MECHANISM OF CAPSID MATURATION IN A DOUBLE-STRANDED DNA VIRUS

Folding mechanisms of proteins incorporated within supramolecular asse mblies, including viruses, are little understood and may differ fundam entally from folding mechanisms of small globular proteins. We describ e a novel Raman dynamic probe of hydrogen-isotope exchange to investig ate directly these protein folding/assembly pathways. The method is ap plied to subunit folding in assembly intermediates of the double-stran ded DNA bacteriophage P22. The icosahedral procapsid-to-capsid maturat ion (shell expansion) of P22 is shown to be accompanied by a large inc rease in exchange protection of peptide beta P-strands. The molecular mechanism of shell expansion involves unfolding of metastable tertiary structure to form more stable quaternary contacts and is governed by a surprisingly high activation energy. The results demonstrate that co at subunit folding and capsid expansion are strongly coupled processes . Subunit structure in the procapsid represents a late intermediate al ong the folding/assembly pathway to the mature capsid. Coupling of fol ding and assembly is proposed as a general pathway for the constructio n of supramolecular complexes.