Natively unfolded domains in endocytosis: hooks, lines and linkers

It is commonly assumed that a protein must adopt a tertiary structure to achieve its active native state and that regions of a protein that are devoid of alpha-helix or beta-sheet structures are functionally inert. Although extended proline-rich regions are recognized as presenting binding motifs to, for example, Src homology 2 (SH2) and SH3 domains, the idea persists that natively unfolded regions in functional proteins are simply 'spacers' between the folded domains. Such a view has been challenged in recent years and the importance of natively unfolded proteins in biology is now being recognized. In this review, we highlight the role of natively unfolded domains in the field of endocytosis, and show that some important endocytic proteins lack a traditionally folded structure and harbour important binding motifs in their unstructured linker regions.