Not many people outside of infectious disease specialists may realize it, but the order Mononegavirales is really bad news for human health. Inside that one you can find measles (the fashionable infection of 2026, damn it all), RSV (always with us), mumps, rabies, and even Ebola, which I very much hope does not become a hot item in any year.
There are plenty of differences between all these (there are eleven families in this order), but something that the Mononegavirales species have in common is the existence of “viral factories” (VFs). These are concentrated blobs of viral proteins that form in infected cells and serve to crank out the pieces of new viral particles for further infection. They are, in fact, phase-separated condensates (which shows again how useful that physical behavior is across different systems - I wrote about these most recently here). But there’s been a mystery about them, as this paper explains well. It’s generally believed, with good reason, that such condensates can only form when the concentration of the proteins that make it up get over a certain threshold. But when an infection is just starting out, there doesn’t seem to be any way for that to be possible. You’d need viral factory condensates to make that much protein, and you can’t condense to get such VFs unless the protein is already there - or can you?
The authors show the way out of this paradox. For RSV, viral factories are formed by the viral nucleoprotein (N), the viral phosphoprotein (P) and also contain the “large” protein (L) and its cofactors, the viral RNA polymerase, and various RNA transcripts. But there are “pre-replication centers” (PRCs) that form before these VFs are able to completely assemble, and these are imaged here for the first time. They are the seeds of the VF condensate formation, what is basically a feed-forward process: protein replication starts at a lower and less efficient level, but these viral proteins are strongly recruited to the PRCs in turn, which makes them even more productive, which makes more protein, and. . .you get the idea! Before long you have the full-fledged viral factories that have been known for some time as a hallmark of RSV-infected cells. This is how the condensates get bootstrapped from low-concentration beginnings.
An unexpected result was that when you look at individual RSV particles (virions) themselves, some of them are much more “PRC-competent” than others. Indeed some of the virions are actually pretty terrible at replication, because they don’t have pre-formed PRCs ready to go in them when they infect a cell. It looks very much like an RSV infection in a whole animal is driven by the virions that do have the PRCs assembled for delivery; the others turn more or less into bystanders (although what viral proteins they do produce probably get recruited over to those other strongly-binding PRCs from other virions that have hit the same cell).
But there’s a lot of cell-to-cell heterogeneity in an RSV infection, and these results suggest why: some of these cells have been hit by far more PRC-active virions and some of them haven’t. This raises a lot of interesting questions, for sure. What exactly are the factors that make PRCs assemble more in some virions than others? Do the PRCs themselves vary in their ability to nucleate viral factories in turn, and if so, what factors drive those differences? A larger question is evolutionary: you’d think that there would be a selection advantage in having efficient PRC formation and that over time you just wouldn’t see those less efficient virions at all. This makes you wonder if there really is an effective selection mechanism at the genetic level or if there’s some random process that’s mixing things up at a slightly later stage.
And moving beyond the Mononegavirales order, there are plenty of other viruses that have to deal with the starting-from-scratch problem when they first infect a cell. Indeed, there are many other kinds that seem to form condensates during their attacks on cells. Do they also do some kind of condensate-seeding trick to get things going? Or will that possibly turn out to be a trick that just the crazily-infectious ones have hit on? And as the authors note, there are certainly also implications for condensate formation in general, as we work out the sequences and interactions that make this feed-forward process work so well. Onward. . .