It is a sad fact of virological life that quite a lot of what we see, in the experiments we do, is artefactual: that is, the way we do experiments leads us to see results that do not necessarily reflect reality, but rather, the scenario we inadvertently selected for.
And it is electron microscopy that is at once our friend and our foe in this regard: over the last thirty years I have revised several aspects of my teaching on how virus particles interact with cells in particular, as what was once considered common knowledge has subsequently been proved to be false. This is usually a consequence of having to use large numbers of virus particles – or high multiplicities of infection – and cultured cells, which may lead to rare events being selected for simply because they may be easier to detect. An important example of this was the revelation that poliovirus (and presumably other picornaviruses) almost certainly enters cells via receptor-mediated endocytosis, rather than via some mysterious direct passage mechanism as is often depicted in textbooks (or here).
One of the long-time models for entry of enveloped viruses into animals has been Sendai paramyxovirus: this ss(-)RNA virus was supposed to fuse its membrane with that of the host cell, and uncoat via diffusion of its envelope glycoproteins into the host membrane, and deposit of virion internal components into the host cell cytoplasm.
Except, it turns out, that this is probably wrong: in a Journal of Virology Minireview published in July of 2010, Anne Haywood of the University of Rochester (NY, USA) describes how Sendai virions uncoat via a “connecting structure” that largely preserves the virion envelope.
Membrane Uncoating of Intact Enveloped Viruses
Anne M. Haywood
JOURNAL OF VIROLOGY, Vol. 84, No. 21, Nov. 2010, p. 10946–10955
Experiments in the 1960s showed that Sendai virus, a paramyxovirus, fused its membrane with the host plasma membrane. After membrane fusion, the virus spontaneously “uncoated” with diffusion of the viral membrane proteins into the host plasma membrane and a merging of the host and viral membranes. This led to deposit of the viral ribonucleoprotein (RNP) and interior proteins in the cell cytoplasm. Later work showed that the common procedure then used to grow Sendai virus produced damaged, pleomorphic virions. Virions, which were grown under conditions that were not damaging, made a connecting structure between virus and cell at the region where the fusion occurred. The virus did not release its membrane proteins into the host membrane. The viral RNP was seen in the connecting structure in some cases. Uncoating of intact Sendai virus proceeds differently from uncoating described by the current standard model developed long ago with damaged virus. A model of intact paramyxovirus uncoating is presented and compared to what is known about the uncoating of other viruses.
Interesting: a whole model for entry of viruses into cells was predicated upon the interactions of a laboratory-derived virus strain which produced damaged particles.
Haywood presents a new model for virus entry, based upon the observation that “early harvest” virions differ substantially form the “late harvest virions” previously used, in that “…the RNP is regularly folded parallel to the long axis of the virions…”, while late-harvest particles “…have RNP strands that are randomly distributed in the virus rather than regularly arranged in relation to the membrane”.
She goes on to review a qualitatively very different alphavirus – Sindbis virus, an enveloped ss(+)RNA virus – for which similar things had been claimed, and shows that virus particles that have been gently treated also make a connector. Moreover, she says that:
“…there is a structure that has no electron-dense material and is released from the cell. It is identified as viral by antibodies conjugated with gold beads. This release of an empty viral membrane has not been noted before, but the use of labeled antibodies meant such a structure would be revealed. If the envelope membrane disengages from the cell instead of merging with the host membrane, then not only would the cell not have viral proteins on its surface until the virus replicates but the released membrane pieces could serve as immunologic decoys.” [my emphasis]
Interestinger and interestinger…so enveloped viruses may have an entry mechanism which serves to hide them more effectively than we knew – by keeping their membranes intact, and possibly even using them as releasable decoys?
I note that in the case of HIV – possibly the best-studied single organism on the planet just recently – it has also recently been shown that virions probably enter cells via endosomal vesicles.
I hear the grinding sound of a shifting paradigm, folks: time for a relook at some other cherished models, possibly??