title: Role of the scaffolding protein in P22 procapsid size determination suggested by T = 4 and T = 7 procapsid structures.
abstract: Assembly of bacteriophage P22 procapsids requires the participation of approximately 300 molecules of scaffolding protein in addition to the 420 coat protein subunits. In the absence of the scaffolding, the P22 coat protein can assemble both wild-type-size and smaller size closed capsids. Both sizes of procapsid assembled in the absence of the scaffolding protein have been studied by electron cryomicroscopy. These structural studies show that the larger capsids have T = 7 icosahedral lattices and appear the same as wild-type procapsids. The smaller capsids possess T = 4 icosahedral symmetry. The two procapsids consist of very similar penton and hexon clusters, except for an increased curvature present in the T = 4 hexon. In particular, the pronounced skewing of the hexons is conserved in both sizes of capsid. The T = 7 procapsid has a local non-icosahedral twofold axis in the center of the hexon and thus contains four unique quasi-equivalent coat protein conformations that are the same as those in the T = 4 procapsid. Models of how the scaffolding protein may direct these four coat subunit types into a T = 7 rather than a T = 4 procapsid are presented.
authors.affiliation: Department of Computational and Applied Mathematics, W. M. Keck Center for Computational Biology, Rice University, Houston, Texas 77005-1892, USA. pthuman@caam.rice.edu
authors.forename: P A
authors.initials: PA
authors.lastname: Thuman-Commike
authors.forename: B
authors.initials: B
authors.lastname: Greene
authors.forename: J A
authors.initials: JA
authors.lastname: Malinski
authors.forename: J
authors.initials: J
authors.lastname: King
authors.forename: W
authors.initials: W
authors.lastname: Chiu
mesh_terms: D017100:Bacteriophage P22 / Q000737:chemistry* / Q000648:ultrastructure; D002213:Capsid / Q000096:biosynthesis / Q000737:chemistry* / Q000648:ultrastructure; D005615:Freezing; D046911:Macromolecular Substances; D008854:Microscopy, Electron; D008958:Models, Molecular; D011487:Protein Conformation; D015678:Viral Structural Proteins / Q000378:metabolism*; D019065:Virus Assembly
chemical_list: D046911:Macromolecular Substances; D015678:Viral Structural Proteins; C111088:scaffolding protein, bacteriophage P22