title: Capsid expansion mechanism of bacteriophage T7 revealed by multistate atomic models derived from cryo-EM reconstructions.
abstract: Many dsDNA viruses first assemble a DNA-free procapsid, using a scaffolding protein-dependent process. The procapsid, then, undergoes dramatic conformational maturation while packaging DNA. For bacteriophage T7 we report the following four single-particle cryo-EM 3D reconstructions and the derived atomic models: procapsid (4.6-Å resolution), an early-stage DNA packaging intermediate (3.5 Å), a later-stage packaging intermediate (6.6 Å), and the final infectious phage (3.6 Å). In the procapsid, the N terminus of the major capsid protein, gp10, has a six-turn helix at the inner surface of the shell, where each skewed hexamer of gp10 interacts with two scaffolding proteins. With the exit of scaffolding proteins during maturation the gp10 N-terminal helix unfolds and swings through the capsid shell to the outer surface. The refolded N-terminal region has a hairpin that forms a novel noncovalent, joint-like, intercapsomeric interaction with a pocket formed during shell expansion. These large conformational changes also result in a new noncovalent, intracapsomeric topological linking. Both interactions further stabilize the capsids by interlocking all pentameric and hexameric capsomeres in both DNA packaging intermediate and phage. Although the final phage shell has nearly identical structure to the shell of the DNA-free intermediate, surprisingly we found that the icosahedral faces of the phage are slightly (∼4 Å) contracted relative to the faces of the intermediate, despite the internal pressure from the densely packaged DNA genome. These structures provide a basis for understanding the capsid maturation process during DNA packaging that is essential for large numbers of dsDNA viruses.
authors.affiliation: Markey Center for Structural Biology, Department of Biological Sciences, Purdue University, West Lafayette, IN 47907;
authors.forename: Fei
authors.initials: F
authors.lastname: Guo
authors.affiliation: Markey Center for Structural Biology, Department of Biological Sciences, Purdue University, West Lafayette, IN 47907;
authors.forename: Zheng
authors.initials: Z
authors.lastname: Liu
authors.affiliation: Markey Center for Structural Biology, Department of Biological Sciences, Purdue University, West Lafayette, IN 47907;
authors.forename: Ping-An
authors.initials: PA
authors.lastname: Fang
authors.affiliation: State Key Laboratory of Biocontrol, School of Life Sciences, Sun Yat-sen University, Guangzhou 510275, China; National Center for Macromolecular Imaging, Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030; and.
authors.forename: Qinfen
authors.initials: Q
authors.lastname: Zhang
authors.affiliation: Department of Biochemistry, The University of Texas Health Science Center, San Antonio, TX 78229.
authors.forename: Elena T
authors.initials: ET
authors.lastname: Wright
authors.affiliation: Markey Center for Structural Biology, Department of Biological Sciences, Purdue University, West Lafayette, IN 47907;
authors.forename: Weimin
authors.initials: W
authors.lastname: Wu
authors.affiliation: Markey Center for Structural Biology, Department of Biological Sciences, Purdue University, West Lafayette, IN 47907;
authors.forename: Ci
authors.initials: C
authors.lastname: Zhang
authors.affiliation: Markey Center for Structural Biology, Department of Biological Sciences, Purdue University, West Lafayette, IN 47907;
authors.forename: Frank
authors.initials: F
authors.lastname: Vago
authors.affiliation: Markey Center for Structural Biology, Department of Biological Sciences, Purdue University, West Lafayette, IN 47907;
authors.forename: Yue
authors.initials: Y
authors.lastname: Ren
authors.affiliation: National Center for Macromolecular Imaging, Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030; and.
authors.forename: Joanita
authors.initials: J
authors.lastname: Jakana
authors.affiliation: National Center for Macromolecular Imaging, Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030; and.
authors.forename: Wah
authors.initials: W
authors.lastname: Chiu
authors.affiliation: Department of Biochemistry, The University of Texas Health Science Center, San Antonio, TX 78229.
authors.forename: Philip
authors.initials: P
authors.lastname: Serwer
authors.affiliation: Markey Center for Structural Biology, Department of Biological Sciences, Purdue University, West Lafayette, IN 47907; jiang12@purdue.edu.
authors.forename: Wen
authors.initials: W
authors.lastname: Jiang
mesh_terms: D017123:Bacteriophage T7 / Q000737:chemistry* / Q000648:ultrastructure; D002213:Capsid / Q000737:chemistry* / Q000648:ultrastructure; D036022:Capsid Proteins / Q000737:chemistry; D020285:Cryoelectron Microscopy; D042003:DNA Packaging; D007091:Image Processing, Computer-Assisted; D008958:Models, Molecular; D011485:Protein Binding; D017433:Protein Structure, Secondary; D019065:Virus Assembly
chemical_list: D036022:Capsid Proteins
keywords: DNA packaging intermediates; bacteriophage T7 maturation; noncovalent topological linking; procapsid; single-particle cryo-EM