The structure and function of a bovine salivary protein - BSP30b

Abstract

The Bovine Salivary Protein 30b (BSP30b) is a member of the palate, lung, and nasal epithelium clone (PLUNC) family which includes human bactericidal/permeability-increasing protein (BPI) and lipopolysaccaride binding protein (LBP). BSP30b is predominantly found in bovine saliva but its function is not understood. Although PLUNC family members are proposed to be lipid binding proteins, in most cases their lipid ligands are also unknown. Here, the X-ray crystal structure of BSP30b is solved at 2 Å resolution using a double methionine mutant and Se-Met MAD phasing. Each molecule adopts a slightly curved cylindrical structure with a hydrophobic channel formed by an α/β wrap, which is highly conserved in the PLUNC family. The structure of BSP30b complexed with oleic acid is also presented where the ligand is accommodated within the hydrophobic channel. The electron density for oleic acid suggests that the ligand is only partially occupied in the binding site implying that oleic acid might not be the preferred ligand of BSP30b. Nonetheless, GFP-tagged BSP30b binds to the surface of olive oil droplets, as observed by fluorescent microscopy. Bacteria extracted directly from rumen samples indicate that the GFP_BSP30b fusion protein binds to a small number of selected bacterial strains in vivo. These results suggest that BSP30b could have a specific range of lipid ligands and may play important roles via its interaction with select rumen bacteria.