Source: http://cegg.unige.ch/insecta/immunodb

LYSs: Lysozymes

Summary

Susan Paskewitz

Russell Labs, Department of Entomology, Madison, USA

Lysozymes are 14-16 kDa basic proteins that hydrolyze peptidoglycan of bacterial cell walls. In insects, both chicken-type (c-type) and invertebrate-type lysozymes occur. Many studies have shown increases in expression of c-type lysozyme genes and of increases in lysozyme activity following exposure to bacteria, especially in the Lepidoptera. Interestingly, Dm lysozymes show little response to bacterial infection [De Gregorio et al. 2001], although several genes were upregulated after infection by the microsporidian parasite, Octosporea [Roxstrom-Lindquist et al. 2004]. Mosquito genes AgLYSC1 and AgLYSC2 and AaLys-A (LYSC11) are upregulated following bacterial challenge [Gesellchen et al. 2005; Ursic Bedoya et al. 2005]. Our analysis indicates that an expansion of the lysozyme gene family has occurred in dipteran flies. There are 13 genes in Dm, 8 in Ag and 7 in Aa. Expansion of the c-type lysozyme gene family also is found in foregut-fermenting vertebrates. For example, ten lysozyme genes occur in Bos taurus. In both invertebrates and vertebrates, expansion is partly due to the use of lysozymes for digestion of bacteria as a food resource. Dipteran insects are associated with moist habitats and often use bacteria as a food. Larval forms of the mosquitoes and larval and adult Dm fit this profile. Several of the Dm lysozymes are expressed in the gut and exhibit acidic pIs, which are presumed to be adaptations to a digestive function in the acidic gut environment[Daffre et al. 1994]. AgLYSC3 and AgLYSC8 are expressed at much higher levels in larvae than in adults and are candidates for a role in digestion [Li et al. 2005], however, no orthologous enzymes were found in Aa. A second group of dipteran lysozymes is characterized by the loss of one of the two amino acids that are critical for muramidase activity. In five of these insect proteins, Asp52 (using vertebrate notation) is altered to Asn52, suggesting the intriguing possibility that these enzymes may become activated upon deglycosylation. Finally, each of the three species has one unusual, long lysozyme that contains 4 or 5 lysozyme domains, these are not shown on the tree. In Ag, LYSC6 is constitutively expressed throughout the life cycle and in many adult tissues [Li et al. 2005] but transcript levels were not altered following immune challenge.