Structural characterization of BPI-modulating 15 kDa proteins from rabbit polymorphonuclear leukocytes: identification of a novel family of leukocyte proteins.

We have previously described the isolation and initial characterization of 15 kDa protein isoforms (p15s) from rabbit polymorphonuclear leukocytes (PMN) that bind to Escherichia coli and modulate the antibacterial actions of other leukocyte proteins on this gram negative organism. We now report that the p15s differ in primary structure. The cloning and sequencing of two distinct p15 cDNAs from a rabbit bone marrow library reveal that two of the isoforms are closely similar in primary structure differing at only two amino acid positions. The p15 cDNAs encode putative signal sequences suggesting a granule-associated localization for these proteins. Analysis of the derived p15 primary structures reveals homology to two leukocyte proteins: CAP-18, an 18 kD lipopolysaccharide (LPS) binding protein from rabbit PMN and cathelin, an 11 kD cysteine protease inhibitor from porcine leukocytes. This structural similarity suggests the existence of a novel family of low molecular weight leukocyte proteins with potential roles in inflammation.