Detecting distant relatives of mammalian LPS-binding and lipid transport proteins

In mammals, a family of four lipid binding proteins has been previously defined that includes two lipopolysaccharide binding proteins and two lipid transfer proteins. The first member of this family to have its three-dimensional structure determined is bactericidal/permeability-increasing protein (BPI). Using both the sequence and structure of BPI, along with recently developed sequence-sequence and sequence-structure similarity search methods, we have identified 13 distant members of the family in a diverse set of eukaryotes, including rat, chicken, Caenorhabditis elegans, and Biomphalaria galbrata. Although the sequence similarity between these 13 new members and any of the 4 original members of the BPI family is well below the "twilight zone," their high sequence-structure compatibility with BPI indicates they are likely to share its fold. These findings broaden the BPI family to include a member found in retina and brain, and suggest that a primitive member may have contained only one of the two similar domains of BPI.