Ideas often recur. It has been pointed out recently that proteins are not always the well-structured entities we have become accustomed to from crystallographic studies, but may be intrinsically unstructured or contain unstructured regions. This feature, far from making these proteins less interesting, is an essential requirement for their function. Fascinating though it may be, the concept of so-called intrinsically unfolded (or unordered) proteins (IUPs), also often referred to as intrinsically disordered proteins (IDPs), is not new: it directly links back to the 1970s when the attention of many structural biologists was focused on biologically active peptides, which like IUPs lack a specific defined conformation. The recurrent nature of this concept may now be of topical interest since it suggests the transfer, upon suitable adaptation, of old tools to develop new ideas. Here, we review some of the approaches that were developed for the study of peptides and discuss how they could inspire powerful new methodologies for the study of IUPs. © 2013 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 100: 592–600, 2013.
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