Identification of a Novel FcγRIIIaα-Associated Molecule That Contains Significant Homology to Porcine Cathelin

The following studies are the first to demonstrate the association of porcine FcgammaRIIIaalpha with a molecule that contains significant homology to the cathelin family of antimicrobial proteins. We performed immunoprecipitation of the porcine FcgammaRIIIaalpha multisubunit complex from Brij 96 lysates of polymorphonuclear leukocytes using the G7 mAb, which binds to FcgammaRIIIaalpha on the surface of porcine NK cells and phagocytes. Previous results indicate that the transmembrane alpha subunit of the FcgammaRIIIa complex is associated with the gamma subunit on the surface of porcine polymorphonuclear leukocytes and with several other unique proteins that surface iodinate and migrate at approximately 15, 20, and 25 kDa when analyzed by reducing SDS-PAGE. Through characterization of the porcine FcgammaRIIIa complex, we identified the 15-kDa molecule as a unique FcgammaR-associated protein that has not been described in other systems. We now report an association between FcgammaRIIIaalpha and a 15-kDa molecule that shares homology to cathelin, a protein of undetermined function initially identified in porcine leukocytes. A domain with a high degree of homology to cathelin is found in the proregions of a family of antibiotic proteins referred to as cathelicidins. The results of our studies indicate the presence of a novel FcgammaRIIIa complex in the porcine system, and may provide new insights into the function of this antimicrobial protein homologue in relation to the variety of responses mediated through FcgammaRs.