Intermediates in the folding equilibrium of repeat proteins from the TPR family

In recent decades, advances in computational methods and experimental biophysical techniques have improved our understanding of protein folding. Although some of these advances have been remarkable, the structural variability of globular proteins usually encountered makes it difficult to extract general features of their folding processes. To overcome this difficulty, experimental and computational studies of the folding of repeat (or modular) proteins are of interest. Because their native structures can be described as linear arrays of the same, repeated, supersecondary structure unit, it is possible to seek  a possibly independent behavior of the different modules without taking into account the intrinsic stability associated with different secondary structure motifs. In this work we have used a Monte Carlo-based simulation to study the folding equilibrium of four repeat proteins belonging to the tetratricopeptide repeat family. Our studies provide new insights into their energy profiles, enabling investigation about the existence of intermediate states and their relative stabilities. We have also performed structural analyses to describe the structure of these intermediates, going through the vast number of conformations obtained from the simulations. In this way, we have tried to identify the regions of each protein in which the modular structure yields a different behavior and, more specifically, regions of the proteins that can stay folded when the rest of the chain has been thermally denatured.