Immune upregulation of novel antibacterial proteins from silkmoths (Lepidoptera) that resemble lysozymes but lack muramidase activity

Study on immune proteins in domesticated and wild silkmoths Bombyx mori and Antheraea mylitta, respectively, led to identification of a new class of antimicrobial proteins. We designated them as lysozyme-like proteins (LLPs) owing to their partial similarity with lysozymes. However, lack of characteristic catalytic amino acid residues essential for muramidase activity in LLPs puts them functionally apart from classical lysozymes. Two LLPs, one from B. mori (BLLP1) and the other from A. mylitta (ALLP1) expressed in a recombinant system, exhibited a broad-spectrum antibacterial action. Further investigation of the antibacterial mechanism revealed that BLLP1 is bacteriostatic rather than bactericidal against Escherichia coli and Micrococcus luteus. Substantial increase in hemolymph bacterial load was observed in B. mori upon RNA interference mediated in vivo knockdown of BLLP1. We demonstrate that the antibacterial mechanism of this protein depends on peptidoglycan binding unlike peptidoglycan hydrolysis or membrane permeabilization as observed with lysozymes and most other antimicrobial peptides. To our knowledge, this is the first report on functional analysis of novel, non-catalytic lysozyme-like family of antibacterial proteins that are quite apart functionally from classical lysozymes. The present analysis holds promise for functional annotation of similar proteins from other organisms.