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Nat Commun. 2017 Jan 30;8:14310. doi: 10.1038/ncomms14310.

Portal protein functions akin to a DNA-sensor that couples genome-packaging to icosahedral capsid maturation.

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Department of Biochemistry and Molecular Biology, Thomas Jefferson University, 233 South 10th Street, Philadelphia, Pennsylvania 19107, USA.
Department of Biochemistry and Molecular Biology, Rutgers University, 683 Hoes lane, Piscataway, New Jersey 08854, USA.
Molecular Biophysics &Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Department of Molecular and Cell Biology Department of Chemistry, University of Connecticut, 91N. Eagleville Road, Storrs, Connecticut 06269, USA.
Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, Michigan 48824, USA.
Institute of Biomembranes and Bioenergetics, National Research Council, Via Amendola 165/A, 70126 Bari, Italy.


Tailed bacteriophages and herpesviruses assemble infectious particles via an empty precursor capsid (or 'procapsid') built by multiple copies of coat and scaffolding protein and by one dodecameric portal protein. Genome packaging triggers rearrangement of the coat protein and release of scaffolding protein, resulting in dramatic procapsid lattice expansion. Here, we provide structural evidence that the portal protein of the bacteriophage P22 exists in two distinct dodecameric conformations: an asymmetric assembly in the procapsid (PC-portal) that is competent for high affinity binding to the large terminase packaging protein, and a symmetric ring in the mature virion (MV-portal) that has negligible affinity for the packaging motor. Modelling studies indicate the structure of PC-portal is incompatible with DNA coaxially spooled around the portal vertex, suggesting that newly packaged DNA triggers the switch from PC- to MV-conformation. Thus, we propose the signal for termination of 'Headful Packaging' is a DNA-dependent symmetrization of portal protein.

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