Opo opposes Numb in the eye

Tissue morphogenesis requires the polarized trafficking of adhesion receptors. Martínez-Morales and colleagues refine the role for the transmembrane protein Opo in this context, and find that it opposes Numb-mediated endocytosis of integrin β₁ (Dev. Cell http://doi.org/jhp; 2012).

The authors previously found that Opo is essential for folding of the optic cup in medaka fish by regulating the polarization of focal adhesion components. To better understand the function of Opo, they performed a yeast two-hybrid screen that identified the encocytic adaptors Numb-like (Numbl) and Dab2 as Opo interacting proteins. Interactions with Numb, Numbl and Dab2 were confirmed in vitro and in mammalian cells, and required an NPxY sequence in Opo also present in integrin β₁. Opo colocalized with clathrin-coated endocytic vesicles and inhibited Numb-mediated endocytosis of integrin β₁ in HeLa cells. Fluorescence recovery after photobleaching (FRAP) analysis demonstrated that basal, but not apical, transport of integrins was affected in medaka opo mutants. Electron microscopy confirmed the basal accumulation of vesicles in opo mutant neuroblasts, and integrin uptake was enhanced in retinae of opo mutants in vivo. Injection of Numb or Numbl RNA into medaka or zebrafish embryos caused morphogenesis defects similarly to those seen in opo mutants, agreeing with Numb and Opo functioning in an antagonistic manner. Genetic interactions were also observed between Numb proteins and Opo in medaka embyros. The precise molecular function of Opo requires further investigation but could involve inhibiting integrin binding to the Numb adaptors.