Folding for binding or binding for folding?

A number of proteins are partially unstructured under physiological conditions and assume a well-defined three-dimensional structure only upon binding to another protein or macromolecular complex. Numerous examples have been found, especially among proteins involved in regulatory functions in eukaryotic cells. Recent computer simulations by Verkhivker et al. yielded an atomic-detail picture of the folding of p27 and its concomitant binding to the phosphorylated cyclin A–cyclin dependent kinase 2.