Cytokine-induced inside-out activation of FcaR (CD89) is mediated by a single serine residue (S263) in the intracellular domain of the receptor

Abstract

Fc receptors play an important role in leukocyte activation and the modulation of ligand binding ("activation") is a criti-cal point of regulation. Previous studies demonstrated that the Fc receptor for IgA (FcaRI/CD89) is regulated by cytokine stimulation, switching it to a high-binding state. To investigate the mechanism by which cytokine-induced signal transduc-tion pathways result in FcaRI activation, cell lines expressing various receptor mu-tants were generated. Binding studies indicated that truncation of the C-termi- nus of the FcaRI resulted in constitutive IgA binding, removing the need for cyto-kine stimulation. Furthermore, mutagen-esis of a single C-terminal serine residue (S263) to alanine (S>A) (single-letter amino acid codes) also resulted in consti-tutive 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 cy-toskeleton, because disruption of the cy-toskeleton results in reduced IgA binding to both FcaRwt and FcaR_S>A. In addi- tion, overexpression of a membrane-targeted intracellular domain of FcaR, and the introduction of cell-permeable CD89 fusion proteins blocked IgA bind-ing, implying a competition for endoge-nous 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.