New currency for old rope: from coiled-coil assemblies to alpha-helical barrels

alpha-Helical coiled coils are ubiquitous protein-protein-interaction domains. They share a relatively straightforward sequence repeat, which directs the folding and assembly of amphipathic helices. The helices can combine in a number of oligonnerisation states and topologies to direct a wide variety of protein assemblies. Although in nature parallel dimers, trimers and tetramers dominate, the potential to form larger oligomers and more-complex assemblies has long been recognised. In particular, complexes above pentamer are interesting because they are barrel-like, having central channels or pores with well-defined dimensions and chemistry. Recent empirical and rational design experiments are beginning to chart this potential new territory in coiled-coil space, leading to intriguing new structures, and possibilities for functionalisation and applications.