Functional analysis and characterization of antimicrobial phosphatidylethanolamine-binding protein BmPEBP in the silkworm Bombyx mori.

Phosphatidylethanolamine-binding proteins (PEBPs) are a class of highly conserved, biologically diverse proteins, which are widely distributed in plants, insects, and mammals. In this study, a Bombyx mori PEBP (BmPEBP) gene was reported, which encodes a protein composed of 209 amino acid residues. BmPEBP includes a predicted signal peptide, indicating that it is an extracellular protein, which differs from the cytoplasmic PEBPs of plants and mammals. Recombinant soluble BmPEBP was successfully synthesized using a prokaryotic expression system and was then purified effectively by Ni2+-NTA affinity chromatography and gel filtration. Far-ultraviolet circular dichroism spectra indicated that BmPEBP had a well-defined β-sheet structure, with the β-sheet content accounting for about 41% of the protein. BmPEBP had a relatively stable structure at temperatures ranging from 15 °C to 57.5 °C. The Tm, ΔH, and ΔS of BmPEBP were 62.27 °C ± 0.14 °C, 570.10 ± 0.17 kJ/mol, and 1.70 ± 0.03 KJ/(mol·K), respectively. Homology modeling analysis suggested that the active sites of BmPEBP were conserved, comprising Pro96, His111, and His143. Quantitative real-time PCR showed that BmPEBP was highly expressed in the silk gland and had very low expression in other tissues. However, BmPEBP expression was significantly upregulated in the larval fat body after infection with two kinds of fungi, Beauveria bassiana and Candida albicans. Moreover, in vitro fungal inhibition tests showed that BmPEBP could significantly inhibit the sporular growth of Saccharomyces cerevisiae, C. albicans, B. bassiana, and Aspergillus fumigatus. To our knowledge, this is the first report to reveal the antifungal role of a PEBP in insects.