Prions existed in yeast billions of years before humanity came on the scene. Why are they still there? What are they for? Immediately we are back in the Aristotelian world of teleology, where everything had a reason for existence and a purpose. http://www.sparknotes.com/philosophy/aristotle/themes.html. Teleology is simply impossible to avoid in biology. “Nothing in Biology Makes Sense Except in the Light of Evolution” is a famous quote from the magnificently named Theodosius Dobzhansky, which clothes naked teleology with respectable scientific garments.

Here’s an example of this sort of thing from back in the day. When I was back in the Denver VA as a neurology resident dealing with the complications of immunosuppressants in Starzl’s early work on transplantation, we wondered what on earth the transplantation antigens were for. All we knew then, is that they were important in transplant rejection. Surely they were not there to prevent cells of the same or another species from finding a new home in us. Only later did we figure out that they were involved in antigen presentation.

A fascinating article from the first Science of the new year — http://science.sciencemag.org/content/359/6371/eaao5654 describes how the yeast organism might be using one of them (Sup35) — e.g. what the prion domains are for. Normally the Sup35 protein functions to terminate messenger RNA (mRNA) translation into protein. However the first 123 amino acids of Sup35 can aggregate forming amyloid fibrils. It contains a series of 9 amino acid repeats with consensus sequence PQGGYQQYN (single letter amino acid code — http://130.88.97.239/bioactivity/aacodefrm.html) which is similar to the human prion protein repeats (PHGGGWGQ).

This work showed that under a variety of stesses (energy depletion, lowering of intracellular pH) Sup35 doesn’t form amyloid-like prions, but something rather different — liquidlike spherical condensates, which subsequently solidify to form a protein gel. Next to the prion domain is a very acidic region, important in formation of the condensate. Low pH is seen in energy depletion, and protonates the acidic amino acids in the acidic region leading to condensate formation. A mutated Sup35 containing only the prion domain and the acidic region will form the condensates as well in a pH dependent manner. The condensates are far from irreversible (like prions) as they quickly disappear when the pH is raised.

If you take out the prion domain from Sup35, the catalytic region (a GTPase) in the carboxy terminal part forms irreversible aggregates — so the prion domain is in some way preventing this.

So basically the two other parts of Sup35 function to protect the business end of Sup35 from being totally put out of commission by irreversible aggregation.

The authors found that yeast cells containing Sup35 lacking the prion domain, after recovering from stress, showed impaired translational activity and a growth defect presumably because there was less functional Sup35 around. This may be what the prion domain is doing.

My guess is that the aggregation of Sup35 into actual prions has a function in yeast that we just haven’t figured out yet.

It will be interesting to see if other yeast prions (there are many) show the similar behavior (condensate formation under stress).