Fc receptors play an important role in leukocyte activation and the modulation of ligand binding ("activation") is a critical point of regulation. Previous studies demonstrated that the Fc receptor for IgA (FcalphaRI/CD89) is regulated by cytokine stimulation, switching it to a high-binding state. To investigate the mechanism by which cytokine-induced signal transduction pathways result in FcalphaRI activation, cell lines expressing various receptor mutants were generated. Binding studies indicated that truncation of the C-terminus of the FcalphaRI resulted in constitutive IgA binding, removing the need for cytokine stimulation. Furthermore, mutagenesis of a single C-terminal serine residue (S263) to alanine (S>A) (single-letter amino acid codes) also resulted in constitutive IgA binding, whereas a serine to aspartate (S>D) mutation was no longer functional. The role of S263 might be in regulating the interaction with the cytoskeleton, because disruption of the cytoskeleton results in reduced IgA binding to both FcalphaRwt and FcalphaR_S>A. In addition, overexpression of a membrane-targeted intracellular domain of FcalphaR, and the introduction of cell-permeable CD89 fusion proteins blocked IgA binding, implying a competition for endogenous proteins. The proposal is made that Fc receptors are activated by cytokines via an inside-out mechanism converging at the cytoplasmic tail of these receptors. (Blood. 2001;97:3478-3483)