Trends in Biochemical Sciences
ReviewThe ankyrin repeat: a diversity of interactions on a common structural framework
Section snippets
Structural organization of ANK repeats
The first three-dimensional structure of an ANK-repeat molecule, that of 53BP2 bound to the p53 cell-cycle tumour suppressor6, was determined almost ten years after the discovery of the motif. This and subsequent structures have shown that the ANK repeat consists of pairs of antiparallel α-helices stacked side by side and connected by a series of intervening β-hairpin motifs (Fig. 1a). The extended β-sheet projects away from the helical pairs almost at right angles to them, resulting in a
Intermolecular interactions
Eight X-ray structures of complexes involving ANK repeats have been determined (Table 1). Despite the overall sequence similarity shared by ANK proteins, the mechanism of binding to specific partner molecules varies considerably. Binding can use much of the available solvent-accessible surface, and contacts are not restricted to any particular secondary structural element within the ANK repeat. Nevertheless, these structures have revealed some common features of molecular interactions. The
Intramolecular interactions
The structure of a central 36-kDa fragment of Swi6 provides the only example so far of intramolecular interactions involving ANK repeats18 (Fig. 2d). In this case, a region that is capable of transcriptional activation in vivo23 interacts with a core domain consisting of five ANK-repeat motifs. The interaction surface on the ANK-repeat domain differs considerably from those of the intermolecular complexes described above. In the cupped-hand analogy, the transcriptional activator interacts with
Summary
Protein evolution demands conservation of key residues to maintain structural integrity, but allows for sequence variations that, in turn, provide functional specificity. In fact, the complex biochemical requirements of a living cell appear to be largely fulfilled by a relatively small number of protein architectures. In some cases, versatility has been achieved through the assembly of multiple copies of amino acid sequences to form families of structurally related but functionally diverse
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