Direct stereoscopic visualization of icosahedral capsid geometry in bacteriophage T4 freeze-fractured, deep-etched replicas
Bacteriophage T4 is one of the most complex of the tailed bacteriophage. Its DNA is packaged inside a protein capsid whose structure has long been the subject of study. All T-even bacteriophage appear to have a capsid structure based upon an icosahedral geometry. This has been established indirectly by a combination of symmetry considerations and physical and biochemical experiments, including some electron microscopy. Perhaps the most persuasive demonstration of the icosahedral capsid geometry in bacteriophage T2 is not based on a direct visualisation of the T2 capsid. Instead, the argument relies critically on the authors' ability to accurately count protein capsomers on the T2 capsid replica of a freeze-fractured, etched sample. Moreover, the icosahedral geometry of T4 capsids is simply inferred from results on T2. In the present study, we visualize directly the prolate icosahedral capsid geometry of T4 bacteriophage from stereopairs of micrographs of replicas produced by the freeze-fracture, deep-etch technique utilizing vertical replication to identify the triangular faces comprising the icosahedral T4 capsid.