Fig. 3. Alternative models of chitin formation in insects. (A) Chitin synthase-loaded vesicles are transported from the transgolgi network to the apical region of epithelial cells by a constitutive secretory pathway and subsequently fuse with the plasma membrane. Upon fusion, they may get activated by proteolytic enzymes present in the molting fluid or the gut lumen. Blue and orange stars indicate the catalytic site facing either the cytoplasm or the extraplasmic space, respectively. (B) In this more speculative model, chitin synthesis has already occurred before the vesicles have fused with the plasma membrane and may continue or cease upon fusion. If the catalytic domain faces the cytoplasm (blue star), nascent chitin polymers have to be transported across the vesicular membrane, presumably involving transmembrane segments of the chitin synthase. By contrast, intravesicular arrangement of the catalytic domain (orange star) would require some uptake mechanism for UDP-GlcNAc.

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