Miranda cargo-binding domain forms an elongated coiled-coil homodimer in solution: implications for asymmetric cell division in Drosophila.

Miranda is a multidomain adaptor protein involved in neuroblast asymmetric division in Drosophila melanogaster. The central domain of Miranda is necessary for cargo binding of the neural transcription factor Prospero, the Prospero-mRNA carrier Staufen, and the tumor suppressor Brat. Here, we report the first solution structure of Miranda central "cargo-binding" domain (residues 460-660) using small-angle X-ray scattering. Ab initio modeling of the scattering data yields an elongated "rod-like" molecule with a maximum linear dimension (D(max)) of approximately 22 nm. Moreover, circular dichroism and cross-linking experiments indicate that the cargo-binding domain is predominantly helical and forms a parallel coiled-coil homodimer in solution. Based on the results, we modeled the full-length Miranda protein as a double-headed, double-tailed homodimer with a long central coiled-coil region. We discuss the cargo-binding capacity of the central domain and propose a structure-based mechanism for cargo release and timely degradation of Miranda in developing neuroblasts.