Distinct cellular and molecular mechanisms contribute to the specificity of the two Drosophila melanogaster chitin synthases in chitin deposition.

Chitin is a major component of arthropod extracellular matrices, including the exoskeleton and the midgut peritrophic matrix. It plays a key role in the development, growth and viability of insects. Beyond the biological importance of this aminopolysaccharide, chitin also receives considerable attention for its practical applications in medicine and biotechnology, as it is a superior biopolymer with excellent physicochemical and mechanical properties. Chitin is synthesised and deposited extracellularly by chitin synthases. Most insects encode two types of chitin synthases: type A, which are presumed to be required for exoskeleton formation, and type B, which are thought to produce the peritrophic matrix. However, the factors that contribute to the specificity of each type of chitin synthase remain unclear. Here, we leverage the advantages of Drosophila melanogaster for functional manipulations to evaluate the mechanisms of activity and the functional requirements of Kkv (Chitin synthase A) and Chs2 (Chitin synthase B). We first demonstrate that Chs2 is expressed and required in a specific region of the larval proventriculus responsible for producing chitin in the peritrophic matrix. We then assess whether the two chitin synthases can functionally substitute for each other. Additionally, we examine their subcellular localisation in different tissues and their ability to deposit chitin in combination with known auxiliary proteins. Our results indicate that these two different chitin synthases are not functionally interchangeable and that they use distinct cellular and molecular mechanisms to deposit chitin. We suggest that the specificity of insect chitin synthases may underlie the production of chitin polymers with different properties, conferring different physiological activities to the extracellular matrices.