Identification and expression pattern of a new digestive invertebrate-type lysozyme from the earthworm

Background The invertebrate type (i-type) lysozyme not showing a clear homology with the known types of lysozyme was first demonstrated from a marine bivalve, conch and earthworm by N-terminal sequence. An i-type lysozyme isolated from the earthworm found to be up-regulated upon bacterial challenge, suggesting this lysozyme to function as an inducible immune factor. However, information on the i-type lysozyme related with digestive function is very limited in the earthworm. Objective The objective of this study is to investigate the molecular characteristics and function of the new i-type lysozyme from the earthworm. Methods To identify a new i-type lysozyme, multiple amino acid sequence alignment and phylogenetic analyses were employed. Its mRNA expression pattern was observed by fluorescent in situ hybridization (FISH). Results A new i-type lysozyme ( Ea-iLys ) from an earthworm, Eisenia andrei with the open reading frame of 678 bp (226 amino acid residues) appeared to comprise conserved 14 cysteine residues for disulfide bridges and amino acid residues for the enzyme activities of lysozyme and isopeptidase, of which mRNA expression is mainly localized in the lining of midgut epithelium. No significant expression signal was detected in immune competent sites such as chloragogue tissue, typhlosole region, body coelom and muscle layers. Conclusion Our results suggest that this enzyme primarily acts as a digestive enzyme rather than an innate immune factor.