Fig. 6. The E-cadherin intracellular domain is required for fusion track assembly and maturation, and maintenance of apical cytoskeletal polarity in tracheal cells. Anterior, leftwards; dorsal, upwards. (A-F) Stage 15 embryos stained with 2A12 to reveal the tracheal lumen (green). In F, the embryo is also stained with anti-E-cadherin (red). Scale bar: 10 µm in A-F. (A) The dorsal trunk is continuous in a wild-type embryo. (B) Numerous breaks and constrictions in the dorsal trunk of a shg²/shg^IH embryo. (C) Expression of a wild-type E-cadherin cDNA in tracheal cells restores fusion in a shg²/shg^IH embryo. (D) Expression of a arm mutant E-cadherin in tracheal cells enhances fusion defects in a shg²/shg^IH embryo. (E) Expression of AAA-JXT mutant E-cadherin in tracheal cells enhances fusion defects in a shg²/shg^IH embryo. (F) Expression of both -arm and AAA-JXT mutant E-cadherin in tracheal cells does not rescue fusion defects in a shg²/shg^IH embryo. Mutant E-cadherins are delocalized and not organized into adherens junctions. Fusion cells maintain finger-like contacts (arrows) but do not form lumenal connections even as epidermal movements and shape changes pull tracheal branches apart. (I,L,O,S) Shot, red; E, cadherin, green; F-actin, blue (S only). (G-I) A stage 13 shg²/shg^IH mutant embryo expressing a arm mutant E-cadherin in tracheal cells. (G) Shot tracks are not detectable at fusion sites (arrows). Scale bar: 10 µm in G-N. (H) E-cadherin is delocalized in tracheal cells. In these embryos, little endogeneous E-cadherin is detectable, so this represents transgene expression. (I) Merge of G and H. (J-L) A stage 13 shg²/shg^IH mutant embryo expressing the AAA-JXT mutant E-cadherin in tracheal cells. (J) Shot tracks are weak (left arrow) or absent (right arrow) in fusion cells. Shot is apically concentrated in other tracheal cells (arrowhead). (K) E-cadherin is delocalized in tracheal cells. (L) Merge of J and K. (M-O) Stage 15 shg²/shg^IH mutant embryo expressing the AAA-JXT mutant E-cadherin in tracheal cells. (M) Shot is delocalized in tracheal cells. (N) E-cadherin is delocalized in tracheal cells. (O) Merge of M and N. (P-S) A stage 13 shg²/shg^IH mutant embryo expressing the AAA-JXT mutant E-cadherin in tracheal cells. (P) Shot tracks are absent from fusion cells (arrows). Scale bar: 10 µm in P-S. (Q) E-cadherin is delocalized. (R) F-actin tracks are absent from fusion cells (arrows). (S) Merge of P-R.

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