Many roads lead to Rome– there is no one ‘right’ way to solve an evolutionary hurdle. Viruses encounter the same evolutionary ‘problems’, but have evolved lots and lots and lots of different solutions to the exact same problems. Random chance of mutations + the bumbling blindness of natural selection (‘good enough’ is selected, not ‘BEST!’) means all that bumbling mess leads to different ‘solutions’. Some might work better than others, but they all work, and thats good enough.
Its easy to comprehend an RNA virus doing something differently than a very distantly related DNA virus. But differences exist between closely related viruses as well. We all know no-new-genes-no-new-functions Michael Behe and his Creationist BFFs hate the evolutionary capacity of new-genes-new-functions HIV-1. They have to hate its cousin Bovine immunodeficiency virus too:
There are lots of different ways to get a protein. Retroviruses operate like a sheet cake– it makes one bit mRNA that gets cuts up into lots of little mRNAs as it leaves the nucleus, which go one to be translated into all the proteins the retrovirus needs.
… But then how to you get a retroviral genome into the babby viruses? The retroviral genome is a big uncut piece of mRNA. If it always gets cut up when it leaves the nucleus, how can you ever get that big uncut RNA genome into new viruses?
Lentiviruses have an answer to this dilemma– “Regulator of Virion Expression”, Rev.
Rev escorts the mRNA out of the nucleus, so it can be chopped up in different ways to get different retroviral proteins, or, prevent the RNA from being cut entirely so full genomes can be packaged into babby viruses.
And of course, its not just about Rev getting out of the nucleus with its RNA companion. It must also be able to get itself into the nucleus. Proteins like Rev are made in the cytoplasm on ribosomes– it needs to perform a few tricks to get itself into the nucleus to pick up its RNA buddy.
So Rev does two things– gets into the nucleus, gets out of the nucleus with RNA. Gets into the nucleus, gets out of the nucleus with RNA. Over and over.
You would think that Rev from HIV and Rev from BIV would look and behave in the same manner. Theyre the same protein from the same family of retrovirus (lentivirus) that accomplish the same function.
Youd be thinking wrong!
1– BIV Rev has a bipartite nuclear localization signal (NLS). HIV Rev has a monopartite NLS (translation, the genes look different).
2– BIV Rev gets into the nucleus via the classical importin alpha/beta nuclear import pathway. It requires a carrier and energy to get in. HIV Rev just needs an importin (beta, 5, 7, 9) or transportin. BIV cannot get in the same ways HIV Rev gets in (these researchers tried).
3– There are two kinds of nuclear export signals (NES), HIV Rev-class, and PKI-class. BIV Rev NES is not an HIV Rev-type. It is PKI-type!
What does this all mean?
In conclusion, we characterized the mechanisms of the nuclear import and export pathways of BIV Rev. The nuclear import of BIV Rev differs from that found in HIV-1 Rev. Although BIV Rev is exported by CRM1, its NES belongs to the PKI-class. In addition to the previously described novel types NLS/NoLS, these mechanisms make of the BIV Rev an unique protein within the retrovirus/lentivirus field.
Sure, HIV and BIV are both ‘retroviruses’. They are both ‘lentiviruses’. They both have a ‘Rev’ protein. The Rev protein ‘does the same thing’ in both– carrying RNA out of the nucleus, running back in, carrying RNA out of the nucleus, running back in.
But that function is performed by Rev proteins that look and behave in totally different manners.
Even the cows are against them.