrees, this restraint is overcome. Pentamers, on the other hand, are more stably anchored and resist this thermal perturbation.
authors.affiliation: Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA.
authors.forename: James F
authors.initials: JF
authors.lastname: Conway
authors.forename: Naiqian
authors.initials: N
authors.lastname: Cheng
authors.forename: Philip D
authors.initials: PD
authors.lastname: Ross
authors.forename: Roger W
authors.initials: RW
authors.lastname: Hendrix
authors.forename: Robert L
authors.initials: RL
authors.lastname: Duda
authors.forename: Alasdair C
authors.initials: AC
authors.lastname: Steven
mesh_terms: D001435:Bacteriophages / Q000648:ultrastructure*; D002152:Calorimetry, Differential Scanning; D002213:Capsid / Q000648:ultrastructure*; D036022:Capsid Proteins / Q000648:ultrastructure*; D020285:Cryoelectron Microscopy; D006358:Hot Temperature; D044367:Phase Transition
chemical_list: D036022:Capsid Proteins