Chitin Prevalence and Function in Bacteria, Fungi and Protists.

Chitin is an important structural polysaccharide, which supports and organizes extracellular matrices in a variety of taxonomic groups including bacteria, fungi, protists, and animals. Additionally, chitin has been recognized as a molecule that is required for Rhizobia-legume symbiosis and involved in arbuscular mycorrhizal signaling in the symbiotic interaction between terrestrial plants and fungi. Moreover, it serves as a unique molecular pattern in the plant defense system against pathogenic fungi and parasites, and in the innate and adaptive immune response of mammals and humans. In this review, we will focus on the prevalence and structural function of chitin in bacteria, fungi, and protists, with a particular focus on the evolution of chitin synthases and the function of chitin oligosaccharides as a signaling molecule in symbiosis and immunity.

Two essential peritrophic matrix proteins mediate matrix barrier functions in the insect midgut.

The peritrophic matrix (PM) in the midgut of insects consists primarily of chitin and proteins and is thought to support digestion and provide protection from abrasive food particles and enteric pathogens. We examined the physiological roles of 11 putative peritrophic matrix protein (PMP) genes of the red flour beetle, Tribolium castaneum (TcPMPs). TcPMP genes are differentially expressed along the length of the midgut epithelium of feeding larvae. RNAi of individual PMP genes revealed no abnormal developmental phenotypes for 9 of the 11 TcPMPs. However, RNAi for two PMP genes, TcPMP3 and TcPMP5-B, resulted in depletion of the fat body, growth arrest, molting defects and mortality. In situ permeability assays after oral administration of different-sized FITC-dextran beads demonstrated that the exclusion size of the larval peritrophic matrix (PM) decreases progressively from >2 MDa to <4 kDa from the anterior to the most posterior regions of the midgut. In the median midguts of control larvae, 2 MDa dextrans were completely retained within the PM lumen, whereas after RNAi for TcPMP3 and TcPMP5-B, these dextrans penetrated the epithelium of the median midgut, indicating loss of structural integrity and barrier function of the larval PM. In contrast, RNAi for TcPMP5-B, but not RNAi for TcPMP3, resulted in breakdown of impermeability to 4 and 40 kDa dextrans in the PM of the posterior midgut. These results suggest that specific PMPs are involved in the regulation of PM permeability, and that a gradient of barrier function is essential for survival and fat body maintenance.