The vast increase in the number of protein structures identified since the first high resolution protein structure was determined has shown that there are relatively few protein folds. The study of the folding of proteins has also expanded significantly since its inception. The contact-order of residues in the native structure has been implicated as important in folding. This has raised the question of whether the common folds observed in protein structures fold via common mechanisms. The protein kinase domain is a large, pharmaceutically important and conserved protein fold, of which many examples fail to fold correctly when overexpressed as recombinant proteins in Escherichia coli. The kinase domain forms an excellent area in which to study the folding of a large conserved domain with varying sequence. To study the refolding of the kinase fold a refolding screen was created, using p38α as a model protein kinase. The results of this screen were compared to the results of refolding a further four protein kinases, AKT2, KIS, PhK and TTK to determine commonalities in the folding of protein kinases. The refolding of the five protein kinases was also examined using a fractional factorial screen which examined combinations of refolding additives. In the screening of the refolding of protein kinases no factors were idenified which were common to the refolding of all five of the tested protein kinases. The equilibrium folding of a single protein kinase, TTK, was also studied. The folding of TTK was determined to proceed via different pathways on folding and unfolding, with a co-operative unfolding pathway through a molten-globule intermediate, and a non-cooperative refolding pathway via an intermediate with different secondary structure content to the unfolding intermediate. The difference between the folding properties of protein kinases determined in the screen and through the analysis of the equilibrium folding of TTK suggest that there may not be a common protein kinase folding mechanism
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