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

Coat as a dagger: the use of capsid proteins to perforate membranes during non-enveloped DNA viruses trafficking

To get access to the replication site, small non-enveloped DNA viruses have to cross the cell membrane using a limited number of capsid proteins, which also protect the viral genome in the extracellular environment. Most of DNA viruses have to reach the nucleus to replicate. The capsid proteins involved in transmembrane penetration are exposed or released during endosomal trafficking of the virus. Subsequently, the conserved domains of capsid proteins interact with cellular membranes and ensure their efficient permeabilization. This review summarizes our current knowledge concerning the role of capsid proteins of small non-enveloped DNA viruses in intracellular membrane perturbation in the early stages of infection.

Simian Virus 40 depends on ER protein folding and quality control factors for entry into host cells

Cell entry of Simian Virus 40 (SV40) involves caveolar/lipid raft-mediated endocytosis, vesicular transport to the endoplasmic reticulum (ER), translocation into the cytosol, and import into the nucleus. We analyzed the effects of ER-associated processes and factors on infection and on isolated viruses and found that SV40 makes use of the thiol-disulfide oxidoreductases, ERp57 and PDI, as well as the retrotranslocation proteins Derlin-1 and Sel1L. ERp57 isomerizes specific interchain disulfides connecting the major capsid protein, VP1, to a crosslinked network of neighbors, thus uncoupling about 12 of 72 VP1 pentamers. Cryo-electron tomography indicated that loss of interchain disulfides coupled with calcium depletion induces selective dissociation of the 12 vertex pentamers, a step likely to mimic uncoating of the virus in the cytosol. Thus, the virus utilizes the protein folding machinery for initial uncoating before exploiting the ER-associated degradation machinery presumably to escape from the ER lumen into the cytosol.

Pairs of Vp1 cysteine residues essential for simian virus 40 infection

Transient disulfide bonding occurs during the intracellular folding and pentamerization of simian virus 40 (SV40) major capsid protein Vp1 (P. P. Li, A. Nakanishi, S. W. Clark, and H. Kasamatsu, Proc. Natl. Acad. Sci. USA 99:1353-1358, 2002). We investigated the requirement for Vp1 cysteine pairs during SV40 infection. Our analysis identified three Vp1 double-cysteine mutant combinations that abolished viability as assayed by plaque formation. Mutating the Cys49-Cys87 pair or the Cys87-Cys254 pair led to ineffective nuclear localization and diminished accumulation of the mutant Vp1s, and the defect extended in a dominant-negative manner to the wild-type minor capsid proteins Vp2/3 and an affinity-tagged recombinant Vp1 expressed in the same cells. Mutating the Cys87-Cys207 pair preserved the nuclear localization and normal accumulation of the capsid proteins but diminished the production of virus-like particles. Our results are consistent with a role for Cys49, Cys87, and Cys254 in the folding and cytoplasmic-nuclear trafficking of Vp1 and with a role for Cys87 and Cys207 in the assembly of infectious particles. These findings suggest that transient disulfide bond formation between certain Vp1 cysteine residues functions at two stages of SV40 infection: during Vp1 folding and oligomerization in the cytoplasm and during virion assembly in the nucleus.

Cys254 and Cys49/Cys87of simian virus 40 Vp1 are essential in formation of infectious virions

The SV40 capsid is composed of pentameric capsomeres of Vp1. We have previously shown that disulfide linkages at Vp1 Cys9, Cys104, and Cys207 are essential in formation of infectious virions. Here, the role of the remaining four cysteines was explored. Single, double, and quadruple cys --> ser mutant genomes at Vp1 Cys49, Cys87, Cys254, and Cys267 codons were generated and transfected into CV-1 cells. The quadruple mutant Vp1 continued to localize to the nucleus and to bind DNA, but resulted in no plaques. SV40Vp1.Cys254 was the only single mutant with complete defect in plaque formation. The double mutant at Vp1.Cys49.Cys87 showed complete defect in plaque formation, while single mutants at the two residues resulted in plaques, suggesting a cumulative effect. All mutants defective in plaque formation continued to localize viral proteins in the nucleus. Taken together, our results suggest that Cys254 and the Cys49/Cys87 combination are essential in late stages of infectious virion formation.