Exobiology and Evolutionary Biology

Temperature effects on protein evolution (2)

PI: Jonathan Silberg

Bioinformatics studies have shown that as the optimal growth temperature for an organism increases, its genes exhibit a reduced tolerance to DNA mutations that cause amino acid substitutions in proteins. This implicates the evolution of functional proteins as hardest for hyperthermophilic organisms that grow optimally at temperatures near the boiling point of water (80-121°C). In addition, this suggests that there may be temperature constraints on the use of proteins as building blocks for advanced life. What cannot be determined from bioinformatics studies, however, is how much harder it is to evolve a functional protein through random mutation and selection as temperatures increase. The genomic sequences used in these studies have diverged through multiple mutational processes, including random mutation, recombination, and horizontal gene transfer. To begin establishing the effect of temperature on the tolerance of proteins to random mutations, we propose to directly evaluate how temperature affects the fraction of proteins with m random amino acid substitutions that retain function after mutation. For these studies, we propose to direct the evolution of adenylate kinases (AK), ubiquitous phosphotransferases that are essential in all known organisms for maintaining cellular energy charge. Using the results from laboratory evolution experiments at 40 and 80°C, we plan to evaluate the quality of predictions made by a thermodynamic model that uses protein structural information to predict the effect of random mutations on protein structure and function. The results from these studies will allow us to directly quantify how the number of proteins that stably fold into a functional AK changes as temperature increases. Furthermore, these unique data sets will allow us to determine if an existing thermodynamic model breaks down when used to predict protein evolution over a range of temperatures, and it will allow for improvements in this model. A better
theoretical model will facilitate calculations of the relative mutational robustness for any protein of known structure within mesophilic, thermophilic, and hyperthermophilic organisms, and it will allow us to better estimate how temperature could limit the use of proteins as biomolecules to support the evolution of advanced life.