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Mystery Rays from Outer Space
Last week, the Effect Measure blog1 talked about a paper that offered a new way of treating influenza.2 Briefly, the approach is to attack the virus by treating the host cell: Eliminating host functions that the virus requires, but that the host cell does not.
The authors of the paper commented that “targeting host cell determinants temporarily dispensable for the host but crucial for virus replication could prevent viral escape,” and Effect Measure observed that “It’s not obvious to me why the virus can’t as easily mutate in ways to adapt to a missing “office tool” as to a drug that affects an important viral function.” In the comments, I said:
I think the fundamental difference is that in the latter case, the virus needs just modify an already-present function; but if the tool is missing altogether, the virus would have to develop a whole new function from scratch, an altogether more difficult task.
That’s not to say that a virus could not do it — we see examples of this all the time, with viruses that have co-opted host functions and in some cases even host genes. But the process is usually much slower; we tend to recognize those events in hindsight, whereas we can often watch viral genes adapting to drugs in real time.
I still think my explanation is generally true, but here’s a counterexample, of what appears to be a virus evolving a brand-new function in just 5 weeks.
Background: Mammals have what seems to be a general defense against retroviruses like HIV. Several members of the APOBEC family of proteins are anti-retroviral;3 they force widespread mutations into the HIV genome, so many mutations that the virus can’t replicate or produce normal proteins.3 The reason HIV is able to replicate in spite of the APOBECs, is that HIV has in turn an anti-APOBEC protein, vif, that causes rapid destruction of several APOBECs. (I’ve mentioned this before, here, here, and here.)
| <img style="float: right; margin-left: 5px; margin-right: 5px;" title="vif structure (Zhang et al, Org Biomol Chem. 2007 Feb 21;5(4):617-26)" …
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