Structural requirements for intracellular processing and sorting of bactericidal/permeability‐increasing protein (BPI): comparison with lipopolysaccharide‐binding protein

The bactericidal/permeability‐increasing protein (BPI), which is stored in the azurophil granules of neutrophils, and the circulating lipopolysaccharide‐binding protein (LBP) share the same structure. Both bind lipopolysaccharide of gram‐negative bacteria through their amino‐terminal domains. The carboxy‐terminal domain of BPI promotes bacterial attachment to phagocytes, whereas the corresponding domain of LBP delivers lipopolysaccharide to monocytes/macrophages. Our aim was to investigate the role of the amino‐ and carboxy‐terminal domains of BPI and LBP for sorting and storage in myeloid cells after transfection of cDNA to two rodent hematopoietic cell lines. Full‐length BPI and LBP were both targeted for storage in these cells. Deletion of the carboxy‐terminal half of BPI resulted in storage followed by degradation while the reciprocal deletion of the amino‐terminal half led to retention in the endoplasmic reticulum for proteasomal degradation. Chimeras between halves of BPI and LBP were also targeted for storage, but those containing carboxy‐terminal BPI had the highest stability, again indicating a role for the carboxy‐terminal domain of BPI in protection against degradation. Therefore, we propose a critical stability function for the hydrophobic carboxy‐terminal domain of BPI during intracellular sorting for storage while the amino‐terminal domain may confer targeting for storage.