Life is an out-of-equilibrium experience. All living matter maintains thermodynamically unstable structures at the expense of external energy. A number of molecular chaperones — including Hsp90 — consume the chemical energy from ATP hydrolysis to aid their protein substrates in folding to their functional states. Is this energy spent driving the protein substrates into thermodynamically unstable structures and keeping them there? I have proposed a mechanism of nonequilibrium protein folding that answers yes to these questions in the case of Hsp90-mediated activation of kinase clients.
The cover image for the October 20 issue of Biophysical Journal depicts the key elements of this nonequilibrium activation process. Hsp90, represented by the effervescent dots, undergoes a conformational cycle through open and closed states. In this cycle, Hsp90 hydrolyzes ATP molecules to ADP molecules and uses the liberated energy to drive the client kinase from its inactive state of low free energy, via a locally unfolded intermediate state, to its active state of high free energy.
In conceptualizing the image, the idea of a waterfall came to me immediately. The steady yet energetic water flow seemed a natural metaphor for the dynamic, steady state of biomolecular reactions in the cells. The landscape of a waterfall — consisting of two different elevations —also provided a suitable illustration for the free energy landscape of client activation: the client is elevated to the ground of high free energy from that of low free energy. It did not take much mental gymnastics to add a water spiral to represent Hsp90’s conformational cycle.
I enlisted the help of a talented artist, Marija Stojkovic, to turn my initial sketch (see image) into an artistic rendition. As we finalized the image, I realized I needed to convey the idea that the efficacy of client activation depended critically on the timings of the different steps in the cycle. It so happened that I had a wall poster of Salvador Dali’s painting The Persistence of Memory right above my desk. “I need one of those distorted clocks in the image.” I told Marija, who graciously and skillfully added the ghostly clock at the bottom of the water spiral.
Recent theoretical and experimental works provided evidence that ATP-driven nonequilibrium protein folding may be a common phenomenon in the cell. It will be intriguing to see what other biomolecular structures might be stabilized out-of-equilibrium by energy consuming processes. Is nonequilibrium protein folding by ATP-consuming molecular chaperones but one example of what might be called nonequilibrium structural biology?
- Huafeng Xu