Which is harder to kill? Bad news wrapped in protein, or in soap?

Viruses have been described* as “bad news wrapped in protein” because many of them are just the genes of the virus – the bad news bit which takes over an infected cell – wrapped in a protein shell**. The wrapping is an elastic shell a few nanometres thick that is assembled from proteins. The image just below shows a reconstruction of such a protein shell:

The colours are false, they just use different colours for different proteins. This protein shell protects the “bad news” inside. But not all viruses are like this. Some, such as SAR-CoV-2 shown at the top of this post, are “bad news wrapped in soap”. Instead of a shell made of an array of protein molecules, it is (mostly) made of soapy/fatty molecules called lipids (shown in grey).

Whatever the wrapping is made of, protein or soap, its job is protect the bad news. So, which offers more protection? I think we are mostly clueless here, and it may well depend on what from. The are some statements in the literature that viruses with protein shells are tougher, but I can’t really see much evidence for this, I think it is pretty speculative.

Part of the problem is that it is not at all easy to understand and quantify how hard it is to break things? The standard model of a virus with a protein shell is basically an egg, a relatively gooey inside (the bad news aka virus genes), surrounded by a thin stiff elastic shell. Whereas as soap is mostly liquid-like, the viruses with soapy shells are more like oil droplets in water. They are easier to deform than the viruses with their elastic shells, but I would guess that just deforming them does not damage their bad-news cargo.

What we can do is poke protein and soapy shells, using a device called an atomic force microscope (AFM). There is a fair bit of variability*** but the force needed to break both protein and soapy shell is around a nanoNewton, i..e., a billionth of a Newton. But even when broken the soapy shell can reform – it is very flexible, while once the elastic shell has fractured that is it. Broken egg shells don’t heal. And the protein shells can only bend by a few % before they break while the soapy ones can be squashed without harm, they just bounce back when the force is removed.

So maybe soapy shells are harder to break and so offer more protection, but they are probably easier to tear. Which of protein or soap offers more protection to the bad news may depend on what the environment is the virus experiences as it travels from one host to another. Most airborne viruses, including SARS-CoV-2 and flu, have soapy shells, so maybe soapy shells offer good protection while be carried across the air. But cold viruses have protein shells, so maybe they work too. We have a lot to learn.

* By Sir Peter Medawar, a pretty well-known biologist from mid-20th century. He shared a Nobel prize in 1960 for work on the immune system.

** There are also some proteins inside as well, Medawar’s expression is a simplification.

*** The exact force depends a fair number of things, how sharp the AFM tip is, how fast it is applied, etc.

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