Solution NMR studies of the maturation intermediates of a 13 MDa viral capsid.

Dynamic regions of proteins often play essential roles in function, assembly, or maturation of macromolecular complexes. When X-ray crystallography and cryo-electron microscopy are used to investigate macromolecular structures, information about these dynamic regions is lost because of conformational disorder or flexibility. Structural studies on the precursor capsid conformations of the lambdoid bacteriophage HK97, a model system for macromolecular maturation, reveal that essential regions tend to be disordered at early maturation stages. In the Prohead II intermediate, the regions that have the greatest disorder are the N-terminal residues and the E-loop, a region involved in the formation of inter-subunit cross-links. The N-terminus of the subunits in the 13 MDa procapsid is sufficiently dynamic to be studied by solution nuclear magnetic resonance (NMR) spectroscopy. NMR measurements enabled the identification and assignment of resonances to specific residues, assessment of the region's behavior within the context of the capsid, and monitoring of changes in these residues during the maturation process. In the precursor Prohead II and immature EI-III states, the N-termini are found to make transient interactions with the interior capsid surface, while at least a subset of N-termini in EI-III becomes more flexible with time. No resonances are observed for the fully mature Head II capsid, which is consistent with its completely ordered structure. NMR spectroscopy complements the current X-ray crystallography and cryo-electron microscopy data of HK97 by providing key information about the behavior of essential dynamic regions only inferred by other techniques.