Synthesis and Assembly of Simian Virus 40 II. Synthesis of the Major Capsid Protein and Its Incorporation into Viral Particles

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African green monkey kidney cells infected by simian virus 40 were analyzed for the presence of the major capsid protein (capsid protein I) by immunological and radiolabeling techniques. Antisera with different specificities were prepared by immunization with intact or denatured viral particles. Antisera prepared against intact virus reacted by complement fixation with viral particles and with an 8S subunit containing the capsid protein I. Antisera prepared against denatured viral particles reacted with unassembled capsid protein(s) as well as with viral particles. These antisera were used to detect 8S viral subunits or unassembled viral capsid protein in soluble extracts of infected cells after centrifugation at 100,000 × g to remove viral particles. The soluble antigen pool was found to be small during infection with wild-type virus or a temperature-sensitive mutant deficient in the synthesis of viral particles. Pulse-chase experiments, performed at a high multiplicity of infection, also indicated a small pool of nonparticle capsid protein I. Radioactive lysine was incorporated into capsid protein I of virus particles during a 2-hr pulse. A subsequent chase with excess unlabeled lysine resulted in only a slight increase in the radio-activity found in capsid protein I of viral particles. Furthermore, in the same experiments, capsid protein I was incorporated preferentially into empty shells during the pulse with a shift in radioactivity to intact virions during the chase period, indicating a possible precursor relationship between the two types of virus particles.

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