Figure 1: Planar cell polarity in Drosophila wing.

Left upper: Instructed by long-range cues, wing hexamer cells generate a single actin-based trichome (hair) at the most distal aspect of each cell. Right upper: Fat (Ft) mRNA is evenly expressed along the tissue, whereas Dachsous (Ds) transcript is expressed in a distal-proximal gradient and Four-jointed (Fj) kinase is expressed in a proximal-distal gradient. Fj phosphorylates both Ft (stronger phosphorylation at proximal side) and Ds (stronger phosphorylation at distal side). Strong Ft phosphorylation translates into a strong Ft-Ds protein-protein interactions at the proximal side, whereas Ds phosphorylation weakens Ft-Ds interactions at the distal part, thereby generating a shallow gradient of Ft-Ds activity across the tissue plane. Right middle: core PCP protein complexes are asymmetrically distributed: Vang-Pk to the proximal and Fz-Dsh-Dg to the distal sides. Cadherin Fmi is localized at both proximal and distal sides and forms homodimers between the extracellular domains of molecules expressed by adjacent cells. Interactions between Fmi and Vang or Fmi and Fz, as well as between extracellular domains of Vang and Fz expressed on surfaces of neighboring cells stabilize Vang-Pk and Fz-Dsh complexes on the opposite cell membranes. Inside the cell, mutual antagonism between Pk and Dsh creates “exclusion” zones where the proteins of the opposite core PCP complex cannot function. Right lower: Core PCP proteins control localization of PCP effectors via direct interactions (e.g. Vang interacts with In and Fy). In Drosophila wing cells, PCP effectors inhibit generation of trichome at the proximal side of the cell, whereas positive actin regulators, such as RhoA GTPase and Drosophila RhoA kinase (DROCK) accumulate at the distal side of the cell where they promote actin polymerization and hair formation.