Cys(9), Cys(104) and Cys(207) of simian virus 40 Vp1 are essential for infectious virion formation in CV-1 celss

Structural studies have implicated Cys(9), Cys(104) and Cys(207) Of simian virus 40 (SV40) Vp1 in disulfide bond formation. Recently, we have shown th e three cysteines to be essential for disulfide linkage of Vp1 complexes in vitro. Here, the role of the three cysteines was explored during the cours e of SV40 infection. Single-, double- and triple-mutant Vp1 at Cys(9), Cys( 104) and Cys(207) continued to localize to the nuclei of transfected CV-1 c ells and to bind DNA, but showed a range of abilities to form plaques. Only mutants containing the Cys(9)--> Ser change showed defects in plaque forma tion. Single mutants at Cys(9) formed small plaques; mutants at Cys(9).Cys( 104), Cys(9).Cys(207) and Cys(9).Cys(104).Cys(207) formed no plaques. All t hree isolated revertants contained back-mutations at the Vp1 Cys(9) codon. These results further confirm the involvement of the three Vp1 cysteines in protein-protein interactions during virus assembly. Cys(9) is critical for production of wild-type infectious virions, whereas Cys(104) and Cys(207) play secondary roles.