Cleavage ensures polarized Bnl sorting to the basal cell surface for signaling. (A–C) High-magnification (40×) images of the exposed basal surfaces (arrowhead) of salivary glands expressing WT or M1 under bnl-Gal4 from an area schematically shown in C. Red, surface αGFP^ex immunostaining; arrow, cell junction. (D) Graph comparing fractions (red surface stain/total GFP) of overexpressed WT (n = 12) and M1 (n = 10) that got externalized on the salivary gland surface. (E–H) Images of sagittal sections of salivary glands expressing WT and M1 under bnl-Gal4. Arrow, apical lumen. (I) Drawings depicting the ASP D-P axis (dashed line; upper panel) and an XZY section along the D-P axis (lower panel) showing the tubular ASP and disc epithelia as shown in J–M. (J and K) Sagittal sections of αDlg immunostained (blue, sub-apical marker) wing disc and ASP when the disc bnl source coexpressed CD8:Cherry with either the WT or M1 construct under bnl-Gal4. Arrow, basal side; arrowhead, apical side. (L) Graph comparing apical and basal percentage of WT and M1 relative to the total amount in the disc source. n = 24 (WT) and 32 (M1). (M) Maximum projections of mid- and para-sagittal sections within ∼3 µm of mid-Y of an αDlg (blue) and αHA (red) stained wing-disc/ASP, where bnl-Gal4 expressed Bnl:HA₁GFP₃. Arrow, truncated Bnl:GFP₃; white dashed line, ASP and wing disc; arrowhead, apical lumen of wing disc. (N–P) Comparison (graph in P) of levels of t-WT^endo (n = 17) and M1^endo (n = 33) on the surface of the disc source and ASP (dashed line). Red and arrowhead, detergent-free αGFP-staining; arrow, intracellular puncta; white staining, phalloidin-Alexa Fluor 647. Scale bars: 50 µm (E–H); 20 µm (all other panels).