Fig. 9. Working models for Fat regulation and signaling. (A,B) Schematics of a cross-section of two cells. Genetic and protein localization studies position Dachs downstream of Fat in a pathway that influences growth and gene expression in imaginal discs. (A) When Ds and Fat are engaged, Fat accumulates at the membrane, where it downregulates Dachs, thereby preventing Dachs-dependent growth and gene expression. (B) Four-jointed is proposed to modify Ds and thereby prevent it from engaging Fat. This allows accumulation of Dachs, and promotion of growth, affinity and gene expression. (C,D) Show schematics of a horizontal section of a field of cells, with localization of Ds (blue), Fat (red) and Dachs (green) schematized. (C) Expression of Fj in a clone of cells (gray) is proposed to antagonize Ds (i) (broken lines). The absence of a good Ds partner within the clone causes Fat in cells at the edge of the clone to localize to the clone perimeter (ii), where it downregulates Dachs (iii). (D) Expression of Ds in a clone of cells (i) (gray) is proposed to recruit Fat in neighboring cells to the edge of the clone (ii). This results in elevated Dachs at the edge of the clone (iii). See text for discussion of possible mechanisms.