Intracellular SV40 nucleoprotein complexes: Synthesis to encapsidation

Abstract

Five forms of SV40 nucleoprotein complexes have been characterized: (i) DNA-replicative intermediates, (ii) a 75 S complex which is the product of replicative forms, (iii) a 200 S previrion structure derived from the 75 S complex shortly after the completion of DNA replication, (iv) 250 S mature virion, and (v) a 75 S complex with accumulates in tsB11-infected cells at the nonpermissive temperature. These complexes were studied by pulse-labeling kinetics of viral DNA and proteins, CsCI gradient analysis, immunoprecipitation with anti-virion antibody, and two-dimensional electrophoretic analysis of the heterogeneity of the major capsid protein. Based upon the results of this study, a biosynthetic pathway for the production and maturation of the SV40 chromosome is proposed. The model involves the gradual addition of virion proteins onto the SV40 75 S chromosome: no capsid proteins are present on the tsB11 75 S complex; 2% of the mature virion VP1 complement and 1% of the VP3 complement are present in the transient wildtype 75 S complex; 84% of the virion VP1 complement, no VP2, and 32% of the VP3 complement are present in the 200 S pre-virion; while the virion contains VP1, VP2, and VP3. The phosphorylation and extent of heterogeneity of the major capsid protein VPI are identical in all of the wildtype SV40 nucleoprotein complexes.