The viruses in our brains

PNMA2 (ParaNeoplastic antigen MA2) is a protein initially found as the target of the immune response (autoantibodies) producing a nasty dementing neurologic disease (Paraneoplastic encephalitis).  The PNMA2 protein is exclusively expressed in neurons which implies that neurons are using it for something.   This is teleological thinking, usually looked down on, but always needed in molecular biology and cellular physiology.

What PNMA2 does is amazing.  It forms icosahedral viral capsids which are released from cells (in culture) as nonEnveloped capsids.  It isn’t clear if this normally happens in our  brains.    Probably it doesn’t, and when the capsid somehow gets out of the producing cell or neuron immunological hell breaks loose and autoimmune encephalitis is the result.

PNMA2 is derived from one of the long terminal repeat retrotransposons (LTR retrotransposons), viral remnants that make up 8% of the human genome (https://en.wikipedia.org/wiki/LTR_retrotransposon). This explains why it makes particles that look like viruses.  Such particles can contain RNA, so big pharma is interested in them as a way of delivering mRNA drugs.

Totally off topic but yesterday I read a paper about E. Coli DNA gyrase, an amazing enzyme which untangles DNA ( Science vol. 384 pp. 227 – 232 ’24 ).

Here is what it does.   If you’ve got some venetian blinds in your home twist it 20 or so times (keeping the ends fixed, and you have the DNA double helix, with two strands winding around each other.  Now to read or copy a single strand, you must grab both strands where you want this to happen  and pull them apart keeping the ends of the venetian blind fixed.  This immediately increases the coiling elsewhere. Since there are only 10 nucleotides/turn of the double helix, copying a gene for a 100 amino acid protein means you are removing 33 twists from the separated strands (and producing new ones elsewhere).   The cords of the venetian blind quickly become a tangled mess when this happens.  This is where DNA gyrase comes in.  It cuts both strands of the DNA double helix, holding on to the cut ends, and slides an intact double helix of the twisted DNA through the cut.   Sounds fantastic doesn’t it?  Hard to see how evolution could come up with something like this but it did.

The paper contains the following passage toward the end

“A second model based on a sign-inversion reaction wassuggested to describe introduction of (–)SC by this enzyme (28). This model proposed that the enzyme binds to a positive crossover followedby a DNA strand passage through a DNA double-strand break that results in a sign inversion.”

(28) is 28. P. O. Brown, N. R. Cozzarelli,Science206, 1081–1083 (1979).

The paper is 45 years old and has now been shown to be correct.  N. R.  Cozzarelli is my late good friend and Princeton classmate Nick, and it is very nice to see him honored here.

A few words about Nick.  Although Princeton was full of rich kids, they still had the brains to take in someone like Nick whose father was an immigrant shoemaker in Jersey City.  Nick worked his way through Princeton waiting on tables in commons (where all Freshmen ate).  I can still see the time that some rich preppie jerk gave him a hard time about the service.

Nick got his PhD at Harvard and later became a professor at Berkeley where he did his great work.  Nick later edited the Proceedings of the National Academy of Sciences (USA) for 10 years before his very untimely death over 20 years ago from Burkitt’s lymphoma.  R. I. P. Nick.

Is there anything in the cell that has just one function — more moonlighting — this time mRNA

Able was I ere I saw Elba said Napoleon. It’s called a palindrome, and can be read either way. So can DNA which brings me to antisense transcription of DNA, particularly in two famous retroviruses –the AIDS virus (HIV1) and HTLV-1.  

Proc. Natl. Acad. Sci. vol. 118 e2014783118 ’21  shows that mRNA can moonlight to do other things than code for protein.  Here’s a direct quote to set the stage.

“Retroviruses share a similar genome structure. The integrated retroviral genome, called the provirus, has two identical long terminal repeats (LTR) located at its 5′ and 3′ ends, respectively. The 5′ LTR acts as the promoter of almost all retroviral genes and thus is indispensable for viral transcription and replication. However, selective methylation of the 5′ LTR and the subsequent viral latency have been observed in HIV-1 and HTLV-1. In contrast, the 3′ LTR of HIV-1 and HTLV-1 remains nonmethylated, and recent findings have shown that novel retroviral genes are transcribed from the 3′ LTR in an antisense direction”.

The 3′ LTR of the AIDS virus enables antisense transcription for  the unimaginatively named ASP (AntiSense Protein).  So the mRNA for ASP is transcribed in the nucleus.  But it doesn’t get out as well as it might, because its 5′ end isn’t polyAdenylated.  So it sticks around in the nucleus and binds to DNA, turning off transcription of the regular HIV1 genome — e.g. helping to maintain viral latency (and preventing a true cure of HIV1 in any individual).

This is unprecedented.  Here is an mRNA with a completely different function (e.g. regulating gene expression).  This is classic moonlighting as something else and the authors call the mRNA for ASP a bifunctional mRNA. 

The other, retrovirus HTLV-1 also has an antisense transcript making a protein called HBZ (your don’t want to know what it stands for). Unlike ASP, HBZ turns on a variety of genes. 

I’ve been fascinated by moonlighting molecules, probably because they show the depths of our ignorance of the biochemical machinations inside the cell.  Even when you think you’ve got the function of a molecule tied down, it goes off and does something else. 

 Here are some links to other posts on the subject.  To get to them just click on the titles

Moonlighting molecules

More moonlighting

A moonlighting quorum sensing molecule