Antibacterial 15-kDa protein isoforms (p15s) are members of a novel family of leukocyte proteins.

We have previously described the isolation and initial characterization of functionally distinct 15-kDa protein isoforms (p15s) from rabbit polymorphonuclear leukocytes (PMN) that bind with high affinity to Escherichia coli and modulate the antibacterial actions of other leukocyte proteins on this Gram-negative bacterium. We now report the cloning and sequencing of two distinct cDNAs from a rabbit bone marrow library that encode p15s differing at only 2 residues (His-3, Arg-88 versus Arg-3, Trp-88). Tryptophan-directed chemical cleavage of two isoforms purified from a single rabbit confirms the existence of multiple isoforms with distinct function and primary structure in a single rabbit. The p15 cDNAs encode putative signal sequences and studies of cellular and subcellular localization indicate that the p15s are granule-associated proteins of PMN. Both purified isoforms bind avidly to lipopolysaccharide (LPS), the major component of the Gram-negative bacterial outer membrane. Analysis of the deduced primary structures of the p15s reveals homology to three other leukocyte proteins: CAP-18, an 18-kDa LPS-binding protein from rabbit PMN, pro-indolicidin, a 16-kDa precursor of an antibacterial peptide of bovine PMN, and cathelin, an 11-kDa cysteine protease inhibitor from porcine leukocytes, suggesting the existence of a novel family of leukocyte proteins with LPS-binding, antimicrobial, and protease-inhibitory activities.