biomolecules Review Folding and Stability of Ankyrin Repeats Control Biological Protein Function Amit Kumar 1,2,* and Jochen Balbach 2,3,* 1 Department of Life Sciences, Faculty of Natural Sciences, Imperial College of Science, Technology and Medicine London, South Kensington, London SW7 2BU, UK 2 Institute of Physics, Biophysics, Martin Luther University Halle–Wittenberg, 06120 Halle, Germany 3 Institute of Technical Biochemistry e.V. and Centre for Structure und Dynamics of Proteins, Martin Luther University Halle–Wittenberg, 06120 Halle, Germany * Correspondence:
[email protected] or
[email protected] (A.K.);
[email protected] (J.B.); Tel.: +44-(0)20-7594-5315 (A.K.); +49-34-5552-8550 (J.B.) Abstract: Ankyrin repeat proteins are found in all three kingdoms of life. Fundamentally, these proteins are involved in protein-protein interaction in order to activate or suppress biological pro- cesses. The basic architecture of these proteins comprises repeating modules forming elongated structures. Due to the lack of long-range interactions, a graded stability among the repeats is the generic properties of this protein family determining both protein folding and biological function. Protein folding intermediates were frequently found to be key for the biological functions of re- peat proteins. In this review, we discuss most recent findings addressing this close relation for ankyrin repeat proteins including DARPins, Notch receptor ankyrin repeat domain, IκBα inhibitor of NFκB, and CDK inhibitor p19INK4d. The role of local folding and unfolding and gradual stability of individual repeats will be discussed during protein folding, protein-protein interactions, and post-translational modifications. The conformational changes of these repeats function as molecular Citation: Kumar, A.; Balbach, J.