Myosin-dependent partitioning of junctional Prickle2 toward the anterior vertex during planar polarization of Xenopus neuroectoderm

Planar cell polarity (PCP) of tissues is established by mutually exclusive partitioning of transmembrane proteins Frizzled and Vangl with their respective binding partners, Dishevelled and Prickle. While the amplification and maintenance of this pattern have been well studied, it remains unclear how the anterior-biased protein localization is initiated. Moreover, PCP protein complexes are located at adherens junctions and their polarization requires the activity of non-muscle myosin II (NMII), but how NMII contributes to PCP is not fully understood. Here we analyze time-lapse images of mNeonGreen-tagged Prickle2 (Pk2) in mid-gastrula stage Xenopus presumptive neuroectoderm and demonstrate that Pk2 puncta move along bicellular apical junctions in a biased manner toward the anterior vertex, where the Vangl-Pk complexes are normally enriched. In addition, length changes of bicellular junction segments flanking each Pk2 punctum are often different from each other, and appear more dynamic near the vertices, suggesting that Pk2 movement is driven by intrinsic junction heterogeneity. Reducing NMII activity eliminates the anterior movement, and surprisingly, increases the motility of Pk2 punta. By assessing the correlation between Pk2 movement and the relative positioning of each Pk2 punctum along apical junctions, we uncovered that NMII activity is required for the anterior Pk2 movement by maintaining the elongation of posterior junction segment while inhibiting Pk2 movement toward both vertices flanking the junctions. Our findings provide the first evidence of biased partitioning of junctional PCP proteins toward the anterior vertex and support the hypothesis that NMII activity facilitates Pk2 polarization not via a direct transport but by regulating intrinsic dynamics of the bicellular junction.