Cryo-Electron Microscopy and Image Analysis of SV40

The discovery that the T=7 icosahedral capsid of polyoma virus is composed of 72 pentameric capsomers rather than 12 pentamers and 60 hexamers as predicted by constraints of quasi-equivalence has prompted an examination of SV40 virus by electron microscopy to determine whether the capsids of other members of the papovavirus family are similarly constructed.Thin layers of buffered aqueous solutions (∼4 mg/ml) of Simian virus 40 (strain WT776) were prepared for cryo-microscopy using recently developed procedures. Images of virus particles suspended in thin layers of vitreous ice over holes in the carbon support film and maintained at -170°C were recorded using minimal irradiation conditions. Figure 1 shows a typical field in which the frozen solution is similar in thickness to the virus particles (∼49 nm diameter). Particles appear to be excluded or squeezed away from the thinnest regions of solution (e.g., the clear region bordered by particles at the top of Fig. 1)