Accumulation of Kkv and Reb.

All images are super-resolution single confocal sections except B, which is a projection of super-resolution confocal sections. (A, B) In the trachea of wild-type embryos, Reb and Kkv do not colocalise, and they show a complementary pattern (A’-A”’) at the local subcellular level. (C) In salivary gland of embryos expressing Reb, the patterns of Kkv and Reb are complementary. (D) Models for the role of kkv and exp/reb in chitin deposition. Kkv oligomerises in complexes that localise to the apical membrane (as proposed in [2]). In the absence of exp/reb activity, Kkv can polymerise chitin from sugar monomers (discontinuous red lines), but it cannot translocate it because the channel is closed, and polymerised chitin remains in the cytoplasm. In addition, Kkv is not homogeneously distributed. Exp/Reb form a complex with other proteins, which localises to the apical membrane. The presence of Exp/Reb complex regulates Kkv apical distribution and activity. In model 1, we propose that a factor/s recruited by Exp/Reb (Factor X) can induce a posttranslation or conformational modification to Kkv protein that opens the channel promoting translocation of chitin fibers to the extracellular domain. In model 2, we propose that a factor/s recruited by Exp/Reb (Factor X’) can induce changes in membrane composition/curvature that will then promote a conformational change in Kkv that opens the channel to translocate chitin. These membrane changes lead to Kkv shedding extracellularly. In model 3, we propose that Exp/Reb complex can bind and relocalise Factor X”, which normally inhibits Kkv-translocating activity. This neutralises the activity of Factor X” allowing chitin translocation. Scale bars: 5 μm.