Functional roles of transiently and intrinsically disordered regions within proteins.

Proteins are structurally heterogeneous and comprise folded regions with variable conformational stabilities and intrinsically disordered protein regions that do not have well-folded structures. Even small, well-folded single-domain proteins are structurally heterogeneous and contain multiple foldon units with different conformational stability. Although the ability of many intrinsically disordered protein regions to undergo at least partial folding at interaction with specific binding partners is a well-established fact, recent studies have revealed that functions of some ordered proteins rely on the decrease in the amount of their ordered structure and require local or even global functional unfolding. This functional unfolding is induced by transient alterations in protein environment or by modification of protein structure and can be reversed as soon as the environment is restored or the modification is removed. Therefore, the important features of these conditionally disordered protein regions (or unfoldons) are the induced nature and the transient character of their disorder. In other words, structurally any protein can be described as a modular assembly of foldons, inducible foldons, semi-foldons, nonfoldons and unfoldons. Obviously, differently ordered/disordered proteins and protein regions can possess very different functional repertoires. This review represents some of the key functions of transiently and intrinsically disordered protein regions.