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 th e non-covalent interaction between two integral type membrane proteins , Fc gamma RI and gamma-chain. Fc gamma RI, the human high affinity re ceptor for immunoglobulin G (IgG), is expressed on the surface of macr ophages and monocytes and mediates a broad range of important immunolo gical 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 phagocy tosis of IgG-coated particles in COS cells when co-expressed with gamm a-chain. We have previously shown that the cytoplasmic domain of Fc ga mma RI is not necessary for this functional interaction, In this study we have used the phagocytosis assay to investigate the role of the tr ansmembrane region of Fc gamma RI in mediating this functional interac tion with gamma-chain by using mutant and chimeric forms of the recept or. Three mutants, which introduce or remove charged residues from a c onserved 10 amino acid stretch of amino acids in the proximal transmem brane 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 o r LFA3, were expressed but failed to interact functionally with gamma- chain to mediate phagocytosis. Thus, these mutants demonstrate that th e interaction between human Fc gamma RI and gamma-chain is mediated so lely via these 21 amino acids in the transmembrane domain of Fc gamma RI.