Par-1, a Ser/Thr kinase, is required for cell polarisation in many organisms. In Drosophila oogenesis, for example, Par-1 is involved in several polarisation events, including the localisation of the anterior determinant bicoid mRNA. Riechmann and Ephrussi now report that Par-1 regulates the localisation of bicoid mRNA by phosphorylating Exuperantia (Exu; see p. ). They identify Exu, a mediator of bicoid mRNA localisation, as a phosphorylation target of Par-1 in a genome-wide, gel-based screen and describe two motifs in Exu that are phosphorylated by Par-1. Mutation of these motifs abolishes bicoid mRNA localisation in mid-oogenesis, but a proportion of the oocytes subsequently recover from these bicoid mRNA localisation defects, indicating that the requirement for Exu phosphorylation becomes less stringent as oogenesis proceeds. Finally, the researchers propose a multi-step model for bicoid mRNA localisation in which redundant mechanisms ensure its anterior accumulation.
