Structural and functional organization of the human neutrophil 60 kDa bactericidal/permeability-increasing protein.

We have isolated, after limited proteolysis of the bactericidal/permeability-increasing protein (BPI) of human polymorphonuclear leukocytes (PMN), two fragments representing roughly the two halves of the BPI molecule. The 25 kDa N-terminal fragment possesses all the antibacterial activities of the 60 kDa parent protein, while the ca. 30 kDa C-terminal fragment has no detectable activity. The 25 kDa fragment is as potent on a molar basis as holo-human BPI against rough Escherichia coli, is more potent than holo-BPI against more resistant smooth E. coli, and retains the specificity of BPI toward Gram-negative bacteria. The findings suggest that all of the molecular determinants of the antibacterial properties of BPI reside within the N-terminal half of the molecule, implying a novel structural/functional organization for a cytotoxic protein.