Role of simian virus 40 Vp1 cysteines in virion infectivity

We have developed a new nonoverlapping infectious viral genome (NO-SV40) in order to facilitate structure-based analysis of the simian virus 40 (SV40) life cycle. We first tested the role of cysteine residues in the formation of infectious virions by individually mutating the seven cysteines in the major capsid protein, Vp1. All seven cysteine mutants-C9A, C49A, C87A, C104 A, C207S, C254A, and C267L-retained viability. In the crystal structure of SV40, disulfide bridges are formed between certain Cys104 residues on neigh boring pentamers. However, our results show that none of these disulfide bo nds are required for virion infectivity in culture. We also introduced five different mutations into Cys254, the most strictly conserved cysteine acro ss the polyomavirus family. We found that C254L, C254S, C254G, C254Q, and C 254R mutants all showed greatly reduced (around 100,000-fold) plaque-formin g ability. These mutants had no apparent defect in viral DNA replication. M utant Vp1's, as well as wild-type Vp2/3, were mostly localized in the nucle us. Further analysis of the C254L mutant revealed that the mutant Vp1 was a ble to form pentamers in vitro. DNase I-resistant virion-like particles wer e present in NO-SV40-C254L-transfected cell lysate, but at about 1/18 the a mount in wild-type-transfected lysate. An examination of the three-dimensio nal structure reveals that Cys254 is buried near the surface of Vp1, so tha t it cannot form disulfide bonds, and is not involved in intrapentamer inte ractions, consistent with the normal pentamer formation by the C254L mutant . It is, however, located at a critical junction between three pentamers, o n a conserved loop (G2H) that packs against the dual interpentamer Ca2+-bin ding sites and the invading C-terminal helix of an adjacent pentamer. The s ubstitution by the larger side chains is predicted to cause a localized shi ft in the G2H loop, which may disrupt Ca2+ ion coordination and the packing of the invading helix, consistent with the defect in virion assembly. Our experimental system thus allows dissection of structure-function relationsh ips during the distinct steps of the SV40 life cycle.