Toroidal surface complexes of bacteriophage ϕ12 are responsible for host-cell attachment

Abstract

Cryo-electron tomography and subtomogram averaging are utilized to determine that the bacteriophage ϕ12, a member of the Cystoviridae family, contains surface complexes that are: toroidal in shape; composed of six globular domains with six-fold symmetry; and have a discrete density connecting them to the virus membrane-envelope surface. The lack of a hexameric spike in a reassortant of ϕ12 demonstrates that the gene for the hexameric spike is located in ϕ12’s medium length genome segment, likely to the P3 open reading frames which are the proteins involved in viral-host cell attachment. Based on this, and on protein mass estimates derived from the obtained averaged structure, it is suggested that each of the globular domains is most likely composed of a total of four copies of P3a and/or P3c proteins. Our findings may have implications in the study of the evolution of the cystovirus species in regard to their host specificity.

A new view into prokaryotic cell biology from electron cryotomography

“…Electron cryotomography (ECT) studies have elucidated the structural features of viral capsids 170,171 and detailed phage attachment to bacterial cells 172–175 , including a particularly elegant mechanism by which phage wrap a filament around the flagellum of Caulobacter crescentus , taking advantage of flagellar rotation to move toward the cell pole where they are concentrated around their pilus entry point, increasing the chance of infection in a dilute environment 176 (see the figure, part a ). Another study of a virus that attacks the conjugative F-pilus used subtomogram averaging to reveal asymmetries in the capsid that may lead to viral entry 177 .…”

Bacteriophage φ6—Structure Investigated by Fluorescence Stokes Shift Spectroscopy

“…However, in a more recent 1992 paper (17), it was suggested the number of P3 proteins per virion is far less than this estimate since only a very small number of P3 are needed to restore infectivity to inactive virions. Recent cryo‐electron tomography of φ6 (Alvin Katz and Paul Gottlieb, unpublished data) indicate that there are 4–10 surface spikes per virion, a number comparable to the average of five surface attachment apparatus observed on the related φ12 virus (36). From this estimate of surface spike number, there are between 284 and 530 Trp residues in the envelope and surface spikes (including the 120 Trp in P5); substantially fewer Trp than the 1968 Trp residues in the PC and NC.…”

Cryo-electron tomography of bacterial viruses

“…In addition to better 3D reconstructions through dual axis tomography, algorithms have been developed for averaging sub-volumes of tomograms (Briggs et al, 2009; Leo-Macias et al, 2011; Schmid and Booth, 2008). Sub-tomogram averaging procedures generate isotropic, higher signal-to-noise structures of a complex of interest.…”

“…To complement these findings, Leo-Macias et al (2011) carried out sub-tomogram averaging of the surface complex of Φ12, averaging 744 copies of this surface density from 158 bacteriophage particles. Approximately five, randomly distributed, toroidal, surface complexes were found to be 13 nm from the membrane of the cystovirus.…”