Molecular cloning and identification of the porcine cytolytic trigger molecule G7 as a Fc gamma RIII alpha (CD16) homologue.

The G7 mAb binds a previously unidentified cytolytic trigger molecule on porcine NK cells and polymorphonuclear cells (PMN). This mAb has been shown to almost completely block PBL-mediated Ab-dependent cellular cytotoxicity and inhibit PMN-mediated Ab-dependent cellular cytotoxicity by approximately one-half, indicating a close functional association of the G7 molecule (to which the G7 mAb binds) with Fc gamma R function. This study was designed to identify the G7 molecule through molecular cloning of its cDNAs. The G7 protein was purified and peptide fragments subjected to amino acid sequencing. This information was then used in the PCR amplification of three cDNA fragments encoding a full-length G7 polypeptide. Amino acid sequences corresponding to peptides derived from G7 are observed within the deduced amino acid sequence of the individual clones confirming the specificity of G7 purification and PCRs. The specificity is also confirmed by reactivity of an antiserum raised against a synthetic G7 peptide to immunoprecipitated G7. Northern blot analysis indicates that a 0.9-kb G7 mRNA is expressed in porcine PMN, PBMC, macrophage, spleen, and lymph nodes but not in thymus or liver. Nucleotide and deduced amino acid sequence analyses indicate that G7 represents a porcine homologue to human Fc gamma RIIIA alpha. This is especially important in that G7 has been previously demonstrated to exist in a molecular complex with a novel molecule, PNK-E. The data suggests the presence of a novel Fc gamma RIII alpha molecular complex (G7/PNK-E) in the porcine system.