Protein fusions of BPI with CETP retain functions inherent to each

Cholesteryl ester transfer protein (CETP), bactericidal/permeability inducing protein (BPI), and lipopolysaccharide binding protein (LBP) are members of the lipid transfer/lipopolysaccharide binding protein (LT/LBP) family of proteins that share a common secondary/tertiary structure. Despite this commonality of structure, very different patterns of lipid binding and protein-protein interactions are observed among the family members. BPI was previously shown to retain aspects of its own function when part of it was fused with LBP to form a chimeric protein. We have extended those observations to CETP. Some aspects of cholesteryl ester transfer function can be maintained in a chimeric protein even when over 40% of the sequence is from BPI. Further replacement of an additional 60 amino acids resulted in a complete loss of CETP function even though the chimera was able to retain some BPI-like properties. These artificial fusions retain BPI functions such as lipopolysaccharide (LPS) binding and protein-protein interactions that are not observed with native CETP. BPI-CETP chimeras are inhibited by LPS but cannot be inhibited by small molecule CETP inhibitors as effectively as native CETP. These results localize the site of LPS binding in BPI to a region no larger than the amino terminal 155 amino acids. This region can participate in some protein-protein interactions similar to intact BPI. Chimeras containing the amino terminus of CETP and the carboxy terminus of BPI did not retain any observable CETP function. These results further confirm the modular nature of the LT/LBP family of proteins but also highlight the discrete nature of their individual functions.