Most insect midguts are lined with a layer of chitin and glycoprotein, called the peritrophic matrix (PM), that performs many of the same functions as mucous secretions in the vertebrate digestive tract. As a barrier, the PM protects the midgut epithelium from abrasion by food particles and damage by ingested pathogens and toxins. As a component of the digestive system, the PM improves the efficiency of nutrient acquisition by compartmentalizing digestive processes allowing for reuse of hydrolytic enzymes. The PM consists of a network of chitin fibrils held together by proteins. Glycans filling the interstitial spaces create a molecular sieve, the porosity of which is dependent on ion content and pH. The suite of proteins associated with the PM from several insect orders, namely Lepidoptera, Diptera and Coleoptera, is now being elucidated. In this review we consolidate recent findings and present a universal scheme to classify PM proteins based firstly on their functional characteristics, either as structural proteins (peritrophins) or enzymes. Structural proteins are then classified according to the organization, characteristics and relatedness of their respective chitin binding and mucin domains as simple, binary, complex or repetitive peritrophins. Homology models of peritrophin A chitin binding domains are used to reveal new features about their structure and interaction with chitin. Finally, the relationship between peritrophin structure and PM function is discussed.
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