My contribution to JustScience 2008 will be a review on a protein with the potential to transform evolution theory as we know it today. The review will be divided into 5 separate blog posts:
1. Introduction to Hsp90 and evolution (this post)
2. Presenting the Hsp90 protein
3. How can chaperones act in evolution
4. Evidence for Hsp90 involvement in rapid evolution of new traits
Here’s the teaser: In one generation you can go from this
to this (a hopeful monster ?).
(from Rutherford SL and Lindquist S, Nature 1998, to see more “monster”-pictures, do a google image search on Hsp90 and evolution)
The protein in focus, Heat Shock Protein 90, is otherwise as normal as a protein can get. It is ubiquitously expressed in all cells and across species, and its function is the same as other heat shock proteins, it’s a chaperone.
A chaperone is a protein that helps other proteins fold correctly (or prevents them to aggregate into non-functional protein junk). Without the chaperone the protein would not achieve an active conformation and end up being degraded. For an excellent video illustrating chaperone assisted protein folding go here.
Illustration of chaperones in action. Picture from Nurse Minerva
Now, it turns out that the Hsp90 chaperone function is important for development and evolution of new traits. To illuminate this, the first paper I will discuss is from 1998 by Susan Lindquist’s lab at the Whitehead Institute.
In this paper they take fruit flies and reduce the expression of Hsp90. As I shall come back to later, this is the experimental equivalent to a stressful condition (like for instance high temperature). They can’t knock out Hsp90, because a complete lack of it is not compatible with life (which demonstrates the importance of this protein). When Hsp90 levels are reduced, the fruit flies are born with a number of different defects ranging from defects in the legs and bristles to defects in the eye (see image above). Now, this could be expected when knocking out a protein, but such extensive morphologic changes are not expected from just reducing the levels of a protein (unless maybe if it’s a transcription factor, which it isn’t). The hypothesis explaining this goes as follows……Since Hsp90 is a heat shock protein it is helping other proteins fold. When stressful conditions occur, there will be more proteins that are in need of folding-help, thus some of the proteins that under normal conditions got their help from Hsp90 will now be left on their own. Since, under such conditions, you see these extensive morphological changes, these proteins must be doing something out of the ordinary when on their own, or if they aggregate, their absence causes abnormality. The theory states that these in-need-of-help-proteins must have accumulated mutations that potentially causes abnormality, but under normal conditions they are still able to perform normally due to the action of Hsp90. The implications for evolution are breathtaking since this means that an organism can accumulate a number of mutations and still function normally, but when exposed to stressful conditions, the changes at the protein level are suddenly exposed in their offspring and appear as physical abnormalities. This allows for extremely rapid evolution and could potentially enable a species to change in just a couple of generations.
Illustration of the process and further evidence for this theory will be presented in the next 4 posts the next 4 days, in SciPhu for just science 08.