The IDGF's (imaginal disc growth factors) are a family of growth factors identified in insects that include at least five members, some of which are encoded by genes in a tight cluster. The IDGF's have an eight-stranded alpha/beta barrel fold and are related to the glycosyl hydrolase family 18 (GH18) chitinases, but they have an amino acid substitution known to abolish chitinase catalytic activity. IDGFs may have evolved from chitinases to gain new functions as growth factors, interacting with cell surface glycoproteins involved in growth-promoting processes.

Glycosyl hydrolases family 18.

Glyco_18 domain.

This conserved domain family includes a large number of catalytically inactive chitinase-like lectins (chitolectins) including YKL-39, YKL-40 (HCGP39), YM1, oviductin, and AMCase (acidic mammalian chitinase), as well as catalytically active chitotriosidases. The conserved domain is an eight-stranded alpha/beta barrel fold belonging to the family 18 glycosyl hydrolases. The fold has a pronounced active-site cleft at the C-terminal end of the beta-barrel. The chitolectins lack a key active site glutamate (the proton donor required for hydrolytic activity) but retain highly conserved residues involved in oligosaccharide binding. Chitotriosidase is a chitinolytic enzyme expressed in maturing macrophages, which suggests that it plays a part in antimicrobial defense. Chitotriosidase hydrolyzes chitotriose, as well as colloidal chitin to yield chitobiose and is therefore considered an exochitinase. Chitotriosidase occurs in two major forms, the large form being converted to the small form by either RNA or post-translational processing. Although the small form, containing the chitinase domain alone, is sufficient for the chitinolytic activity, the additional C-terminal chitin-binding domain of the large form plays a role in processing colloidal chitin. The chitotriosidase gene is nonessential in humans, as about 35% of the population are heterozygous and 6% homozygous for an inactivated form of the gene. HCGP39 is a 39-kDa human cartilage glycoprotein thought to play a role in connective tissue remodeling and defense against pathogens.

The class V plant chitinases have a glycosyl hydrolase family 18 (GH18) domain, but lack the chitin-binding domain present in other GH18 enzymes. The GH18 domain of the class V chitinases has endochitinase activity in some cases and no catalytic activity in others. Included in this family is a lectin found in black locust (Robinia pseudoacacia) bark, which binds chitin but lacks chitinase activity. Also included is a chitinase-related receptor-like kinase (CHRK1) from tobacco (Nicotiana tabacum), with an N-terminal GH18 domain and a C-terminal kinase domain, which is thought to be part of a plant signaling pathway. The GH18 domain of CHRK1 is expressed extracellularly where it binds chitin but lacks chitinase activity.