Molecular characterization and functional analysis of the bactericidal permeability-increasing protein/LPS-binding protein (BPI/LBP) from roughskin sculpin (Trachidermus fasciatus)

Bactericidal permeability-increasing protein (BPI) and lipopolysaccharide (LPS) binding proteins (LBP) both play important roles in innate immunity against bacterial infection. Herein, we identified a novel full-length cDNA sequence of BPI/LBP from Trachidermus fasciatus (designated as TfBPI/LBP). The full-length cDNA sequence of TfBPI/LBP was 1594bp, which contains an open reading frame (ORF) of 1422bp encoding a secreted protein with 473 amino acid residues. Similar to BPI/LBPs from other teleost and mammals, the peptide of TfBPI/LBP contains an N-terminal BPI/LBP/CETP domain with an LPS-binding motif and a C-terminal BPI/LBP/CETP domain BPI2. Multiple alignments and phylogenetic analysis supported that TfBPI/LBP was a new member of the vertebrate BPI/LBP family. TfBPI/LBP gene was ubiquitously expressed in all detected tissues, with the most abundant in the liver, and could be significantly induced in the skin, blood, liver, spleen post LPS challenge. The recombinant N-terminal domain of TfBPI/LBP (designated as rTfBPI/LBPN) was successfully expressed in Escherichia coli. Sugar binding assay showed that rTfBPI/LBPN could bind to LPS, peptidoglycan (PGN), and lipoteichoic acid (LTA), with the highest affinity to LPS. The results of bacteria binding and agglutinating assay revealed that rTfBPI/LBPN could bind and agglutinate to all of the 9 kinds of bacteria we used. Moreover, membrane integrity analysis indicated that rTfBPI/LBPN could increase the membrane permeability of bacteria. These results suggested that BPI/LBP may play crucial roles in host defense against microorganisms, possibly through nonselective bacterial recognition and induction of membrane penetration.