In Drosophila embryos, hunchback ( hb ) messenger RNA is tightly regulated by recruitment of a complex that represses its translation at the posterior of the embryo. Pumilio (Pum) binds hb mRNA through a defined element in its 3′ untranslated region, and then recruits two other repressive factors, Nanos ( Nos) and Brain Tumour ( Brat). How is the sequential formation of this complex controlled? The region of Pum that is required for this has been narrowed down to the region that contains Puf repeats, so named after Pum and another protein that contains these repeats, FBF. This critical region is highly conserved in human Pum1, stressing its importance. So how similar are their structures? Very is the answer. The crystal structure of the crucial region of Pum from both is now reported by two groups in Cell and Molecular Cell; and, in addition to revealing a conserved 'rainbow-like' structure, they reveal some intriguing observations about their similarity to helical-repeat proteins, and the common surface used to mediate interactions with both proteins and RNA.

Both crystal structures show that Puf repeats align to form an extended, curved molecule. Each repeat is related to the next by ∼20°, leading to a rainbow-like arc structure. Intriguingly, this structure is similar to repeats found in the family of helical-repeat proteins, including β-catenin and karyopherin-α. Moreover, positive charge is concentrated along the inner concave surface — equivalent to the surface where many helical-repeat proteins show the greatest sequence conservation and interact with their binding partners.

This concentration of charge, together with data from previously characterized mutations in this region, led both groups to propose that the concave surface might provide the binding site for RNA. To investigate this, Edwards and colleagues used mutagenesis to identify the sites important for binding, and found two lines of evidence showing that the concave surface contacts RNA. Does this reflect a general property of the Puf-domain protein family? From an alignment with other members, Wang and colleagues show that the basic nature of the concave surface is conserved, and predict that it probably has the same function in binding RNA.

Next Edwards and colleagues asked which regions bind the two other members of the complex — Nos and Brat. To do this, they tested sets of mutants that bind RNA but cannot interact with one or the other factor. They found that the Pum surface that interacts with Nos includes the eighth repeat and the carboxy-terminal tail region. Focusing on the site that interacts with Brat, they showed that this is limited to repeats seven, eight and nine. The position of this site relative to the Nos-binding region indicates that Nos and Brat might bind cooperatively, and could explain why Brat is recruited only after Nos is present in the complex.

This observation — that Pum binds RNA through its concave surface — indicates that the helical-repeat family, previously characterized by an extended surface that is proposed to mediate protein–protein interactions, is more versatile, and also uses this surface for protein–RNA interactions.