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

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. Besides the biological importance of this aminopolysaccharide, chitin also receives a lot of attention for its practical applications in medicine and biotechnology as a superior biopolymer with excellent physicochemical and mechanical properties. Chitin is produced and deposited extracellularly by chitin synthases. Most insects encode two types of chitin synthases, presumably with type A being required for exoskeletons and type B to produce the peritrophic matrix. It is not fully understood, however, which factors contribute to the specificity of each type of chitin synthase. Here we leverage the advantages of Drosophila melanogaster for functional manipulations to evaluate the mechanisms of activity and functional requirements of Kkv (Chitin synthase A) and Chs2 (Chitin synthase B). We first demonstrate that Chs2 is expressed and required in a region in the larval proventriculus that produces the chitin deposited in the peritrophic matrix. We then analyse whether the two chitin synthases can replace each other. We also investigate the subcellular localisation of these chitin synthases in different tissues and their ability to deposit chitin in combination with known auxiliary proteins. Our results indicate that the two different chitin synthases are not functionally equivalent, and that they use specific cellular and molecular mechanisms to deposit chitin. We suggest that the specificity of the different insect chitin synthases may underly the production of chitin polymers with different properties, conferring different physiological activities to the extracellular matrices.