Cys9, Cys104 and Cys207 of simian virus 40 Vp1 are essential for infectious virion formation in CV-1 cells

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Abstract
Structural studies have implicated Cys9, Cys104 and Cys207 of simian virus 40 (SV40) Vp1 in disulfide bond formation. Recently, we have shown the three cysteines to be essential for disulfide linkage of Vp1 complexes in vitro. Here, the role of the three cysteines was explored during the course of SV40 infection. Single-, double- and triple-mutant Vp1 at Cys9, Cys104 and Cys207 continued to localize to the nuclei of transfected CV-1 cells and to bind DNA, but showed a range of abilities to form plaques. Only mutants containing the Cys9→Ser change showed defects in plaque formation. Single mutants at Cys9 formed small plaques; mutants at Cys9 . Cys104, Cys9 . Cys207 and Cys9 . Cys104 . Cys207 formed no plaques. All three isolated revertants contained back-mutations at the Vp1 Cys9 codon. These results further confirm the involvement of the three Vp1 cysteines in protein–protein interactions during virus assembly. Cys9 is critical for production of wild-type infectious virions, whereas Cys104 and Cys207 play secondary roles.