title: Structure and size determination of bacteriophage P2 and P4 procapsids: function of size responsiveness mutations.
abstract: Bacteriophage P4 is dependent on structural proteins supplied by a helper phage, P2, to assemble infectious virions. Bacteriophage P2 normally forms an icosahedral capsid with T=7 symmetry from the gpN capsid protein, the gpO scaffolding protein and the gpQ portal protein. In the presence of P4, however, the same structural proteins are assembled into a smaller capsid with T=4 symmetry. This size determination is effected by the P4-encoded protein Sid, which forms an external scaffold around the small P4 procapsids. Size responsiveness (sir) mutants in gpN fail to assemble small capsids even in the presence of Sid. We have produced large and small procapsids by co-expression of gpN with gpO and Sid, respectively, and applied cryo-electron microscopy and three-dimensional reconstruction methods to visualize these procapsids. gpN has an HK97-like fold and interacts with Sid in an exposed loop where the sir mutations are clustered. The T=7 lattice of P2 has dextro handedness, unlike the laevo lattices of other phages with this fold observed so far.
authors.affiliation: Department of Microbiology, University of Alabama at Birmingham, Birmingham, AL 35294, USA.
authors.forename: Altaira D
authors.initials: AD
authors.lastname: Dearborn
authors.forename: Pasi
authors.initials: P
authors.lastname: Laurinmaki
authors.forename: Preethi
authors.initials: P
authors.lastname: Chandramouli
authors.forename: Cynthia M
authors.initials: CM
authors.lastname: Rodenburg
authors.forename: Sifang
authors.initials: S
authors.lastname: Wang
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: Butcher
authors.forename: Terje
authors.initials: T
authors.lastname: Dokland
mesh_terms: D017901:Bacteriophage P2 / Q000737:chemistry* / Q000235:genetics / Q000648:ultrastructure*; D002213:Capsid / Q000737:chemistry* / Q000000981:diagnostic imaging*; D020285:Cryoelectron Microscopy; D008954:Models, Biological; D009154:Mutation; D017900:Myoviridae / Q000737:chemistry* / Q000235:genetics / Q000648:ultrastructure*; D017433:Protein Structure, Secondary; D014463:Ultrasonography