Mimivirus and the Stargate

Two fundamental events in virus replication cycles are the delivery of virus genomes into host cells and the packaging of these genomes into virus protein capsids. In bacteriophages and herpesviruses these processes occur linearly along the genome, base pair after base pair, through a single portal located at a unique site in the viral capsid. New research has addressed the question of whether such a linear translocation through a single portal also takes place for viruses with very large genomes by studying genome delivery and packaging in the amoeba-infecting virus Acanthamoeba polyphaga Mimivirus. With 1.2 million base pairs, this double-stranded DNA genome is the largest documented viral genome. Using electron tomography and cryo-scanning electron microscopy researchers identified a large tunnel in the Mimivirus capsid that is formed shortly after infection, following a large-scale opening of the capsid. The tunnel allows the whole virus genome to exit in a rapid, one-step process. DNA encapsidation is mediated by a transient aperture in the capsid that, they suggest, may promote concomitant entry of multiple segments of the viral DNA molecule. These unprecedented modes of viral genome translocation imply that Mimivirus - and potentially other large viruses - evolved mechanisms that allow them to cope effectively with the exit and entry of particularly large genomes.

Distinct DNA exit and packaging portals in the virus Acanthamoeba polyphaga mimivirus. 2008 PLoS Biol 6(5): e114
Icosahedral double-stranded DNA viruses use a single portal for genome delivery and packaging. The extensive structural similarity revealed by such portals in diverse viruses, as well as their invariable positioning at a unique icosahedral vertex, led to the consensus that a particular, highly conserved vertex-portal architecture is essential for viral DNA translocations. Here we present an exception to this paradigm by demonstrating that genome delivery and packaging in the virus Acanthamoeba polyphaga mimivirus occur through two distinct portals. By using high-resolution techniques, including electron tomography and cryo-scanning electron microscopy, we show that Mimivirus genome delivery entails a large-scale conformational change of the capsid, whereby five icosahedral faces open up. This opening, which occurs at a unique vertex of the capsid that we coined the ‘‘stargate’’, allows for the formation of a massive membrane conduit through which the viral DNA is released. A transient aperture centered at an icosahedral face distal to the DNA delivery site acts as a non-vertex DNA packaging portal. In conjunction with comparative genomic studies, our observations imply a viral packaging pathway akin to bacterial DNA segregation, which might be shared by diverse internal membrane–containing viruses.

Oh my God, it’s full of stars…

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• Marine mimivirus relatives are large algal viruses
• DNA Viruses

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