A novel transmembrane protein recruits numb to the plasma membrane during asymmetric cell division.

Numb, an evolutionarily conserved cell fate-determining factor, plays a pivotal role in the development of Drosophila and vertebrate nervous systems. Despite lacking a transmembrane segment, Numb is associated with the cell membrane during the asymmetric cell division of Drosophila neural precursor cells and is selectively partitioned to one of the two progeny cells from a binary cell division. Numb contains an N-terminal phosphotyrosine-binding (PTB) domain that is essential for both the asymmetric localization and the fate specification function of Numb. We report here the isolation and characterization of a novel PTB domain-binding protein, NIP (Numb-interacting protein). NIP is a multipass transmembrane protein that contains two PTB domain-binding, NXXF motifs required for the interaction with Numb. In dividing Drosophila neuroblasts, NIP is colocalized to the cell membrane with Numb in a basal cortical crescent. Expression of NIP in Cos-7 cells recruited Numb from the cytosol to the plasma membrane. This recruitment of Numb to membrane by NIP was dependent on the presence of at least one NXXF site. In Drosophila Schneider 2 cells, NIP and Numb were colocalized at the plasma membrane. Inhibition of NIP expression by RNA interference released Numb to the cytosol. These results suggest that a direct protein-protein interaction between NIP and Numb is necessary and sufficient for the recruitment of Numb to the plasma membrane. Recruitment of Numb to a basal cortical crescent in a dividing neuroblast is essential for Numb to function as an intrinsic cell fate determinant.

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