The apical plasma membrane of chitin-synthesising epithelia

Chitin is the second most abundant polysaccharide on earth. It is produced at the apical side of epidermal, tracheal, fore- and hindgut epithelial cells in insects as a central component of the protective and supporting extracellular cuticle. Chitin is also an important constituent of the midgut peritrophic matrix that encases the food supporting its digestion and protects the epithelium against invasion by possibly ingested pathogens. The enzyme producing chitin is a glycosyltransferase that resides in the apical plasma membrane forming a pore to extrude the chains of chitin into the extracellular space. The apical plasma membrane is not only a platform for chitin synthases but, probably through its shape and equipment with distinct factors, also plays an important role in orienting and organising chitin fibres. Here, I review findings on the cellular and molecular constitution of the apical plasma membrane of chitin producing epithelia mainly focussing on work done in the fruit fly Drosophila melanogaster.