The interaction between human Fc gamma RI and the gamma-chain is mediated solely via the 21 amino acid transmembrane domain of Fc gamma RI

We have established a biological assay to investigate the nature of the non-covalent interaction between two integral type membrane proteins, Fc gamma RI and gamma-chain. Fc gamma RI, the human high affinity receptor for immunoglobulin G (IgG), is expressed on the surface of macrophages and monocytes and mediates a broad range of important immunological functions. Fc gamma RI relies on a functional interaction with a second integral type I membrane protein, gamma-chain, to mediate many of these functions. For example, Fc gamma RI can only mediate phagocytosis of IgG-coated particles in COS cells when co-expressed with gamma-chain. We have previously shown that the cytoplasmic domain of Fc gamma RI is not necessary for this functional interaction, In this study we have used the phagocytosis assay to investigate the role of the transmembrane region of Fc gamma RI in mediating this functional interaction with gamma-chain by using mutant and chimeric forms of the receptor. Three mutants, which introduce or remove charged residues from a conserved 10 amino acid stretch of amino acids in the proximal transmembrane region of Fc gamma RI, were able to mediate phagocytosis of IgG-coated particles. In contrast, two chimeric receptors, in which 21 of the amino acids in the distal transmembrane region of Fc gamma RI were replaced with the transmembrane region of the related receptors CD2 or LFA3, were expressed but failed to interact functionally with gamma-chain to mediate phagocytosis. Thus, these mutants demonstrate that the interaction between human Fc gamma RI and gamma-chain is mediated solely via these 21 amino acids in the transmembrane domain of Fc gamma RI.