Calcium Bridge Triggers Capsid Disassembly in the Cell Entry Process of Simian Virus 40

Abstract

The calcium bridge between the pentamers of polyoma viruses maintains capsid metastability. It has been shown that viral infection is profoundly inhibited by the substitution of lysine for glutamate in one calcium-binding residue of the SV40 capsid protein, VP1. However, it is unclear how the calcium bridge affects SV40 infectivity. In this in vitro study, we analyzed the influence of host cell components on SV40 capsid stability. We used an SV40 mutant capsid (E330K) in which lysine had been substituted for glutamate 330 in protein VP1. The mutant capsid retained the ability to interact with the SV40 cellular receptor GM1, and the internalized mutant capsid accumulated in caveolin-1-mediated endocytic vesicles and was then translocated to the endoplasmic reticulum (ER) region. However, when placed in ER-rich microsome, the mutant capsid retained its spherical structure in contrast to the wild type, which disassembled. Structural analysis of the mutant capsid with cryo-electron microscopy and image reconstruction revealed altered pentamer coordination, possibly as a result of electrostatic interaction, although its overall structure resembled that of the wild type. These results indicate that the calcium ion serves as a trigger at the pentamer interface, which switches on capsid disassembly, and that the failure of the E330K mutant capsid to disassemble is attributable to an inadequate triggering system. Our data also indicate that calcium depletion-induced SV40 capsid disassembly may occur in the ER region and that this is essential for successful SV40 infection.Viruses use their capsid to transmit genome from cell to cell and thus gain access to cellular synthesis machinery. To achieve this, the capsids of non-enveloped viruses perform a myriad of tasks during viral infection, such as viral genome encapsidation, binding to host cell surface receptors, selection of the appropriate entry route, and disassembly to release the genome into a suitable replication environment. The viral capsid must be robust enough to protect the genome against a harsh extracellular environment while being able to disassemble easily within the cell in order to release its genome. Viruses have therefore evolved various strategies to balance the stability of the capsid against its capacity to disassemble. The switch between these states is triggered by the cell.SV40, a polyomavirus, is composed of a circular doublestranded DNA genome enclosed within an icosahedral capsid. This virus was discovered in the early 1960s as a frequent contaminant of the rhesus macaque kidney cell cultures used to grow poliomyelitis vaccines (1-3). SV40 induces tumors in hamsters and has been used extensively as a model for tumorcausing viruses (2). SV40 capsid is composed of three structural proteins: VP1, VP2, and VP3. The major structural protein, VP1, is arranged in 72 pentameric rings that are assembled into an icosahedral T ϭ 7d surface lattice (4 -6). The two minor structural proteins, VP2 and VP3, are located on the inner surface of the capsid in a r...

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