The E3 Ubiquitin Ligase Mindbomb1 controls zebrafish Planar Cell Polarity

Vertebrate Delta/Notch signaling involves multiple ligands, receptors and transcription factors. Delta endocytosis – a critical event for Notch activation – is however essentially controlled by the E3 Ubiquitin ligase Mindbomb1 (Mib1). Due to its position at a molecular bottleneck of the pathway, Mib1 inactivation is often used to inhibit Notch signaling. However, recent findings indicate that the importance of Mib1 extends beyond the Notch pathway. We report an essential role of Mib1 in Planar Cell Polarity (PCP). mib1 null mutants or morphants display impaired gastrulation stage Convergence Extension (CE) movements. Comparison of different mib1 mutants and functional rescue experiments indicate that Mib1 controls CE independently of Notch. In contrast, Mib1-dependent CE defects can be rescued using the PCP downstream mediator RhoA. Mib1 regulates CE through the RING Finger domains that have been implicated in substrate ubiquitination, suggesting that Mib1 may control PCP protein trafficking. Accordingly, we show that Mib1 controls the endocytosis of the PCP component Ryk and that Ryk internalization is required for CE. Numerous morphogenetic processes involve both Notch and PCP signaling. We show that Mib1, a known Notch signaling regulator, is also an essential PCP pathway component. Care should therefore be taken when interpreting Mib1 loss of function phenotypes.